Workshop on Nanostructured Graphene - CMT group - Universiteit ...

Organizers:
<strong>Workshop</strong> on
Nanostructured Graphene
Book of Abstracts
Antwerp
21st-24th May 2013
Antwerp 21-24 May 2013
Prof. Dr. François Peeters, Universiteit Antwerpen, Condensed Matter Theory
Prof. Dr. Pawel Hawrylak, NRC Canada, Quantum Theory Group
Dr. Lucian Covaci, Universiteit Antwerpen, Condensed Matter Theory

Supported by:
European Science Foundation
(Eurocores project: EuroGraphene)
Flemish Science Foundation
Scientific Research Communities (WOG):
- Computational modelling of materials
- Tuning the functional properties of nanoparticles and nanowires
University of Antwerp

Programme of the workshop on nanostructured graphene
Antwerp, 21 st -24 th May 2013
Tuesday 21 May Wednesday 22 May Thursday 23 May Friday 24 May Time
Registration 8.30 – 9.00
L. Vandersypen M. Morgenstern C. Stampfer J. Folk 9.00 - 10.00
F. Amet P. Potasz S. Yuan M. Connolly 10.00 - 10.30
Coffee Break Coffee Break Coffee Break Coffee Break 10.30 - 11.00
E. Andrei K. Ensslin A. Geim A.-P. Jauho 11.00 – 12.00
B. Trauzettel D. Guclu V. Pereira A. Chaves 12.00 - 12.45
Lunch & discussions Lunch & discussions Lunch & discussions Lunch & discussions 12.45 – 14.15
S. Ulloa J. Fernandez-Rossier R. Egger M. Neek-Amal 14.15 – 15.00
T. Chakraborty G. Burkard J. McGuire L. Covaci 15.00 – 15.45
Coffee Break &
Poster session 1
Coffee Break &
Poster session 2
Visit historical
Antwerp
Conference dinner
(Hof van Liere)
End of workshop
15:30
15.45 – 17.45
20.00 – 22.00
55 + 5 mins
40 + 5 mins
25 + 5 mins

Tuesday May 21 2013
Session: Morning session
Part 1: Invited talks
9:00 - 10:00 Lieven Vandersypen, Delft Technical University, Netherlands
Ballistic transport in CVD and natural graphene
10:00 - 10:30 Francois Amet, Stanford University, USA
Insulating behavior in monolayer graphene on boron-nitride
10:30 - 11:00 Coffee Break
11:00 - 12:00 Eva Andrei, Rutgers University, USA
Screening Charged Impurities in Graphene: From Cloaking to Supercriticality
12:00 - 12:45 Björn Trauzettel, University of Wuerzburg, Germany
Transport properties of graphene nanostructures
12:45 - 14:15 Lunch and discussions
Session: Afternoon session
14:15 - 15:00 Sergio Ulloa, Ohio University, USA
Spin Scattering in Graphene: Impurity Characterization and Birefringent Electron
Optics
15:00 - 15:45 T. Chakraborty, University of Manitoba, Canada
Novel physics of interacting Dirac fermions in a quantizing magnetic field
15:45-17:45 Coffee Break and Poster Session 1

Wednesday May 22 2013
Session: Morning session
9:00 - 10:00 Markus Morgenstern, RWTH Aachen, Germany
Probing Graphene on the Nanoscale by Scanning Tunneling Microcopy
10:00 - 10:30 Pawel Potasz: Wroclaw University of Technology, Poland
Electronic and magnetic properties of triangular graphene quantum dots and
rings
10:30 - 11:00 Coffee Break
11:00 - 12:00 Klaus Ensslin, ETH Zürich, Switzerland
Graphene nanodevices with reduced disorder
12:00 - 12:45 Devrim Guclu, Izmir Institute of Technology, Turkey
Optical and electrical control of magnetic properties of graphene quantum
dots and Mobius nanoribbon rings
12:45 - 14:15 Lunch and discussions
Session: Afternoon session
14:15 - 15:00 Joaquin Fernadez-Rossier, Iberian Nanotechnology Laboratory, Spain
Anisotropic intrinsic spin relaxation in graphene
15:00 - 15:45 Guido Burkard, University of Konstanz, Germany
Spin and valley in graphene quantum dots
15:45-17:45 Coffee Break and Poster Session 2

Thursday May 23 2013
Session: Morning session
9:00 - 10:00 Christoph Stampfer, RWTH Aachen, Germany
Progresses in etched graphene quantum dots
10:00 - 10:30 Shengjun Yuan, Radboud University, Netherlands
Modeling Electronic, Optical and Magnetic Properties of Graphene
10:30 - 11:00 Coffee Break
11:00 - 12:00 Andre Geim, Manchester University, UK
Some interaction, magnetic and superlattice effects in graphene
12:00 - 12:45 Vitor Pereira, National University of Singapore, Singapore
Resonant tunneling in graphene pseudomagnetic quantum dots
12:45 - 14:15 Lunch and discussions
Session: Afternoon session
14:15 - 15:00 Reinhold Egger, University of Dusseldorf, Germany
Strongly interacting Dirac fermions in graphene quantum dots
15:00 - 15:45 John McGuire, Michigan State University, USA
Excitonic Interactions in Colloidal Graphene Quantum Dots
15:45-17:45 Visit historical Antwerp
20:00-22:00 Conference dinner at the University Club: Hof van Liere

Friday May 24 2013
Session: Morning session
9:00 - 10:00 Josh Folk, University of British Columbia, Canada
Defect-Mediated Spin Relaxation in Graphene
10:00 - 10:30 Malcolm R. Connolly, National Physical Laboratory, UK
Rapid time-controlled emission of single Dirac fermions from graphene quantum
dots
10:30 - 11:00 Coffee Break
11:00 - 12:00 Antti-Pekka Jauho, Technical University of Denmark, Denmark
Transport in nanostructured graphene
12:00 - 12:45 Andrey Chaves, Universidade Federal do Ceara, Brazil
Wave packet dynamics in graphene: strain effects, edge scattering and valley filtering
12:45- 14:15 Lunch and discussions
Session: Afternoon session
14:15 - 14:45 Mehdi Neek-Amal, Universiteit Antwerpen, Belgium
Boron nitride monolayer under non-uniform strain
14:45 - 15:15 Lucian Covaci, Universiteit Antwerpen, Belgium
Andreev quantum dots in graphene
15:30 End of the workshop

Abstracts of invited talks

Ballistic transport in CVD and natural graphene
Lieven Vandersypen ∗
Kavli Institute of Nanoscience, TU Delft
We will present unpublished and recently published work on transport measurements
of graphene devices in the ballistic regime. Most recently, we have observed
transverse magnetic focussing in micron size Hall bars created from single layer
CVD graphene. The synthesis is done under highly controlled conditions, resulting
in single crystals with up to 0.5 mm in lateral dimensions, and electron mobilities
over 100.000 cm 2 /Vs at 4K (on hBN). For the first time, ballistic transport over
1 µm was observed in CVD graphene. Comparison of atomic force microscopy
and transport measurements indicate that when wrinkles are present, they spoil
ballistic transport. Earlier, we have created split-gate bilayer graphene devices
(using graphene exfoliated from natural graphite), sandwiched in between hBN dielectrics.
Transport through electrostatically induced constrictions shows signs of
quantized conductance, also a signature of ballistic transport. Transport through
two constrictions in series shows clear Coulomb blockade at low temperature.
Finally, Coulomb blockade was observed even at room temperature, in devices
created by the controlled rupture of a graphene sheet subjected to a large electron
current in air. The size of the quantum dot islands is estimated to be in the 1 nm
range, giving addition energies up to 1.6 eV.
Measurements and sample fabrication were carried out by Victor Calado,
Stijn Goossens and Amelia Barreiro. CVD graphene was synthesized by Shou-En
Zhou in the group of Guido Janssen (TU Delft).
[1] Ballistic transport in CVD graphene. In preparation.
[2] Gate defined zero- and one-dimensional confinement in bilayer graphene, A.M.
Goossens, S.C.M. Driessen, T.A. Baart, K. Watanabe, T. Taniguchi, L.M.K. Vandersypen,
Nano letters 12 , 4656 (2012).
[3] Quantum Dots at Room Temperature Carved out from Few-Layer Graphene,
A. Barreiro, H. S. J. van der Zant, and L. M. K. Vandersypen, Nano Lett. 12,
6096 (2012).
∗ Electronic address: l.m.k.vandersypen@tudelft.nl

Insulating behavior in monolayer graphene on
boron-nitride
Francois Amet ∗ and David Goldhaber-Gordon †
Stanford University
The conductivity at the neutrality point in monolayer graphene is known to
saturate on the order of e 2 /h due to disorder-induced density-fluctuations. In
this study, in contrast, we observed a diverging peak resistivity in high-mobility
graphene devices with a boron-nitride substrate and a suspended top-gate. At low
temperature, the peak resistivity under the top-gate increases by three orders of
magnitude and becomes as high as several megohms. In a perpendicular magnetic
field, the device remains insulating and directly transitions to the ν = 0 quantum
Hall phase. We discuss the roles of substrate-induced valley symmetry-breaking
and external screening of density fluctuations by the top-gate as causes of our
observations.
∗ Electronic address: amet@stanford.edu
† Electronic address: goldhaber-gordon@stanford.edu

Screening Charged Impurities in Graphene:
From Cloaking to Supercriticality
Eva Y. Andrei ∗
Rutgers Unversity
We study a charged impurity in graphene and its screening by the Dirac electrons
in the presence of a magnetic field. Using scanning tunneling microscopy
and spectroscopy we show that the effective charge of the impurity can be tuned
by employing a gate voltage to control the occupancy of Landau levels. At low
occupancy strong screening by the conduction electrons renders the impurity essentially
invisible. Screening progressively diminishes with increased occupancy
until, for fully occupied Landau-levels, the unscreened impurity significantly perturbs
the spectrum in its vicinity, lifting the orbital degeneracy and causing the
Landau-levels to split into discrete states. In this regime the impurity displays signatures
of supercritical behavior associated with electronic orbits collapsing into
its core. Thus the magnetic field makes it possible to tune the strength of interactions
from subcritical to supercritical, providing direct experimental access to
Coulomb criticality.
∗ Electronic address: eandrei@physics.rutgers.edu

Transport properties of graphene nanostructures
Björn Trauzettel ∗
Würzburg University, Germany
We discuss transport properties of nanostructured graphene, for instance,
graphene nanoribbons and graphene rings. In our theoretical analysis, these systems
are either coupled to normal metal or to superconducting electron reservoirs.
We discover interesting fingerprints of graphene-specific physics. Particularly, we
will present a color-dependence of the conductance in disordered graphene due
to adatoms as well as the possibility to distinguish specular from retro Andreev
reflection in graphene rings coupled to one normal metal and one superconducting
lead.
∗ Electronic address: trauzettel@physik.uni-wuerzburg.de

Spin Scattering in Graphene: Impurity
Characterization and Birefringent Electron
Optics
Mahmoud M. Asmar ∗ and Sergio E. Ulloa †
Ohio University and Freie Universitat Berlin
An important element on the dynamics of spins in materials is the spin-orbit
interaction (SOI), which reflects/arises from intrinsic lack of inversion symmetry
in the lattice structure, or via broken symmetries in the system due to external
or interfacial fields (Rashba interaction). Although intrinsic SOI is weak in
graphene, the Rashba SOI can in fact be large due to strong local hybridizations
by impurities of defects or by manipulation of substrates or applied gates [1]. We
have studied electron/hole transport in graphene under sizeable SOI and address
theoretically some of the anticipated observables due to this effect.
We have developed analytical spinor solutions of the Dirac equation that include
spin dependent observables, and use these to examine the role of SOI on scattering
cross sections. By calculating the ratio of total to transport cross section at low
energy we are able to probe the degree of isotropy of the scattering processes,
and consequently probe the nature of the impurities and defects present in the
graphene sample. We show that at low energies, this ratio of cross sections
(equivalent to the ratio of scattering times obtainable from experiments) can be
nearly 1 (instead of 2, as expected with no SOI), to a degree that depends on the
Rashba SOI strength. This suggests then a sample specific measurement of the
important effective size of the SOI, especially if one is to consider spin transport.
We also show that Rashba SOI in graphene gives rise to optical birefringence
in electron optics, which in essence reflects the intrinsic crystal structure even
at long electronic wavelengths. This effect requires the presence of Rashba SOI,
where different group velocities depend on the chirality of the electronic states,
mimicking the light polarization dependence of the group velocities in optical
birefringent materials. This can in principle be achieved via gated regions, and
result in the formation of spinful cusps and caustics caused by the Veselago
lens defined by the gate. Interestingly, this would be evident by the doubling of
caustics and cusps produced by circular birefringent lenses, where the spacing
∗ Electronic address: MA236408@ohio.edu
† Electronic address: ulloa@ohio.edu

etween the two different chiral cusps is proportional to the strength of the
Rashba interaction in the system [2].
[1] D. Marchenko et al., Nature Commun. 3, 1232 (2012).
[2] M. M. Asmar and S. E. Ulloa, Phys. Rev. B 87, 075420 (2013).

Novel physics of interacting Dirac fermions in a
quantizing magnetic field
Tapash Chakraborty ∗
University of Manitoba, Winnipeg, Canada
The relativistic-like behavior of electrons in graphene exhibits novel interactioninduced
properties in a quantizing magnetic field. Here one observes many intriguing
properties of the fractional quantum Hall effect states that are absent
in conventional (non-relativistic) semiconductor systems. In bilayer graphene the
interaction strength can be controlled by a bias voltage and by the orientation of
the magnetic field. The finite bias voltage between the graphene monolayers can
in fact, enhance the interaction strength in a given Landau level. As a function of
the bias voltage, a graphene bilayer system shows transitions from a state of weak
electron-electron interactions to a state with strong interactions. Interestingly,
the in-plane component of a tilted magnetic field can also alter the interaction
strength in bilayer graphene. Tuning of the interaction strength of Dirac fermions
in graphene therefore provides a unique route to a profound understanding of interacting
fermions in a magnetic field. The nature of the Pfaffian state in bilayer
graphene will also be discussed and shown that the stability of this state can be
greatly enhanced by applying an in-plane magnetic field.
∗ Electronic address: tapash@physics.umanitoba.ca

Probing Graphene on the Nanoscale by Scanning
Tunneling Microcopy
Markus Morgenstern ∗
Institute of Physics and JARA-FIT, RWTH Aachen, D. 52074 Aachen, Germany
We use ultra-high-vacuum scanning tunneling microscopy at low temperature
(6 K) to probe basic features of graphene relevant to mechanical and quantum
mechanical applications.
Firstly, I present wave function mapping in graphene quantum dots deposited
on Ir(111) [1]. The quantum dots are confined exclusively by zig-zag edges. However,
edge states are absent due to an exchange interaction of the π-bands of
graphene with the dz2 surface states of the Ir(111) [2]. The exchange interaction
gets continuously smaller away from the edges, which leads to weak confinement
of the quantum dot states being decisive for a rather regular wave function shape
observed experimentally. The wave functions are additionally influenced by the
penetration of an sp-like surface state of Ir(111) into graphene [1]. Concerning
the edge state, DFT calculations reveal that H-terminated graphene nanoribbons
on Au(111) should exhibit magnetic edge states [3]. First experiments will be
shown. Secondly, the mechanical properties of graphene flakes deposited by the
scotch tape method are probed by STM. The graphene is partly not conformal to
the substrate, but provides areas which are freely suspended [4]. The valleys of
this corrugation can be further lifted by the van-der Waals forces and the electrostatic
forces of the tip of the STM [5]. During continuous lifting, we often observe
a breaking of atomic symmetry, i.e. the atomic structure switches reproducibly
between a hexagonal and a triangular appearance.
[1] D. Subramaniam et al., Phys. Rev. Lett. 108 (2012) 046801.
[2] Y. Li et al., Adv. Materials, 25, 1967 (2013).
[3] Y. Li et al., arXiv:1210.2876.
[4] V. Geringer et al., Phys. Rev. Lett. 102 (2009) 076102.
[5] T. Mashoff et al., Nano. Lett. 10 (2010) 461.
∗ Electronic address: mmorgens@physik.rwth-aachen.de

Electronic and magnetic properties of triangular
graphene quantum dots and rings
Pawel Potasz ∗
Institute of Physics, Wroclaw University of Technology
The theory of electronic and magnetic properties of triangular graphene quantum
dots (TGQD) and rings (TGQR) with zigzag edges is presented. Single particle
properties are investigated using the tight-binding model and the effect of
electron-electron interactions are treated using both mean-field Hubbard model
and a combination of tight-binding, Hartree-Fock, and configuration interactions
methods (tb+HF+CI). We show that TGQDs break symmetry between the two
sublattices of a graphene bipartite honeycomb lattice, which leads to an appearance
of a shell of degenerate states at the Fermi level Ef = 0 in the middle of
the energy gap [1-6]. We derive an analytical form of the eigenfunctions corresponding
to the zero-energy shell and analyze shell degeneracy as a function of
size of TGQDs and width of TGQRs [5,6]. The stability with respect to disorder
and the influence of the external magnetic field is considered [5, 7, 8]. Next, the
total spin and electronic correlations are investigated using tb+HF+CI method.
We show that for the charge neutral TGQD, the degenerate shell is half-filled and
the electron-electron interactions lead to the appearance of a finite magnetic moment
[1-9]. This result is consistent with Lieb’s theorem for a Hubbard model
on a bipartite lattice [10]. Using mean-field Hubbard model, we analyze width
and size dependence of the magnetic moment in TGQR. The stability of the magnetic
properties in TGQD and TGQR with respect to shell filling controlled by
the external gate is investigated using tb+HF+CI [4, 9]. We show that both the
many-body energy gap and total spin oscillates as a function of the filling fraction.
In particular, spin polarized ground state at half-filling is depolarized by addition
of a single electron for TGQDs of sizes below a critical size. The effect of electronic
correlations on Coulomb and Spin blockade in transport through a TGQD
are discussed [4].
Collaborators: A. Guclu, O. Voznyy, M. Korkusinski, and P. Hawrylak.
[1] M. Ezawa, Phys. Rev. B 76, 245415 (2007).
[2] J. Fernandez-Rossier and J. J. Palacios, Phys. Rev. Lett. 99, 177204 (2007).
[3] W. L. Wang, S. Meng, and E. Kaxiras, Nano Letters 8, 241 (2008).
[4] A. D. Güçlü, P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak, Phys.
Rev. Lett. 103, 246805 (2009).
∗ Electronic address: pawel.potasz@pwr.wroc.pl

[5] P. Potasz, A. D. Güçlü, and P. Hawrylak, Phys. Rev. B 81, 033403 (2010).
[6] P. Potasz, A. D. Güçlü, and P. Hawrylak, Phys. Rev. B 83, 174441 (2011).
[7] M. Ezawa, Physica E 42 , 703 (2010).
[8] O.Voznyy, A. D. Güçlü, P. Potasz and P. Hawrylak, Phys. Rev. B 83, 165417
(2011).
[9] P. Potasz, A. D. Güçlü, A. Wojs, and P. Hawrylak, Phys. Rev. B 85, 075431
(2012).
[10] E. H. Lieb, Phys. Rev. Lett. 62, 1201 (1989).

Graphene nanodevices with reduced disorder
Klaus Ensslin ∗
ETH Zurich
Graphene quantum devices, such as nanoribbons, quantum rings and quantum
dots often display electronic properties which are governed by disorder potentials
arising from bulk and edge contributions.
In order to reduce bulk disorder, single layer graphene micron-sized devices and
nanoribbons have been fabricated on hexagonal boron nitride substrates. These
micron-sized devices have significantly higher mobility and lower disorder density
compared to devices fabricated on silicon dioxide substrate in agreement with previous
findings. The transport characteristics of the reactive-ion-etched graphene
nanoribbons on hexagonal boron nitride, however, appear to be very similar to
those of ribbons on a silicon dioxide substrate. A detailed study reveals both similarities
as well as differences between the two types of devices. This suggests that
the edges have an important influence on transport in reactive-ion-etched graphene
nanodevices. In an alternative approach with the goal to minimize edge disorder
graphene quantum structures are prepared on bilayer graphene. Using a homogeneous
back gate and a nanostructured top gate energy gaps can be locally induced
into the bilayer graphene. This way the edges are determined by electrostatics and
possibly screening in graphene which should lead to smoother edges compared to
etched samples.
This work was done in collaboration with D. Bischoff, A. Varlet, P. Simonet, C.
Barraud and T. Ihn.
∗ Electronic address: ensslin@phys.ethz.ch

Optical and electrical control of magnetic
properties of graphene quantum dots and
Mobius nanoribbon rings
A. Devrim Güçlü ∗
Izmir Institute of Technology, Izmir, Turkey
We present a theory of optical, magnetic and topological properties of graphene
quantum dots and graphene Möbius strips using numerical techniques combining
tight-binding, Hartree-Fock and configuration interactions methods. Triangular
graphene quantum dots with zigzag edges [1-6] exhibit robust magnetic moment
and optical transitions simultaneously in the THz, visible and UV spectral ranges
due to the existence of a band of degenerate states lying at the Fermi level. The
magnetic and optical properties [3-6] are determined by strong electron-electron
and excitonic interactions. We show that the magnetization of the zigzag edges can
be manipulated optically. The magnetic moment can be first erased by addition
of a single electron spin with a gate, then restored by absorption of a photon.
The conversion of a single photon to a magnetic moment results in a many-body
effect, optical spin blockade. In graphene Möbius nanoribbon rings [7], the finite
width opens a gap and nontrivial topology of the Möbius ring leads to a single
edge with an induced, effective gauge field, in analogy to topological insulators.
The single zigzag edge leads to a shell of degenerate states at the Fermi level
and a ferromagnetic (FM) ground state at half-filling, due to electron-electron
interactions. For fractional fillings, the magnetic moment is found to oscillate as
a function of the shell filling.
Collaborators: P. Potasz, I. Özfidan, O. Voznyy, M. Korkusinski, M. Grabowski
and P. Hawrylak.
[1] J. Fernandez-Rossier, and J. J. Palacios, Phys. Rev. Lett. 99, 177204 (2007).
[2] M. Ezawa, Phys. Rev. B 76, 245415 (2007).
[3] A. D. Güçlü, P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak, Phys
Rev. Lett. 103, 246805 (2009).
[4] A. D. Güçlü, P. Potasz, and P. Hawrylak, Phys. Rev. B, 82, 155445 (2010).
[5] P. Potasz, A. D. Güçlü, A. Wojs,P. Hawrylak, Phys. Rev. B 85, 075431
(2012).
[6] A. D. Güçlü, P Hawrylak, Phys. Rev. B 87, 035425 (2013).
[7] A. D. Güçlü, M Grabowski, P Hawrylak, Phys. Rev. B 85, 035435 (2013).
∗ Electronic address: devrimguclu@iyte.edu.tr

Anisotropic intrinsic spin relaxation in graphene
J. Fernandez-Rossier ∗ and D. Gosalbez
International Iberian Nanotechnology Laboratory (INL), Braga, Portugal and
Universidad de Alicante, Alicante, Spain
P. Merodio-Camara and S. Fratini
Institut Neel, CNRS, Grenoble, France
In this talk I discuss an intrinsic spin scattering mechanism in graphene originated
by the interplay of atomic spin-orbit interaction and the local curvature
induced by flexural distortions of the atomic lattice. Starting from a multiorbital
tight-binding Hamiltonian with spin-orbit coupling considered non-perturbatively,
we derive an effective Hamiltonian for the spin scattering of the Dirac electrons
due to flexural distortions. We compute the spin lifetime due to flexural phonons
and we find values in the microsecond range at room temperature. Interestingly,
these spin lifetimes are anisotropic on two counts. First, they are different for
different spin quantization axis. Second, they depend on the in-plane momentum
direction of the initial state. Our results set upper limits for the spin lifetimes
in graphene that will be dominant if extrinsic sources of spin relaxation can be
removed from the samples.
∗ Electronic address: joaquin.fernandez-rossier@inl.int

Spin and valley in graphene quantum dots
Guido Burkard ∗
University of Konstanz, Germany
Graphene has emerged as an interesting material for coherent spin physics and
spin qubits, due to the low concentration of nuclear spins and relatively weak spinorbit
coupling. However, the localization of electrons in quantum dots in graphene
is a non-trivial task due to the absence of a band gap and the related effect of Klein
tunneling [1]. Among the possible solutions to this problem are electrostatically
defined quantum dots in armchair graphene nanoribbons [2] or gapped graphene
[3]. Interestingly, the valley degeneracy present in graphene modifies the spin-orbit
induced spin relaxation [4,5] as well as hyperfine interaction with C-13 nuclear spins
and plays an important role in the spin-valley blockade in double quantum dots
[6]. The exchange coupling in graphene mixes spin and valley degrees of freedom
and calls for special procedures for spin-based quantum information processing [7].
[1] M. I. Katsnelson, K. S. Novoselov, A. K. Geim, Nature Phys. 2, 620 (2006).
[2] B. Trauzettel, D. Bulaev, D. Loss, and GB, Nature Phys. 3, 192 (2007).
[3] P. Recher, J. Nilsson, GB, and B. Trauzettel, Phys. Rev. B 79, 085407
(2009).
[4] P. R. Struck and GB, Phys. Rev. B 82, 125401 (2010).
[5] M. Droth and GB, Phys. Rev. B 84, 155404 (2011).
[6] A. Palyi and GB, Phys. Rev. B 80, 201404 (2009).
[7] N. Rohling and GB, New Journal of Physics 14, 083008 (2012).
∗ Electronic address: Guido.Burkard@uni-konstanz.de

Progresses in etched graphene quantum dots
Christoph Stampfer ∗
JARA-FIT and II. Institute of Physics B,
RWTH Aachen, 52074 Aachen, Germany
Graphene quantum dots are interesting and promising systems for studying longliving
spin states of confined electrons. In particular nanostructured graphene
quantum devices based on (reactive ion) etching processes have been studied extensively
in the last few years. In these quantum devices, Coulomb blockade, excited
states, spin states and electron-hole crossover have been successfully demonstrated.
However, due to bulk disorder and edge roughness the behavior of all these devices
is irregular and control proves to be a challenge [1-2].
Here we report our recent progress on quantum transport studies on etched
graphene quantum dots. In particular, we focus on graphene quantum dots with
highly tunable tunneling barriers, which allow for the first time pulse-gated transient
current spectroscopy measurements of excited states. These experiments
provide insights to relaxation times of confined electrons in graphene and allow us
to extract a lower limit of the charge relaxation rates on the order of 60-100 ns
[3]. Furthermore, we show a detailed analysis of transport measurements taken on
etched graphene quantum dots resting on hexagonal boron nitride substrates. In
agreement with earlier experiments we find that nanostructures with dimensions
smaller 150 nm are strongly limited by edge roughness. Consequently an improvement
of the substrate induced disorder is not helping to improve the overall device
performance. Finally, we report on recent progress on a disorder potential reducing
edge functionalization of graphene nanodevices.
[1] B. Terres, J. Dauber, C. Volk, S. Trellenkamp, U. Wichmann, and C.
Stampfer, Appl. Phys. Lett. 98, 032109 (2011)
[2] C. Volk, S. Fringes, B. Terrs, J. Dauber, S. Engels, S. Trellenkamp, and C.
Stampfer, Nano Lett. 11, 3581 (2011)
[3] C. Volk, C. Neumann, S. Kazarski, S. Fringes, S. Engels, F. Haupt, A. Mueller,
and C. Stampfer, Nature Communication DOI: 10.1038/ncomms2738, in press
(2013).
∗ Electronic address: stampfer@physik.rwth-aachen.de

Modeling Electronic, Optical and Magnetic
Properties of Graphene
Shengjun Yuan ∗ and Mikhail I. Katsnelson
Institute for Molecules and Materials, Radboud University of Nijmegen,
NL-6525AJ, Nijmegen, The Netherlands
One of the most important problems in graphene is to understand the influence
of disorder on its physical properties. Motivated by recent experiments, we
performed a systemic study of the electronic, optical and magnetic properties
of single-layer, multilayer and nanostructured graphene [1-12], including different
types of imperfection such as vacancies, adatoms, admolecules, ripples, puddles
and coulomb impurities.
The methods are based on the numerical simulations of electrons in the framework
of full pi-band tight-binding model, without diagonalization of the Hamiltonian
matrix. The density of states is calculated by the time-evolution method,
which is based on the simulation of wave packet evolution according to the timedependent
Schrdinger equation, with additional averaging over random superposition
of basis states. We extend the time-evolution method by using the Kubo
formula to the calculation of polarization function, dielectric function, response
function, energy loss function, electronic and magnetic susceptibility, static and
dynamical (optical) conductivity, diffusion coefficients, mean free path, localization
length, charge velocity and mobility. The magnetic field is introduced by
means of the Peierls substitution, and the effect of electron-electron interaction
is considered within the random phase approximation. The Klein tunneling and
quantum interference are studied by direct simulation of the wave packet propagation.
Our numerical methods allow us to carry out calculations for rather large
systems, up to hundreds of millions of atoms, with a computational effort that
increases only linearly with the system size.
[1] S. Yuan, H. De Raedt, and M. I. Katsnelson, Phys. Rev. B 82, 115448 (2010).
[2] S. Yuan, H. De Raedt, and M. I. Katsnelson, Phys. Rev. B 82, 235409 (2010).
[3] S. Yuan, R. Roldn, and M. I. Katsnelson, Phys. Rev. B 84, 035439 (2011).
[4] S. Yuan, R. Roldn, and M. I. Katsnelson, Phys. Rev. B 84, 125455 (2011).
[5] S. Yuan, R. Roldn, and M. I. Katsnelson, Solid State Comm. 152, 1446
(2012).
[6] S. Yuan et al, Phys. Rev. B 84, 195418 (2011).
∗ Electronic address: s.yuan@science.ru.nl

[7] S. Yuan et al, Phys. Rev. Lett. 109, 156601 (2012).
[8] S. Yuan et al, Phys. Rev. B 87, 085430 (2013).
[9] A. Singha et al, Science 332, 1176 (2011).
[10] T. O. Wehling et al, Phys. Rev. Lett. 105, 056802 (2010).
[11] R. R. Nair et al, Small 6, 2877 (2010).
[12] M. A. Akhukov et al, New J. of Phys. 14, 123012 (2012).

Some interaction, magnetic and superlattice
effects in graphene
∗ Electronic address: andre.k.geim@manchester.ac.uk
André Geim ∗
University of Manchester

Resonant tunneling in graphene pseudomagnetic
quantum dots
Vitor M. Pereira ∗
National University of Singapore
Endowed with the strongest covalent bonding in nature, graphene exhibits the
largest tensional strength ever registered (E ≈ 1 TPa), and a record range of
elastic deformation for a crystal, which can be as high as 15-20 %. Such outstanding
mechanical characteristics are complemented by an unusual coupling of
lattice deformations to the electronic motion, that can be captured by the concept
of a fictitious or pseudo-magnetic field (PMF) arising as a result of non-uniform
local changes in the electronic hopping amplitudes. Since electrons in graphene
respond to these local PMFs as they would to a real magnetic field, this specific
strain-induced perturbation is not screened by the free electrons in the same
way that the usual displacement field coupling can be. Consequently, the ability
to manipulate the strain distribution in graphene opens the enticing prospect
of strain-engineering its electronic and optical properties, as well as of enhancing
interaction and correlation effects. The recent experimental confirmation that
PMFs in the striking range 300-600 T are possible with spontaneously occurring
deformations in structures spanning only a few nm brings this prospect of strongly
impacting graphenes electronic properties by strain closer to fruition.
Despite this recent experimental evidence for strong PMF-induced Landau quantization
and various studies addressing implications of PMF for the graphene electronic
spectrum, the nature (and opportunities) of electronic transport in such
nanostructures remains scarcely explored. In this context a combination of analytical,
numerical, and simulation approaches will be used to describe features
of quantum transport in a representative situation where tailored local strain distributions
are used to confine electrons at the nanoscale. The fact that local
non-uniform strain manifests itself as a PMF in graphene which can easily exceed
100 T opens the prospect for local pseudo-magnetic confinement of carriers.
This allows for analogues of magnetic quantum dots, where transport proceeds
by resonant tunneling assisted by Landau levels in the confinement region, with
the important difference that there is no real magnetic field, and the effect can
be spatially restricted to a few nm. The transport characteristics of one such
strained pseudo-magnetic quantum dot structure in the y-junction geometry will
be presented and discussed.
∗ Electronic address: vpereira@nus.edu.sg

Strongly interacting Dirac fermions in graphene
quantum dots
Reinhold Egger ∗
University of Duesseldorf, Germany
Low-energy quasi-particles in monolayer graphene provide a realization of massless
Dirac fermions in two spatial dimensions. Electron-electron interactions in
that case should be quite strong, with effective fine structure constant of order
unity. Artificial atoms can then be formed by confining N quasiparticles (on top
of the filled Dirac sea) in a quantum dot. We discuss the electronic structure
and stability of this interacting N particle Dirac system. In particular, a finitesize
version of the bulk excitonic instability is predicted [1] and Wigner molecule
formation should be detectable [2].
[1] T. Paananen and R. Egger, Phys. Rev. B 84, 155456 (2011) .
[2] T. Paananen, R. Egger, and H. Siedentop, Phys. Rev. B 83, 085409 (2011).
∗ Electronic address: egger@thphy.uni-duesseldorf.de

Excitonic Interactions in Colloidal Graphene
Quantum Dots
John A. McGuire ∗
Department of Physics and Astronomy, Michigan State University
We present results of optical studies of single- and biexcitons in graphene quantum
dots (GQDs) consisting of 132 and 168 C atoms in fully aromatic structures,
i.e., having no unpaired pz electrons, and having HOMO-LUMO transitions of 1.6-
1.8 eV. Bottom-up synthesis allows the creation of such GQDs with well defined
lattice structure.[1] GQDs of > 100 sp 2 hybridized C atoms are intriguing systems
both for applications in solar energy conversion and for fundamental studies of
carrier interactions and dynamics in strongly confined two-dimensional systems.
We focus here on the earliest stages of the exciton lifetime after optical excitation.
Compared to traditional semiconductor quantum dots, the weak screening associated
with the two-dimensional lattice of C atoms leads to strong carrier-carrier
interactions. In both time-resolved photoluminescence and transient absorption
(TA) measurements of GQDs dispersed in toluene, we see fast cooling of carriers
with electron-hole pairs generated with >1 eV excess energy relaxing to the band
edge within a few hundred femtoseconds. Following initial cooling, we identify
biexciton binding energies of >200 meV for the lowest-energy optically accessible
biexcitons. Biexcitons are expected to be short lived in such strongly confined
structures. In 380-nm long carbon nanotubes, nonradiative Auger recombination
of biexcitons to a single exciton occurs on a timescale of ∼1 ps.[2] A naive linear
scaling of Auger lifetime with nanotube length extended to the size of our GQDs
would suggest a biexciton lifetime of the order of 5 fs GQDs.[3] However, on increasing
the excitation intensity from the single- to multi-photon-absorption regime, we
observe the emergence of dynamics on the ∼300 fs timescale. Excitation-fluencedependence
of this component suggests that Auger recombination of biexcitons in
our GQDs occurs on a ∼300 fs timescale.
[1] Yan, X.; Cui, X.; Li, L. S., Synthesis of Large, Stable Colloidal Graphene
Quantum Dots with Tunable Size. J. Am. Chem. Soc. 2010, 132, 5944-5945.
[2] Wang, F.; Dukovic, G.; Knoesel, E.; Brus, L. E.; Heinz, T. F., Observation
of Rapid Auger Recombination in Optically Excited Semiconducting Carbon
Nanotubes. Phys. Rev. B 2004, 70, 241403.
[3] Wang, F.; Wu, Y.; Hybertsen, M. S.; Heinz, T. F., Auger Recombination of
Excitons in One-Dimensional Systems. Phys. Rev. B 2006, 73, 245424.
∗ Electronic address: mcguire@pa.msu.edu

Defect-Mediated Spin Relaxation in Graphene
Josh Folk
University of British Columbia ∗
This talk with describe a transport measurement that disentangles mechanisms
of spin and orbital phase relaxation in graphene. The measurement is based
on well-known quantum interference phenomena–weak localization and universal
conductance fluctuations. We show that a careful analysis of the in-plane magnetic
field and temperature dependences of these effects can separately quantify
spin-orbit and magnetic scattering rates; this technique works especially well in
graphene due to its single-atom thickness. Spin relaxation in exfoliated graphene
on SiO2 is found to be dominated by magnetic scattering (scattering off of magnetic
defects), with a smaller contribution from in-plane spin-orbit interaction. A
similar measurement is performed in graphene on SiC; interestingly, both magnetic
scattering and spin-orbit interaction are a factor of 10 stronger than in exfoliated
graphene.
∗ Electronic address: jfolk@physics.ubc.ca

Rapid time-controlled emission of single Dirac
fermions from graphene quantum dots
M. R. Connolly, ∗ S. P. Giblin, M. Kataoka, and J. D. Fletcher
National Physical Laboratory, UK
K. L. Chiu, C. Chua, J. P. Griffiths, and G. A. C. Jones
University of Cambridge, UK
V. I. Fal’ko
University of Lancaster, UK
C. G. Smith
University of Cambridge
T. J. B. M. Janssen
National Physical Laboratory
Electronic excitations in undoped monolayer graphene behave dynamically like
chiral massless Dirac fermions. Electrical transport measurements traditionally
probe the unique nature of scattering, interference, and localization of these quasiparticles
by driving randomly generated wavepackets through a graphene layer and
monitoring the evolution of the time-averaged current density with magnetic field,
temperature, and carrier concentration. In this work we describe a different mechanism
for generating currents which is based on the time-controlled release of single
charge-quantized excitations from lithographically defined graphene quantum dots
[1]. The application of phase-shifted radiofrequency signals to all-graphene side
gates results in a single charge being transferred between the dots each cycle, generating
a net current equal to the fundamental electronic charge times the drive
frequency [2]. Quantized charge transfer is observed up to GHz frequencies, an
order of magnitude higher than previously achieved using conventional metallic or
semiconductor adiabatic charge pumps. We discuss the role played by the physical
and electronic properties of graphene in facilitating such efficient charge transfer,
the character of the emitted wavepacket, and the conformity of the experimental
data to theoretical models based on adiabatic transfer through metallic islands
[3]. High-speed emission of tailorable wavepackets provides the foundations for exploring
and manipulating the quantum nature of individual electronic excitations
in graphene, paving the way towards single Dirac fermion quantum optics [4] and
read-out of spin-based graphene qubits for quantum information processing [5].
∗ Electronic address: mrc1@npl.co.uk

[1] M. R. Connolly, et al., Nature Nanotechnology (2013) (in press);
arXiv:1207.6597 (2012)
[2] H. Pothier, et al., Europhysics Letters, 17(3) 249-254 (1992).
[3] N. Winkler, et al., Phys. Rev. B 79, 235309 (2009).
[4] E. Bocquillon, et al., Science 339, 1054 (2013).
[5] B. Trauzettel, et al., Nature Phys. 3, 192-196 (2007).

Transport in nanostructured graphene
Antti-Pekka Jauho ∗ and Mikkel Settnes †
CNG - Center for Nanostructured Graphene, DTU Nanotech
Stephen Power ‡
CNG - Center for Nanostrcutured Graphene, DTU Nanotech
We address two different situations. (1) Regular perforations of graphene with
nanoscale holes (Graphene antidot lattices, GAL) can be used to locally modify
the electronic structure, e.g., to introduce an energy gap [1]. Recent research has
shown that it is possible to create photonic crystal-like electron wave-guides in
graphene by using GALs as the confining agent [2]. Here, we report on our recent
simulations of such structures, employing a recursive Green’s function technique.
We focus on two issues: the efficiency of the wave-guide in confining the electron
wave-function for different Fermi energies, and the role of antidot disorder on the
conductance profile of such systems. (2) We have developed a theoretical formulation
to interpret measurements of the conductive properties of nanostructured
graphene, using multiple STM-type probes. The situation differs conceptually
from the standard Landauer configuration: instead of having a finite sample between
(multiple) probes, we consider a large sample, but which is probed locally
by several probes. This is a microscopic extension of the standard four-point spectroscopy
[3]. Results will be presented for various geometries with one fixed probe,
with a local impurity (impurities), and a scannable second probe.
[1] T. G. Pedersen et al., Phys. Rev. Lett. 100, 136804 (2008).
[2] J. G. Pedersen et al., Phys. Rev. B 86, 245410 (2012).
[3] P. Boggild, et al, Rev. Sci. Instr., 71, 2781-2783 (2000).
∗ Electronic address: Antti-Pekka.Jauho@nanotech.dtu.dk
† Electronic address: mikse@nanotech.dtu.dk
‡ Electronic address: spow@nanotech.dtu.dk

Wave packet dynamics in graphene: strain
effects, edge scattering and valley filtering
Andrey Chaves ∗
Universidade Federal do Ceara, Brazil
Theoretical models based on wave packet propagation have been extensively applied
in the study of transport properties of electrons in low dimensional structures.
In this work, we demonstrate the use of wave packet propagation for the study of
a series of interesting phenomena in monolayer and bilayer graphene. The time
evolution of the wave packet is calculated by means of a split-operator technique
[1] adapted for the tight-binding Hamiltonian describing electrons in graphene, as
well as for the Dirac Hamiltonian for low energy electrons in the continuum model.
We discuss how wave packets can be used to investigate several well known phenomena
in graphene in greater details, such as Klein tunneling [2], zitterbewegung
[3] and the inter- and intra-valley scattering in zigzag and armchair boundaries, respectively
[4]. Most importantly, we will show how the dynamics of a wave packet
have been used to predict other interesting effects in graphene, for example (i) the
existence of a non-propagating state in the armchair boundary of a strained monolayer
graphene nanoribbon, and the valley filtering (ii) by a quantum point contact
in a bilayer graphene and (iii) by a monolayer-bilayer [5] graphene junction.
[1] A. Chaves et al., Phys. Rev. B 82, 205430 (2010);
[2] J. M. Pereira Jr. et al., Semicond. Sci. Technol. 25 033002 (2010);
[3] T. M. Rusin and W. Zawadzki, Phys. Rev. B 80, 045416 (2009);
[4] A. R. Akhmerov and C. W. J. Beenacker, Phys. Rev. B 77, 085423 (2008);
[5] T. Nakanishi et al., Phys. Rev. B 82, 125428 (2010).
∗ Electronic address: andrey@fisica.ufc.br

Boron nitride monolayer under inhomogeneous
stress
Mehdi Neek-Amal ∗
Universiteit Antwerpen
The electronic properties of a strained hexagonal boron-nitride sheet are investigated
using density functional theory and classical molecular dynamics simulations.
Triaxial in-plane strain is applied to a hexagonal shaped boron-nitride flake with
zig-zag edges. Different from graphene, the triaxial strain localizes the molecular
orbital close to the Fermi level in the center of the sheet depending on the type
of edge atoms (i.e. B versus N). The energy gap decreases with increasing strain.
We show how inhomogeneous strain on the boron-nitride sheet can be used to
selectively detect various gas molecules.
∗ Electronic address: neekamal@srttu.edu

Andreev quantum dots in graphene
Lucian Covaci ∗
Department of Physics, Universiteit Antwerpen
Although graphene is not intrinsically superconducting, Cooper pairs can diffuse
from a superconducting contact. The superconducting proximity effect was
observed experimentally in graphene Josephson junctions with contacts made of
various superconducting materials like Al, Pb, Nb and even layered materials like
NbSe2 [1,2]. When the energy of the electrons in the graphene layer is below the
superconducting gap of the contacts, they will be bound in the normal region.
These are the well known Andreev bound states. We consider a graphene layer
deposited on top of a rough superconducting surface such that the graphene layer
can be considered to be partially freestanding and strained. It was recently shown
that strain has a peculiar effect on the electronic properties in graphene, namely
that it will coupled exactly like a gauge field [3,4]. Under certain conditions it is
thus possible to have strong pseudo-magnetic fields and even pseudo-Landau levels
coexisting with superconducting correlations [5]. By using an efficient numerical
method [6] we solve the Bogoliubov-de Gennes equations for a tight binding model
of the graphene layer. Deformations of the graphene layer are considered as Gaussian
bumps around imperfections of the substrate. We show that in the regions
where the sheet is freestanding, bound states due to Andreev reflections appear.
[1] H. B. Heersche, P. Jarillo-Herrero, J. B. Oostinga, L. M. K. Vandersypen,
and A. F. Morpurgo, Nature (London) 446, 56 (2007).
[2] A. Kanda, T. Sato, H. Goto, H. Tomori, S. Takana, Y. Ootuka, and K.
Tsukagoshi, Physica C 470, 1477 (2010).
[3] F. Guinea, M. I. Katsnelson, and A. K. Geim, Nat. Phys. 6, 30 (2009).
[4] N. Levy, S. A. Burke, K. L. Meaker, M. Panlasigui, A. Zettl, F. Guinea, A.
H. C. Neto, and M. F. Crommie, Science 329, 544 (2010).
[5] L. Covaci and F. M. Peeters, Phys. Rev. B 84, 241401(R) (2011).
[6] L. Covaci, F. M. Peeters, and M. Berciu, Phys. Rev. Lett. 105, 167006
(2010).
∗ Electronic address: lucian@covaci.org

Abstracts of poster presentations

Part 2: Contributed poster presentations
Poster Session 1 ( Tuesday - 21 May)
P01. Z. Ao Electric field induced hydrogenation of graphene
P02. S. Ulloa Doping dependence of Kondo states in bilayer graphene
P03. S. Barraza-Lopez Strain-engineering of graphene electronic structure beyond continuum elasticity
P04. G. R. Berdiyorov Structural properties and stability of silicon clusters intercalated bilayer graphene
P05. B. Beschoten Spin transport in single and bilayer graphene
P06. C. De Beule Gapless interface states at the junction between two topological insulators
P07. P. Butti Efficiency of graphene based rectifiers
P08. P. Clark Graphene Quantum Point Contacts in the Ballistic Regime
P09. S. Costamagna Life-time of acoustic phonon modes in Graphene
P10. M. Droth Electron Spin Relaxation in Graphene Nanoribbon Quantum Dots
P11. B. Van Duppen Chiral tunneling in graphene multilayers
P12. D. Faria Persistent currents and pseudomagnetic fields in strained graphene rings
P13. O. Leenaerts Tunable Dirac cone in bilayer graphyne
P14. S. P. Milovanovic Graphene Hall bar with an asymmetric pn-junction
P15. D. Moldovan Electronic states in a graphene flake strained by a Gaussian bump
Poster Session 2 (Wednesday - 22 May)
P16. W. A. Munoz Superconducting correlations in multilayer graphene
P17. G. Nanda Contacting of graphene devices by direct-write atomic layer deposition
P18. I. Ozfidan Optical properties of colloidal graphene quantum dots
P19. S. R. Power RKKY interactions in uniaxially strained graphene
P20. S. Reichardt Relaxation times and electron-phonon interaction in graphene quantum dots
P21. N. Sandler Pumping in graphene ribbons: transport in adiabatic and non-adiabatic regimes
P22. S. E. Savel'ev Current resonances in graphene with time dependent potential barriers
P23. A. Shylau Interaction-induced enhancement of g factor in graphene
P24. S. K. Singh Melting of graphene clusters
P25. J. Sivek
Adsorption and absorption of Boron, Nitrogen, Aluminium and Phosphorus on
Silicene: stability, electronic and phonon properties
P26. H. Söde Band gap evolution and termini of short graphene nanoribbons
P27. E. Tovari Large scale nanopatterning of graphene
P28. H. Xu Andreev reflection by magnetic barriers in superconductor contacted graphen
P29. M. Zarenia Snake states in graphene quantum dots in the presence of a p-n junction
Anomalous Raman Spectra and Thickness Dependent
P30. H. Sahin Electronic properties of WSe2
Phonon Softening and Direct to Indirect Bandgap Crossover in
P31. S. Sahin Strained Single Layer MoSe2

P01: Electric field induced hydrogenation of
graphene
Zhimin Ao ∗
The University of New South Wales
Due to the importance of hydrogenation of graphene for several applications, we
present an alternative approach to hydrogenate graphene based on density functional
theory calculations. We find that a perpendicular electric field F can act
as a catalyst to reduce the energy barrier for molecular H2 dissociative adsorption
on both pristine and N-doped graphene. For the case of pristine graphene,
increasing -F would decrease this dissociation barrier, and above 0.02 a.u. (1 a.u.
= 5.14 ∗ 10 11 V/m) this process occurs smoothly without any potential barrier.
However, F should been removed after the H2 dissociation to complete this hydrogenation
process. For the case of N-doped graphene, the dissociative adsorption
energy barrier of a H2 molecule on a pristine graphene layer changes from 2.7 eV
to 2.5 eV on N-doped graphene, and to 0.88 eV on N-doped graphene under an
electric field of 0.005 a.u.. When increasing the F above 0.01 a.u. the barrier disappears.
This hydrogenation process can be completed automatically. Therefore,
applying an electric field and N doping have catalytic effects on hydrogenation of
graphene, which can be used for hydrogen storage purposes and nanoelectronic
applications.
∗ Electronic address: zhimin.ao@unsw.edu.au

P02: Doping dependence of Kondo states in
bilayer graphene
D. Mastrogiuseppe, Sergio E. Ulloa, ∗ and N. Sandler
Department of Physics and Astronomy, and Nanoscale
and Quantum Phenomena Institute, Ohio University and
Dahlem Center for Complex Quantum Systems and Fachbereich Physik, Freie Universität Berlin
A. Wong and K. Ingersent
Department of Physics, University of Florida
One of the remarkable manifestations of cooperative phenomena in condensed
matter physics is the many-body screening of a magnetic impurity placed in a
metallic system, the Kondo effect. Although the physics underlying this effect
in ordinary metals is well understood, microscopic symmetries can give rise to
intricate features in the effective density of states of the host with profound consequences
in the Kondo regime. Bilayer graphene (BLG) is an example of such
a material with a gate-dependent gap and large pseudospin symmetry that provides
an ample set of different microscopic environments for intercalated magnetic
impurities. Combined to its easy tunability, BLG is an ideal material to study
quantum phase transitions into various types of Kondo states.
We provide a full characterization of these transitions for a magnetic impurity
intercalated in Bernal-stacked BLG and symmetrically coupled to carbon atoms
on each layer, as a function of doping level of the system. Two factors determine
the wealth of phases predicted: 1) the particular dispersion relation of BLG that
gives rise to an interesting density of states with a discontinuity at the interlayer
hopping energy; and 2) the properties of the microscopic coupling between the
impurity and the layers that define different symmetries for the possible phases.
A multiband Anderson Hamiltonian that includes interaction and different hybridization
environments describes the system. After an appropriate Schrieffer-
Wolff transformation, we find the effective single-channel Kondo model with a
strongly energy-dependent exchange coupling between conduction electron and
impurity spins. This effective Kondo Hamiltonian reveals the possibility of driving
the system through quantum phase transitions via changes in the chemical
potential through gating or doping.
We use numerical renormalization group calculations to accurately describe the
Kondo regime. Our calculations reveal zero-temperature transitions between localmoment
and singlet strong-coupling phases under variation of band filling and/or
energy of the impurity level. The latter show different regimes, such as con-
∗ Electronic address: ulloa@ohio.edu

ventional Kondo, pseudogap Kondo, and local-singlet ground states, distinguishable
by their thermodynamic and spectral properties. We also obtain the Kondo
temperature dependence with the chemical potential within the different regimes,
which would be accessible via STM experiments.

P03: Strain-engineering of graphene electronic
structure beyond continuum elasticity
Salvador Barraza-Lopez ∗
University of Arkansas
Alejandro Pacheco Sanjuan †
Universidad del Norte
Zhengfei Wang ‡
University of Utah
Mihajlo Vanevic §
University of Belgrade
The discussion of strain engineering of graphenes electronic structure has always
been given in a context where the mechanics enters within first-order continuum
elasticity. Is it possible to re-express the theory at a different level of approximation
on the mechanics? In following this program, is there something new and
fundamental to be learned? When derived, how can we use this new formulation?
It is possible to express the theory dispensing with continuum elasticity, and we
will present a new first-order approach to strain-engineering of graphenes electronic
structure where no continuous displacement field u(x,y) is ever required. The
theory is directly expressed in terms of atomic displacements under mechanical
load. The approach is valid for negligible curvature. In this formulation of the
theory the deformation potential and pseudo-magnetic field take discrete values at
any given unit cell.
In following this program we learn two fundamental things: Guinea, Katsnelson,
and Geim (Nature, 2010) indicate in opening statements that in order to obtain
gauge fields one may want to verify that mechanical strain actually varies smoothly
at each unit cell, such that sublattice symmetry holds, and pseudospin Hamiltonians
can be laid out. No such check exists to date, and we provide it. The second
piece of fundamental understanding comes by demonstrating that the purported
correction to the theory (Kitt, PRB, 2012) does not hold. We are not aware of
such proof being given elsewhere. This is a contentious issue, and addressing it
would help in moving the field forward.
The approach has been validated, cross-checked, and the theory in the continuum
∗ Electronic address: sbarraza@uark.edu
† Electronic address: alepach75@yahoo.com
‡ Electronic address: zfwang1981@gmail.com
§ Electronic address: mihajlo.vanevic@gmx.com

appears as a limiting case, when the scale of the mechanical deformation is small
in comparison with the lattice constant.
We illustrate the formalism by providing strain-derived fields and local density
of electronic states on graphene membranes with large numbers of atoms.
We wish to convey that in considering the mechanics from actual displacements
-as opposed to deformation fields obtained from idealizations of graphene as a
continuum media- one charters new ways to think of the interrelation between the
mechanics and the electronic structure of graphene. The present method leads to
important insight, and complements the prevalent understanding of strain engineering
of graphene’s electronic structure from first-order continuum elasticity.
References:
[1] Strain gauge fields for rippled graphene membranes under central mechanical
load: an approach beyond first-order continuum elasticity. J. V. Sloan, A. A.
Pacheco Sanjuan, Z. Wang, C. Horvath, and S. Barraza-Lopez. Physical Review
B (Accepted on April 04, 2013).
[2] Strain-engineering of graphene electronic structure beyond continuum elasticity.
S. Barraza-Lopez, A.A. Pacheco Sanjuan, Z. Wang, and M. Vanevic. (In
preparation.)

P04: Structural properties and stability of
silicon clusters intercalated bilayer graphene
G. R. Berdiyorov, ∗ M. Neek-Amal, and F. M. Peeters †
Departement Fysica, Universiteit Antwerpen
Adri C. T. van Duin
Department of Mechanical and Nuclear Engineering, the Pennsylvania State University
Structural properties of Sin clusters (n ≤ 24) intercalating bilayer graphene
are studied using reactive molecular dynamics simulations. A confinement from
graphene layers results in a planar clustering of Si atoms, which is energetically unfavorable
for free standing Si nanostructures. Hexagonal arrangement of Si atoms
is observed for larger n with slight buckling in agreement with recent first principles
calculations. The graphene layers considerably increases thermal stability
of these quasi-two dimensional structures; they are found to be stable well above
the room temperature. Our findings, which are also supported by the density
functional tight binding method, can be useful in understanding the mechanisms
of epitaxial growth of few layer graphene on SiC substrate.
∗ Electronic address: golibjon.berdiyorov@ua.ac.be
† Electronic address: francois.peeters@ua.ac.be

P05: Spin transport in single and bilayer
graphene
B. Beschoten, ∗ F. Volmer, M. Drogeler, E.
Maynicke, T.-Y. Yang, and G. Guntherodt
2. Physikalisches Institut, RWTH Aachen University
Graphene is considered to be a promising candidate for spintronics applications.
The reason is the weak intrinsic spin-orbit coupling, the negligible hyperfine interaction
and the observation of micrometer long spin relaxation lengths [1]. So far
most spin transport studies have focused on single layer graphene (SLG). However,
bilayer graphene (BLG) has unique electronic properties, which differ greatly from
those of SLG by its effective mass of carriers, interlayer hopping and electric-field
induced band gap. Our studies of spin transport in BLG as a function of mobility,
minimum conductivity, charge carrier density and temperature reveal the importance
of the Dyakonov - Perel (DP)-type spin scattering mechanism [2]. In BLG
samples, the spin dephasing time scales inversely with the charge carrier mobility
both at room temperature and at low temperatures. Spin dephasing times of up
to 2 ns are observed in samples with the lowest mobility. Remarkably, we observe
a similar inverse scaling of the spin dephasing times and the charge carrier mobility
in SLG samples [3]. We discuss the role of intrinsic and extrinsic factors that
could lead to the dominance of the DP-type spin scattering mechanism in both
SLG and BLG. Furthermore, we demonstrate the role of adatoms and of tunneling
vs. transparent Co/MgO/graphene contacts for the manipulation of the spin
dephasing times. Work supported by DFG through FOR 912.
[1] N. Tombros et al., Nature 448, 571 (2007).
[2] T.-Y. Yang et al., Phys. Rev. Lett. 107, 047206 (2011).
[3] F. Volmer et al., in preparation.
∗ Electronic address: bernd.beschoten@physik.rwth-aachen.de

P06: Gapless interface states at the junction
between two topological insulators
Christophe De Beule ∗ and Bart Partoens †
University of Antwerp
A topological insulator is a symmetry protected quantum state of matter, that is
insulating in the bulk, but conducting on the surface. Moreover, the surface electrons
form a helical liquid, and remain conducting for any time-reversal invariant
perturbation that does not close the bulk energy gap.
Protected bound states also exist at the junction between two topological insulators
with surface states that have opposite helicity direction [1]. The low-energy
physics of this system resembles that of bilayer graphene, as both result from the
hybridization of Dirac cones.
We use a low-energy model [2] for a topological insulator with a single Dirac
cone on the surface, to calculate the properties of the interface states. We find
gapless interface states when the surface states of the topological insulators have
opposite, as well as equal helicity [3]. We note that the bulk gap closes when the
helicity changes orientation, allowing for a topological phase transition along the
junction [1]. Recently, superluminal tachyon-like excitations were also predicted to
exist in this system [4]. We found, however, that these states are artifical and due
to a wrong implementation of the model under certain pathological conditions.
[1] R. Takahashi and S. Murakami, Phys. Rev. Lett. 107, 166805 (2011).
[2] H. Zhang et al., Nat. Phys. 5, 438 (2009).
[3] C. De Beule and B. Partoens, Phys. Rev. B 87, 115113 (2013).
[4] V. M. Apalkov and T. Chakraborty, Eur. Phys. Lett. 100, 17002 (2012).
∗ Electronic address: christophe.debeule@ua.ac.be
† Electronic address: bart.partoens@ua.ac.be

P07: Efficiency of graphene based rectifiers
Pascal Butti, ∗ Ivan Shorubalko, † and Urs Sennhauser ‡
EMPA - Electronics/Metrology/Reliability Laboratory,
Swiss Federal Laboratories for Materials Science and Technology, 8600 Duebendorf, Switzerland
Klaus Ensslin §
Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland
One class of graphene based nanodevices promising for future electronics is ballistic
switches and rectifiers. Recently, voltage rectification in graphene threeterminal
nanojunctions was demonstrated at room temperature [1]. The mechanisms
influencing the efficiency of rectification are still not well understood.
The influence of different geometrical designs of those junctions is studied in this
work. Graphene monolayer flakes are obtained by mechanical exfoliation of natural
graphite and identified by Raman spectroscopy. Electrical contacts and junction
designs are fabricated by electron beam lithography, followed by metallization
(Ti/Au) and reactive ion etching, respectively. Electrical characterization of the
devices is done at room temperature, 77K and 4K. In parallel, finite element simulations
are performed to model diffusive transport in the devices, taking into
account temperature, fringing effects from the electric field and potential disorder.
[1] A. Jacobsen, I. Shorubalko, L. Maag, U.Sennhauser and K. Ensslin, Appl.
Phys. Lett. 97, 032110 (2010).
∗ Electronic address: pascal.butti@empa.ch
† Electronic address: ivan.shorubalko@empa.ch
‡ Electronic address: urs.sennhauser@empa.ch
§ Electronic address: ensslin@phys.ethz.ch

P08: Graphene Quantum Point Contacts in the
Ballistic Regime
Paul Clark ∗ and Hendrik Ulbricht †
School of Physics and Astronomy, University of Southampton,
Southampton, United Kingdom, SO17 1BJ
Graphene offers a new and unique opportunity to recreate and adapt quantum
optics experiments with electrons. Work in coherent manipulation of electrons has
been historically focused on the quantum Hall regime in structured GaAs devices.
Recent advances in substrates for graphene allow the investigation of quantum
optics in the ballistic regime. Nano structuring graphene allows a direct method to
control the allowed motional quantum states of electrons in such solids. A quantum
point contact (QPC) is an important building block to realise quantum optics
experiments in graphene as it can act as a coherent source and as an electronic
beam splitter. Graphene devices have been fabricated with structures for a QPC
using both electron beam and Helium ion lithography with a varying constriction
width from 250nm down to 8nm. Results from electrical measurements made at
4.2K will be presented and discussed.
∗ Electronic address: P.Clark@soton.ac.uk
† Electronic address: H.Ulbricht@soton.ac.uk

P09: Life-time of acoustic phonon modes in
Graphene
S. Costamagna ∗
Instituto de Fsica Rosario, Boulevard 27 de Febrero 210 bis, 2000 Rosario, Argentina
K. Michel † and F. M. Peeters ‡
Departement Fysica, Universiteit Antwerpen,
Groenenborgerlaan 171, BE-2020 Antwerpen, Belgium
We study the phonon damping of acoustic phonon modes in graphene by using
the perturbation approximation to the conventional theory of phonons. Considering
cubic anharmonic energy as a perturbation to the harmonic system we
calculated the temperature dependence of the phonon life-times along different
crystallographic directions using a suited discretization of the first Brillouin zone
of graphene. Contributions from Normal and Umklapp processes were obtained
and the relevant damping processes between different acoustic modes identified.
These results help to elucidate the partial contributions of acoustic phonon modes
to the large thermal conductivity of graphene.
∗ Electronic address: costagol@gmail.com
† Electronic address: ktdm@skynet.be
‡ Electronic address: francois.peeters@ua.ac.be

P10: Electron Spin Relaxation in Graphene
Nanoribbon Quantum Dots
Matthias Droth ∗ and Guido Burkard †
University of Konstanz, 78457 Konstanz, Germany
Graphene is promising as a host material for electron spin qubits because of its
predicted potential for long coherence times. In armchair graphene nanoribbons
(aGNRs) a small bandgap is opened, allowing for electrically gated quantum dots,
and furthermore the valley degeneracy is lifted. The spin lifetime T1 is limited
by spin relaxation, where the Zeeman energy is absorbed by lattice vibrations,
mediated by spin-orbit and electron-phonon coupling. We have calculated T1 by
treating all couplings analytically and find that T1 can be in the range of seconds for
several reasons: (i) low phonon density of states away from Van Hove singularities;
(ii) destructive interference between two relaxation mechanisms; (iii) Van Vleck
cancelation at low magnetic fields; (iv) vanishing coupling to out-of-plane modes
in lowest order due to the electronic structure of aGNRs. Owing to the vanishing
nuclear spin of 12 C, T1 may be a good measure for overall coherence. These results
and recent advances in the controlled production of graphene nanoribbons make
this system interesting for spintronics applications.
∗ Electronic address: matthias.droth@uni-konstanz.de
† Electronic address: guido.burkard@uni-konstanz.de

P11: Chiral tunneling in graphene multilayers
Ben Van Duppen ∗ and Francois Peeters †
Departement fysica, Universiteit Antwerpen,
Groenenborgerlaan 171, B-2020 Antwerpen
Charge carriers in monolayer graphene are known to have a lattice induced pseudospin.
One of the consequences of this peculiar property is the occurrence of
Klein tunneling, unimpeded transmission through a pn junction irrespective of
its height, when they hit the junction perpendicularly. In bilayer graphene the
pseudospinorial nature of the charge carriers is perserved, but it now suppresses
the transmission (anti-Klein tunneling) at normal incidence while enhancing it
at oblique incidence. We have investigated at which angles Klein and anti-Klein
tunneling occurs for arbitrary graphene multilayers and present it as an algebraic
formula that depends on the number of layers and their stacking. Furthermore, we
present numerical results for the transmission in bi- and trilayer graphene samples
taking into account all the possible modes of propagation and the intermode scattering.
These results confirm the occurrence of Klein tunneling at the previously
found angles in the low energy regime while showing new transmission features at
high energy. Finally we show that the application of an interlayer bias to bi- and
trilayer samples gives rise to an angular asymmetric transmission probability.
[1] B. Van Duppen and F. M. Peeters, Appl. Phys. Lett., 101, 226101 (2012).
[2] B. Van Duppen and F. M. Peeters, arxiv: 1302.5623 (accepted in Europhys.
Lett.)
[3] B. Van Duppen and F. M. Peeters, arxiv: 1303.6876 (submitted to Phys.
Rev. B)
[4] B. Van Duppen, S.H.R. Sena and F. M. Peeters, arxiv: 1303.0533 (submitted
to Phys. Rev. B)
∗ Electronic address: ben.vanduppen@ua.ac.be
† Electronic address: francois.peeters@ua.ac.be

P12: Persistent currents and pseudomagnetic
fields in strained graphene rings
Daiara Faria ∗
Universidade Federal Fluminense / Ohio University/ Freie Universitt Berlin
Andrea Latge †
Universidade Federal Fluminense
Sergio Ulloa ‡ and Nancy Sandler §
Ohio University/ Freie Universitt Berlin
The ability to apply effective magnetic fields on graphene by manipulating externally
imposed strains has opened an exciting area of research. Experimental
reports indicate the existence of these pseudomagnetic fields in graphene under
tension and recent explorations controlling deformations on its surface in a variety
of shapes in different substrates have proven quite successful [1]. We would like to
explore what other observable consequences these pseudomagnetic fields produce,
especially as they do not break time-reversal symmetry.
We show that curved graphene rings are excellent systems to visualize the competition
between pseudomagnetic fields and real magnetic fluxes in Aharonov-Bohm
(AB) geometries. In order to address this problem we study the states of rings
under the influence of both AB fluxes and strain-induced pseudofields, by numerically
obtaining the strain and field dependence of the ground and low-lying excited
states. We show that the local probability density for different states is strongly
modulated by the presence of pseudofields. Interestingly, even when pseudofields
cannot generate persistent currents by themselves, we show that the strain of
a Gaussian deformation of the ring produces strongly inhomogeneous persistent
current patterns. The observation of structural effects on the current is then only
possible because of the AB field that changes confinement in the inner and outer
edges of the ring. One can say that AB fields then allow the spatial visualization of
the pseudomagnetic fields in the system via the distribution of persistent current
amplitudes induced by both types of fields. We also present detailed comparisons
of different boundary conditions for the states of the ring.
[1] Georgiou et al., Appl. Phys. Lett. 99, 093103 (2011).
∗ Electronic address: daiara.faria@gmail.com
† Electronic address: andrea.latge@gmail.com
‡ Electronic address: ulloa@ohio.edu
§ Electronic address: sandler@ohio.edu

P13: Tunable Dirac cone in bilayer graphyne
Ortwin Leenaerts, ∗ Bart Partoens, † and Franois Peeters ‡
Universiteit Antwerpen, Departement Fysica,
Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
Graphene has some fascinating electronic properties which follow from the special
behavior of its charge carriers that mimic massless Dirac fermions. However, the
existence of Dirac cones in the electronic band structure of graphene is a rare, but
not unique, property. Other materials such as topological insulators, silicene and
some particular graphyne crystalhave been found, or predicted, to show similar
electronic dispersion around the Fermi-level. In this paper we investigate the
electronic structure of the recently proposed α-graphyne structure [1] in its bilayer
form. α-graphyne can be thought of as a layer of graphene in which every bond
is interspaced with two more carbon atoms with sp hybridization. The resulting
crystal has hexagonal symmetry and its electronic band structure shows linear
dispersion close to the Fermi-level. Using first-principles calculations, we show that
bilayer α-graphyne has an electronic band structure that is qualitatively different
from its monolayer form and depends crucially on the stacking mode of the two
layers. Two stable stacking modes are found: a configuration with a gapless
parabolic band structure, similar to AB stacked bilayer graphene, and another
one which exhibits a doubled Dirac-cone spectrum. The latter can be tuned by an
electric field with a gap opening rate of 0.3 e˚A.
[1] D. Malko, C. Neiss, F. Viñes, and A. Görling, Phys. Rev. Lett. 108, 086804
(2012).
∗ Electronic address: Ortwin.Leenaerts@ua.ac.be
† Electronic address: Bart.Partoens@ua.ac.be
‡ Electronic address: Francois.Peeters@ua.ac.be

P14: Graphene Hall bar with an asymmetric
pn-junction
S. P. Milovanovic, ∗ M. R. Masir, † and F. M. Peeters ‡
Universiteit Antwerpen
We investigated the magnetic field dependence of the Hall and the bend resistances
in the ballistic regime for a single layer graphene Hall bar structure containing
a pn-junction. When both regions are n-type the Hall resistance dominates
and Hall type of plateaus are formed. These plateaus occur as a consequence of
the restriction on the angle imposed by Snell’s law allowing only electrons with
a certain initial angles to transmit though the potential step. The size of the
plateau and its position is determined by the position of the potential interface
as well as the value of the applied potential. When the second region is p-type
the bend resistance dominates which is asymmetric in field due to the presence of
snake states. Changing the position of the pn-interface in the Hall bar strongly
affects these states and therefore the bend resistance is also changed. Changing
the applied potential we observe that the bend resistance exhibits a peak around
the charge-neutrality point (CNP) which is independent of the position of the pninterface,
while the Hall resistance shows a sign reversal when the CNP is crossed,
which is in very good agreement with a recent experiment [J. R. Williams et al.,
Phys. Rev. Lett. 107, 046602(2011)].
∗ Electronic address: slavisa.milovanovic@gmail.com
† Electronic address: mrmphys@gmail.com
‡ Electronic address: francois.peeters@ua.ac.be

P15: Electronic states in a graphene flake
strained by a Gaussian bump
D. Moldovan, ∗ M. Ramezani Masir, † and F. M. Peeters ‡
Departement Fysica, Universiteit Antwerpen,
Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
The effect of strain in graphene can be modeled using the pseudo-magnetic vector
potential. Traditionally, this vector potential is derived with only small values
of strain in mind. That is at odds with graphene’s very high strain tolerance,
which can be as high as 25%. Here we evaluate the pseudo-magnetic field model
specifically for high strain and we show significant differences as compare with
numerical tight-binding results. Furthermore, we investigate the properties of
confined electronic states in the strained region for a hexagon shaped flake with
armchair edges. We find that the six-fold symmetry of the wave functions inside
the Gaussian bump is directly related to the different effect of strain along the
fundamental directions of graphene: zigzag and armchair. Low energy electrons
are strongly confined in the armchair directions and are localized on the carbon
atoms of a single sublattice.
∗ Electronic address: dean.moldovan@ua.ac.be
† Electronic address: mrmphys@gmail.com
‡ Electronic address: Francois.Peeters@ua.ac.be

P16: Superconducting correlations in multilayer
graphene
W.A. Munoz, ∗ L. Covaci, † and F.M. Peeters ‡
Universiteit Antwerpen
Using highly efficient GPU-based simulations of the tight-binding Bogoliubov-de
Gennes equations we solve self-consistently for the pair correlation in two different
kidn of based-graphene structures: (i) a superconducting - bilayer graphene
(BLG) - superconducting Josephson junction and (ii) a rhombohedral (ABC) and
Bernal (ABA) multilayer graphene with a finite intrinsic s-wave pairing potential.
For BLG Josephson junction we considered different doping levels for the nonsuperconducting
link where self-consistent results for the pair correlation ands superconducting
current resemble those reported previously for single layer graphene
except at the Dirac point where remarkable differences in the proximity effect are
found as well as a suppression of the superconducting current in long junction
regime. Inversion symmetry is broken by considering a potential difference between
the layers and we found that the supercurrent can be switched if the junction length
is larger than the Fermi length. In the another hand, solving self-consistently for
the pair correlation in multilayer graphene, we find that the two different stacking
configurations (ABC and ABA) have opposite bulk/surface behavior for the
order parameter. Surface superconductivity is robust for ABC stacked multilayer
graphene even at very low pairing potentials for which the bulk order parameter
vanishes, in agreement with a recent analytical approach. In contrast, for Bernal
stacked multilayer graphene, we find that the order parameter is always suppressed
at the surface and that there exists a critical value for the pairing potential below
which no superconducting order is achieved. We considered different doping
scenarios and find that homogeneous doping strongly suppresses surface superconductivity
while non-homogeneous field-induced doping has a much weaker effect on
the superconducting order parameter. Finally, we find that surface superconductivity
survives throughout the bulk of a few-layer structures with hybrid stacking
(ABC and ABA) due to proximity effect between ABC/ABA interfaces where the
order parameter is enhanced.
∗ Electronic address: wallaz85@gmail.com
† Electronic address: lucian@covaci.org
‡ Electronic address: Francois.Peeters@ua.ac.be

P17: Contacting of graphene devices by
direct-write atomic layer deposition
Gaurav Nanda ∗ and Paul Alkemade †
Kavli Institute of Nanoscience, TU Delft, Delft, The Netherlands
In the growing field of nano-electronics, there is an utter need for a reliable
and flexible fabrication method. Lately, there has been a tremendous growth in
patterning of graphene and graphene nano-ribbons by e-beam lithography, but
the inevitable issue of contamination imposed by the resist is not yet resolved.
Therefore, alternative nano-patterning methods need to be exploited. In this work,
an alternate technique is explored to make contacts to graphene devices. The
contact pattern is defined as a seed layer by focused-helium-ion-beam-induced
deposition of platinum. The low mass and low dose of helium ions do not induce
any significant damage to the crystal structure of graphene, making it an ideal
technique for contacting and patterning of graphene devices. The actual contacts
are made by selective platinum growth via atomic layer deposition on the seed
patterns.
Keywords: graphene, atomic layer deposition, helium ion microscope.
∗ Electronic address: g.nanda@tudelft.nl
† Electronic address: P.F.A.Alkemade@tudelft.nl

P18: Optical properties of colloidal graphene
quantum dots
Isil Ozfidan, ∗ Marek Korkusinski, † and Pawel Hawrylak ‡
Quantum Theory Group, Security and Disruptive Technologies,
National Research Council of Canada, Ottawa, Canada
Alev Devrim Guclu §
Department of Physics, Izmir Institute of Technology IZTECH, TR35430, Izmir, Turkey
The electronic, optical and magnetic properties of graphene can be modified by
engineering lateral size, shape, and edge [1-4]. Here we present new results describing
the role of electron-electron and final state interactions in the optical properties
of small colloidal graphene quantum dots (GQD)[1]. Building on our previous work
[2-4] we describe the single-particle energy spectra of Pz carbon orbitals using the
tight-binding model. All direct and exchange two-body Coulomb matrix elements
are computed using Slater Pz orbitals for on-site and nearest and next nearest
neighbors and approximated for farther neighbors. All Coulomb matrix elements
are screened by a dielectric constant of external medium controlling the ratio of
Coulomb interactions to the tunneling matrix element. For a given GQD with a
defined shape, size, edge, and dielectric constant we start with the tight-binding
calculation of single-particle states followed by a fully self-consistent Hartree-Fock
treatment. We construct a HF phase diagram of the GQD as a function of the
interaction strength V relative to the tunneling matrix element t. We find a semiconducting
state originating from the semi-metallic ground state of bulk graphene,
followed by a Mott-insulating state with decreasing screening. The ground state
wavefunction and energy is improved by inclusion of a limited number of pair excitations
using CI+Lanczos technique. For a semiconducting GQD ground state
the singlet and triplet optical spectra are obtained by creating quasi-electron-hole
pair excitations from the HF state and solving the Bethe-Salpeter equation. The
bandgap renormalization and excitonic effects are analyzed as a function of GQD
size, shape, and edge and compared with experiments on colloidal graphene quantum
dots [1].
[1] M. L. Mueller, X. Yan, J. A. McGuire, and L.-S. Li, Nano Letters 10,
2679 (2010); X. Yan, B. Li, and L.-S. Li, Acct. Chem. Research, DOI :
∗ Electronic address: ozfidani@gmail.com
† Electronic address: marek.korkusinski@nrc-cnrc.gc.ca
‡ Electronic address: pawel.hawrylak@nrc-cnrc.gc.ca
§ Electronic address: devrim.guclu@gmail.com

10.1021/ar300137p (2012).
[2] P. Potasz, A. D. Guclu, and P. Hawrylak, Phys. Rev. B 81, 033403 (2010);
O. Voznyy, A.D. Guclu, P. Potasz, and P. Hawrylak, Phys. Rev. B 83. 165417
(2011).
[3] A. D. Guclu, P. Potasz, and P. Hawrylak, Phys. Rev. B 82, 155445 (2010).
[4] A. D. Guclu, P. Potasz, O. Voznyy, M. Korkusinski, and P. Hawrylak, Phys.
Rev. Lett. 103, 246805 (2009); A. D. Guclu and P. Hawrylak, Phys. Rev. B 87,
035425 (2013).

P19: RKKY interactions in uniaxially strained
graphene
Stephen R. Power ∗
Center for Nanostructured Graphene (CNG), DTU Nanotech,
Technical University of Denmark, DK-2800 Kongens Lyngby
Paul D. Gorman, John. M. Duffy, and Mauro S. Ferreira
School of Physics, Trinity College Dublin, Dublin 2, Ireland
The ease with which the physical properties of graphene can be tuned suggests a
wide range of possible applications. Recently, strain engineering of these properties
has been of particular interest [1]. Possible spintronic applications of magnetically
doped graphene systems have motivated recent theoretical investigations of the
Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between localized
moments in graphene [2]. In this work a combination of analytic and numerical
techniques are used to examine the effects of uniaxial strain on such an interaction.
A range of interesting features are uncovered depending on the separation and
strain directions, and on how the localized magnetic impurity connects to the
graphene lattice. For substitutional impurities we see a range of amplification and
suppression effects as a function of strain, which maintain the sublattice dependent
sign rules for the interaction in unstrained graphene. These features suggest that
the strength of the interaction between two moments can be tuned or even switched
on and off by minor levels of strain [3]. For adsorbed impurities a further range of
features, including significant modification of the decay rate and the possibility of
changing the sign of the interaction, are predicted [4].
In all cases, mathematically transparent expressions describing these features are
derived which allow reliable predictions in agreement with numerical calculations.
Since a wide range of effects, including overall moment alignment and magnetotransport
response, are underpinned by such interactions, the ability to manipulate
the coupling by applying strain may lead to interesting spintronic applications.
[1] V. M. Pereira and A. H. Castro Neto, Phys. Rev. Lett., 103 (2009), 046801.
[2] S. Saremi, Phys. Rev. B, 76 (2007), 184430, S. R. Power and M. S. Ferreira,
Crystals, 3 (2013), page. 49-78 and references within.
[3] S. R. Power, P. D. Gorman, J. M. Duffy and M. S. Ferreira, Phys. Rev. B
86 (2012), 195423.
[4] P. D. Gorman, J. M. Duffy, M. S. Ferreira and S. R. Power, in preparation.
∗ Electronic address: spow@nanotech.dtu.dk

P20: Relaxation times and electron-phonon
interaction in graphene quantum dots
S. Reichardt, ∗ C. Volk, † C. Neumann, ‡ S. Engels, § and C. Stampfer
JARA-FIT and II. Institute of Physics B,
RWTH Aachen, 52074 Aachen, Germany
Quantum devices made out of graphene received increasing attention during the
past few years. In particular, graphene quantum dots have been suggested as
an interestng system for implementing spin qubits. Its advantages compared to
state-of-the-art GaAs based quantum dots are a smaller spin-orbit coupling and a
smaller hyperfine interaction giving rise to possibly more favourable spin relaxation
times. Although various phenomena like the Coulomb blockade and excited state
spectra have already been studied, the investigation of relaxation times in graphene
quantum dots is still in its infancy.
Here, we present measurements of charge relaxation times in graphene quantum
dots (QDs) and compare them to a model based on electron-phonon interaction.
We study etched graphene QDs surrounded by electrostatic gates and nearby
graphene charge detectors [1]. The QDs measure typically around 100 nm in diameter.
So-called Coulomb diamond measurements allow us to investigate the excited
state spectrum. We study charge relaxation processes by applying high frequency
pump-and-probe pulse schemes to a lateral graphene gate [2]. The experimental
relaxation data are compared to model elctron-phonon relaxation calculations.
Our model takes into account the mechanisms of deformation potential and of
bond length change for both longitudinal acoustic (LA) and transversal acoustic
(TA) phonons. We provide an analytical expression for the relaxation rate of an
electron confined in a QD and study its dependence on the QD radius and the
energy level spacing.
[1] C. Neumann, C. Volk, S. Engels, and C. Stampfer. submitted (2013).
[2] C. Volk, C. Neumann, S. Kazarski, S. Fringes, S. Engels, F. Haupt, A. Mueller,
and C. Stampfer. Nature Communication DOI: 10.1038/ncomms2738, in press,
arXiv:1303.5297 (2013).
∗ Electronic address: sven.reichardt@rwth-aachen.de
† Electronic address: volk@physik.rwth-aachen.de
‡ Electronic address: christoph.neumann@rwth-aachen.de
§ Electronic address: stephan.engels@rwth-aachen.de
Electronic address: stampfer@physik.rwth-aachen.de

P21: Pumping in graphene ribbons: transport in
adiabatic and non-adiabatic regimes.
Tejinder Kaur ∗
Department of Physics and Astronomy, Ohio University, Athens, OH, USA
Liliana Arrachea †
Departamento de Fsica, Univ. Nac. de Bs. As., Bs. As., Argentina
Nancy Sandler ‡
Department of Physics and Astronomy, Ohio University, Athens, OH,
USA and Dahlem Center for Complex Quantum Systems, Freie Universitt, Berlin, Germany.
The interest in the development of devices at the nano-scale has intensified the
search for mechanisms that provide control of transport properties while reducing
effects of heat dissipation and contact resistance. Charge pumping, in which
dc currents are generated in open-quantum systems by applying time-dependent
potentials, may achieve these goals. Since the theoretical proposal by Thouless
[1], the application of a periodic perturbation to pump dc charge or spin current
was achieved in various experimental settings. Most of these works focused
on the adiabatic regime (low driving), in configurations with two or more periodically
changing parameters that yield dc currents proportional to the driving
frequency. New insights into pumping have appeared from studies of models of
two-dimensional graphene systems. The solution of a two-parameter pumping
model in the adiabatic regime, based on the Dirac Hamiltonian, showed an enhanced
pumped current (as compared with semiconductor materials), which was
attributed to the unusual persistence of evanescent modes in the presence of Dirac
points [2]. Green’s function methods were used to analyze the effects of resonant
tunneling in a similar configuration, showing the persistence of anomalous
behavior in this regime [3]. Among the extraordinary properties of graphene,
there are those arising from confinement effects with significant consequences for
the conductance of finite samples. With the purpose to understand the effect of
boundaries and geometry in realistic experimental settings, we have analyzed the
properties of non-equilibrium zero-bias currents through graphene nanoribbons
using a tight-binding Hamiltonian and the Keldysh non-equilibrium formalism.
Using a numerical implementation with two local single-harmonic time-dependent
potentials, we provide detailed analysis of transport through armchair and zigzag
∗ Electronic address: TK169607@ohio.edu
† Electronic address: lili@df.uba.ar
‡ Electronic address: sandler@ohio.edu

ibbons, attached to metallic leads (modeled by semi-infinite square lattice contacts)
as a function of chemical potential of the leads and pumping parameters.
The model fully describes adiabatic and non-adiabatic regimes and the crossover
between both for finite size ribbons. Furthermore, it provides a detailed account of
the contribution of evanescent modes to the current, which it is shown to strongly
depend on the aspect ratio of the ribbon. Most importantly, the analysis of different
boundaries and contact geometries reveals the fundamental role played by
space inversion symmetry in the value of the pumped current that can vanish for
appropriate setups [4].
[1] D. J. Thouless, Phys. Rev. B 27, 6083, (1983).
[2] E. Prada, P. San-Jose, and H. Schomerus, Phys. Rev. B 80, 245414 (2009).
[3] E. Grichuk and E. Manykin, Europhys. Lett. 92, 47010 (2010).
[4] T. Kaur, L. Arrachea and N. Sandler. Submitted for publication;
arXiv:1203.3952

P22: Current resonances in graphene with time
dependent potential barriers
Sergey E. Savel’ev
Department of Physics, Loughborough University,
Loughborough LE11 3TU, United Kingdom ∗
Wolfgang Husler † and Peter Hänggi
Institut fr Physik, Universitt Augsburg, D-86135 Augsburg, Germany
A method is derived to solve the massless Dirac-Weyl equation describing electron
transport in a mono-layer of graphene with a scalar potential barrier U(x, t),
homogeneous in the y-direction, of arbitrary x- and time dependence. Resonant
enhancement of both electron backscattering and currents, across and along the
barrier, is predicted when the modulation frequencies satisfy certain resonance
conditions. These conditions resemble those for Shapiro-steps of driven Josephson
junctions. Surprisingly, we find a non-zero y-component of the current for carriers
of zero momentum along the y-axis.
[1] Sergey E. Savelev, Wolfgang Husler, Peter Hnggi, Phys. Rev. Lett. 109,
226602 (2012).
∗ Electronic address:
† Electronic address: Wolfgang.Haeusler@Physik.Uni-Augsburg.DE

P23: Interaction-induced enhancement of g
factor in graphene
Artsem Shylau ∗
DTU Nanotech, Department of Micro- and Nanotechnology,
Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
Recent measurements of magnetotransport in graphene show the enhancement
of the spin-splitting of Landau levels [1]. This behavior is attributed to the effect
of electron-electron interaction and can be described by introducing a phenomenological
effective g factor.
In the present work we use Thomas-Fermi approach to study the spin splitting
in realistic two-dimensional graphene sheets in a perpendicular magnetic field situated
on a dielectric surface and subjected to a smooth confining potential due to
charged impurities. The effect of electron-electron interaction is taken into account
using the Hubbard and the Hartree approximations [2].
We show that the effective g factor is enhanced in comparison to its free electron
value g = 2 and oscillates as a function of the filling factor ν in the range 2 ≤ g∗ 4
reaching maxima at ν = 4N = 0, ±4, ±8, . . . and minima at ν = 4 N + 1
=
2
±2, ±6, ±10, . . ., with N being the Landau level index. We outline the role of
charged impurities in the substrate, which are shown to suppress the oscillations
of the g∗ factor. This effect becomes especially pronounced with the increase of
the impurity concentration, when the effective g factor becomes independent of
the filling factor reaching a value of g∗ ≈ 2.3.
[1] E. V. Kurganova, H. J. van Elferen, A. McCollam, L. A. Ponomarenko, K.
S. Novoselov, A. Veligura, B. J. van Wees, J. C.Maan,and U. Zeitler, Phys. Rev.
B 84, 121407 (2011).
[2] A. V. Volkov, A. A. Shylau, and I. V. Zozoulenko, Phys. Rev. B 86, 155440
(2012).
∗ Electronic address: arts@nanotech.dtu.dk

P24: Melting of graphene clusters
Sandeep Kumar Singh, ∗ M. Neek-Amal, † and F.M. Peeters ‡
University of Antwerpen
The study of the melting of crystals is one of the important subjects in the field of
phase transitions. Nano-scale molecular clusters due to their size-dependent properties
show melting processes different from those of bulk materials and infinite size
two-dimensional materials. The melting of nano-clusters has received considerable
attention recently and it was found that nano-clusters melt typically below their
corresponding bulk melting temperature [1,2]. This is due to the higher chemical
reactivity of nano-clusters which is the consequence of the increased accessible
surface and the presence of more free dangling bonds.
Density-functional tight-binding and classical molecular dynamics simulations
are used to investigate the structural deformations and melting of planar carbon
nano-clusters CN with N=2-55. The minimum energy configurations for different
clusters [3,4] are used as starting configuration for the study of the temperature effects
on the bond breaking/rotation in carbon lines (N

P25: Adsorption and absorption of Boron,
Nitrogen, Aluminium and Phosphorus on
Silicene: stability, electronic and phonon
properties
J. Sivek, ∗ H. Sahin, † B. Partoens, ‡ and F. M. Peeters §
Departement Fysica, Universiteit Antwerpen,
Groenenborgerlaan 171, B-2020 Antwerpen, Belgium
Since the first reports on the successful isolation of a stable monolayer of
graphene special efforts have been invested in the exploration of similar materials
with novel properties resulting from their two-dimensional nature. Those
alternative materials have the potential to bypass some of the obstacles existing
in the usage of graphene in contemporary electronics i.e., incompatibility with
present day silicon technology and lack of an energy bandgap which is essential for
all semiconductor devices. One of such compound is the graphene-like monolayer
of silicon – silicene – semimetal with linearly crossing bands and a zero electronic
band gap similar to graphene [1]. The recent experimental observations and synthesis
of silicene have opened a new path for such nanoscale materials [2].
We investigate the adsorption and absorption of B, N, Al and P atoms on the
surface of pristine free-standing silicene within the density-functional theory formalism.
Our interest is pointed towards the compounds’ structure, electronic,
magnetic and vibrational properties. The aforementioned elements have been chosen
because of their chemical propinquity to silicon and carbon. These elements
are the subject of many theoretical studies of functionalization of graphene [3]
accompanied by the experimental realizations [4]. Interaction of these elements
with silicene and induced properties in adsorbed/substituted compounds are thus
a matter of high interest.
We find the most preferable adsorption sites to be valley, bridge, valley and hill
site for B, N, Al and P adatoms, respectively. All the silicene systems with adsorbed/substituted
atoms exhibit metallic behaviour with strongly bonded atoms
accompanied by an appreciable electron transfer from silicene to the B, N and P
adatom/substituent. The Al atoms exhibit opposite charge transfer, with n-type
doping of silicene and weaker bonding. We report a potential route for chemical
∗ Electronic address: jozef.sivek@ua.ac.be
† Electronic address: hasan.sahin@ua.ac.be
‡ Electronic address: bart.partoens@ua.ac.be
§ Electronic address: francois.peeters@ua.ac.be

doping of silicene via chemical decoration. The adatoms/substituents induce characteristic
branches in the phonon spectrum of silicene, which can be probed by
Raman measurements. To address the thermal stability of the modified silicene
monolayer we consider the effect of temperature by employing ab initio MD calculations.
The systems under study are stable up to at least T = 500 K. Our results
demonstrate that silicene has a very reactive and functionalizable surface.
[1] K. Takeda and K. Shiraishi, Phys. Rev. B 50, 14916 (1994).
[2] P. D. Padova, et al., Appl. Phys. Lett. 96, 261905 (2010); P. Vogt, et al.,
Phys. Rev. Lett. 108, 155501 (2012).
[3] A. Lherbier, et al., Phys. Rev. Lett. 101, 036808 (2008); P. A. Denis, Chem.
Phys. Lett. 492, 251 (2010).
[4] L. S. Panchakarla, et al., Adv. Mater. (Weinheim, Ger.) 21, 4726 (2009).

P26: Band gap evolution and termini of short
graphene nanoribbons
Hajo Söde, ∗ Leopold Talirz, † Carlo Pignedoli, ‡
Roman Fasel, § and Pascal Ruffieux
Empa, Swiss Federal Laboratories for Materials Science and Technology - Dübendorf, Switzerland
Xinliang Feng ∗∗ and Klaus Müllen ††
Max Planck Institute for Polymer Research - Mainz, Germany
Its remarkable properties make graphene attractive for use in electronic devices.
Yet, graphene is semimetallic and thus not directly suitable for most electronic or
optoelectronic switching devices, which require a semiconductor with a specific,
finite band gap. In armchair graphene nanoribbons (AGNRs), band gaps suitable
for room temperature applications open at widths smaller than 3 nm. However,
atomically precise control over the edges is crucial to produce AGNRs with reliable
electronic transport properties. We recently developed a bottom-up method
providing this level of control over the edges. This allows, in particular, to grow
atomically precise 7-AGNRs (7 carbon dimers across the width) for which we have
studied the cyclodehydrogenation step of the synthesis and determined a band gap
of 2.3 eV as well as the occupied band structure of the ribbons adsorbed on a gold
surface. Here, we present the atomic structure found at the termini of 7-AGNRs
obtained via this bottom-up approach. By combining STM experiments with
STM simulations based on large scale density functional theory calculations, we
demonstrate that the termini are passivated by hydrogen. Additionally, we present
detailed scanning tunneling spectroscopy (STS) and angle-resolved photoemission
spectroscopy (ARPES) results on the electronic structure of the 7-AGNRs. GNRs
of different length have been synthesized in order to explore the length dependence
of the electronic band gap. STS data for ribbons of varying length reveals substantially
larger band gaps for ribbons shorter than 5 nm. Fourier transformed STS was
used to determine the band dispersion of occupied and unoccupied bands. Based
in these data, we determined the effective masses and energy-dependent charge
carrier velocities for the frontier bands of 7-AGNRs. In addition, the experimental
results are discussed by a detailed comparison with ab initio simulations of the
length-dependent band gap and the band structure.
∗ Electronic address: hajo.soede@empa.ch
† Electronic address: leopold.talirz@empa.ch
‡ Electronic address: carlo.pignedoli@empa.ch
§ Electronic address: roman.fasel@empa.ch
Electronic address: pascal.ruffiuex@empa.ch
∗∗ Electronic address: feng@mpip-mainz.mpg.de
†† Electronic address: muellen@mpip-mainz.mpg.de

P27: Large scale nanopatterning of graphene
E. Tovari, ∗ S. Csonka, † and G. Mihaly
Department of Physics, Technical University of Budapest, Hungary
P. Neumann, ‡ P. Nemes-Incze, G. Dobrik, and L. P. Biro
Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences
Introducing a band gap into graphenes dispersion relation is essential for applying
it in logistic circuits. There are several ways to do this, such as chemical
doping, or using transversal confinement, in other words, fabricating nanoribbons.
The energy band gap has to be on the order of eV, and reproducible with low
standard deviation. The roughness and crystallographical orientation of edges, for
example, are of great importance in this respect. Low-temperature measurements
have shown that the transport gap of ribbons made by lithography and plasma
etching is independent of orientation[1], emphasising the role of the substrate and
edge roughness. If the edges are well-defined on or near the atomic scale, the two
edge types armchair and zigzag are expected to show very different transport
characteristics: zigzag graphene nanoribbons are half-metallic in a transverse electric
field, making them a good candidate for spintronics applications[2]. I present
a previously introduced technique, carbothermal etching (CTE)[3] for producing
regular ribbons, but combined with e-beam lithography (EBL)[4]. The original
technique needs natural or artificial defects - like holes - either made by oxidation,
or by AFM-indentation. At high temperatures ( 700C) and in a neutral atmosphere
the holes start to grow into hexagons with edges parallel with the zigzag
orientation of the lattice, if the substrate is made of SiO2. Under such conditions
the carbon atoms of the edges selectively react with oxygen from the substrate,
making the process anisotropic. The combination of CTE and EBL means fabricating
holes by plasma etching at sites defined by lithography, and then annealing
them at the necessary high temperatures. We have performed low-temperature
transport measurements on an antidot lattice fabricated by using the combination
of EBL and CTE, and on a single nanoribbon between hexagons formed by
CTE from AFM-indented holes in the graphene flake. I compare the results with
measurements done on plasma-etched ribbons, and also with ribbons made by
AFM-lithography. Weak localisation on the antidot lattice shows a phase coherence
length of 200 nm, and an intervalley scattering length around 50 nm, less
than the average ribbon width. The CTE-ribbon shows low conductance, unlike
∗ Electronic address: tovari@dept.phy.bme.hu
† Electronic address: csonka@dept.phy.bme.hu
‡ Electronic address: neumann.peter@ttk.mta.hu

expectations, but the AFML and plasma-etched ribbons conductance maps have
diamond-like structures, as expected.
[1] PRL 98, 206805 (2007)
[2] Nature Letters 444, 347 (2006)
[3] Nano Research (2010) 3: 110
[4] Nuclear Instruments and Methods in Physics Research B 282 (2012) 130

P28: Andreev reflection by magnetic barriers in
superconductor contacted graphene
Hengyi Xu ∗ and Thomas Heinzel
Condensed Matter Physics Laboratory, Heinrich-Heine-Universitaet, Duesseldorf, Germany
The Andreev reflection of normal-metal-superconductor junction in both monolayer
and bilayer graphene with a single magnetic barrier is investigated by means
of the Green’s function formalism. Within the periodic tight-binding model along
the transverse direction, we study the the angle-dependent Andreev reflection of
two-dimensional graphene-superconductor junction in the retroreflection and specular
reflection regimes depending upon the energy of the incident electron relative
to the Fermi energy. In the fully reflected regime in which the energy of an incident
electron is smaller than the pair potential of the superconductor, our calculations
are consistent with the analytical results based on the Dirac theory. The
normal transmissions emerge when the energy of an incident electron is higher
than the superconducting pair potential, and the presence of magnetic barriers in
this case suppresses the normal transmission while it enhances the Andreev retroand
specular reflections significantly. Moreover, the differential conductances of
the normal-superconductor interface with a magnetic barrier in various transport
regimes are considered within the Blonder-Tinkham-Klapwijk formula.
∗ Electronic address: hengyi.xu@uni-duesseldorf.de

P29: Snake states in graphene quantum dots in
the presence of a p-n junction
M. Zarenia ∗ and F. M. Peeters †
Department of Physics, University of Antwerp,
Groenenborgerlaan 171, B-2020 Antwerpen, Belgium.
J. M. Pereira Jr. ‡ and G. A. Farias §
Departamento de Física, Universidade Federal do Ceará, Fortaleza, Ceará, 60455-760, Brazil.
We present numerical results within a tight-binding study, for the energy spectrum
and magnetic field dependence of the eigenstates of graphene quantum dots,
on which a p-n junction creates electron and hole-doped regions. The presence
of the magnetic field, together with the coupling between electron and hole states
across the potential barrier due to Klein tunneling leads to the appearance of localized
states at the potential interface, known as snake states [1,2]. These states,
which have previously been investigated for p-n junctions on infinite graphene
sheets, can influence the transport properties of graphene-based nanodevices [2,3].
We have obtained results that show that for the case of quantum dots the low
energy dynamics of the system is dominated by hybridized states that arise due
to the overlap between quantum Hall edge states and the snake states at the p-n
junction, with the snake states allowing the superposition of quantum Hall edge
states at the p and n sides of the dot [4]. These states are characterized by an energy
spectrum that displays an oscillating behavior as function of the electrostatic
potential and magnetic field at the vicinity of the Fermi energy. Furthermore, the
energy spectrum was shown to depend on the specific alignment of the potential
interfaces with regard to the graphene lattice, as well as on the geometry of the
gates. Remarkable localized states are found at the crossing of the p-n junction
with the zigzag edge having a dumb-bell-shaped electron distribution.
[1] J. M. Pereira Jr., F. M. Peeters, and P. Vasilopoulos, Phys. Rev. B 75,
125433 (2007).
[2] J. R. Williams and C. M. Marcus, Phys. Rev. Lett. 107, 046602 (2011).
[3] D. A. Abanin and L. S. Levitov, Science 317, 641 (2007).
[4] M. Zarenia, J. M. Pereira Jr., F. M. Peeters, and G. A. Farias, Phys. Rev.
B 87, 035426 (2013).
∗ Electronic address: m.zarenia@gmail.com
† Electronic address: francois.peeters@ua.ac.be
‡ Electronic address: pereira@fisica.ufc.br
§ Electronic address: gil@fisica.ufc.br

Anomalous Raman Spectra and Thickness
Dependent Electronic properties of WSe2
Hasan Sahin ∗ and Francois Peeters †
University of Antwerp
Typical Raman spectra of transition-metal dichalcogenides (TMDs) display two
prominent peaks, E2g and A1g, that are well separated from each other. We find
that these modes are degenerate in bulk WSe2 yielding one single Raman peak
in contrast to other TMDs. As the dimensionality is lowered, the observed peak
splits in two. In contrast, our ab initio calculations predict that the degeneracy
is retained even for WSe2 monolayers. Interestingly, for minuscule biaxial strain,
the degeneracy is preserved, but once the crystal symmetry is broken by a small
uniaxial strain, the degeneracy is lifted. Our calculated phonon dispersion for
uniaxially strained WSe2 shows a good match to the measured Raman spectrum,
which suggests that uniaxial strain exists in WSe2 flakes, possibly induced during
the sample preparation and/or as a result of the interaction between WSe2 and the
substrate. Furthermore, we find that WSe2 undergoes an indirect-to-direct bandgap
transition from bulk to monolayers, which is ubiquitous for semiconducting
TMDs. These results not only allow us to understand the vibrational and electronic
properties of WSe2, but also point to effects of the interaction between the
monolayer TMDs and the substrate on the vibrational and electronic properties.
∗ Electronic address: hasan.sahin@ua.ac.be
† Electronic address: francois.peeters@ua.ac.be

Phonon Softening and Direct to Indirect
Bandgap Crossover in Strained Single Layer
MoSe2
Seyda Horzum Sahin ∗ and Francois Peeters †
University of Antwerp
Motivated by recent experimental observations of Tongay et al. [ Nano Lett.
12 5576 (2012)] we show how the electronic properties and Raman characteristics
of single layer MoSe2 are affected by elastic biaxial strain. We found that with
increasing strain: (1) the E ′ and E ′′ Raman peaks (E2g and E1g in bulk) exhibit
significant redshifts (up to 30 cm1), (2) the position of the A1 peak remains at 180
cm1 (A1g in bulk) and does not change considerably with further strain, (3) the
dispersion of low energy flexural phonons crosses over from quadratic to linear, and
(4) the electronic band structure undergoes a direct to indirect band gap crossover
under 3% biaxial tensile strain. Thus the application of strain appears to be a
promising approach for a rapid and reversible tuning of the electronic, vibrational,
and optical properties of single layer MoSe2 and similar MX2 dichalcogenides.
∗ Electronic address: seyda.horzumsahin@ua.ac.be
† Electronic address: francois.peeters@ua.ac.be

How to get to Antwerp
Useful information
The conference venue, Klooster van de Grauwzusters, is located within walking
distance the Antwerpen Centraal train station.
To get to/near the train station from the airport in Brussels you have the following
options:
1) take the train from "Brussels Airport" to "Antwerpen Centraal". Please consult
the schedule at www.BelgianRail.be. The trains arrive and leave from level -2, with
ticket desks on level -1 in the airport terminal. The trip takes about 50 minutes.
2) take the bus from the airport directly to Antwerpen Centraal. This bus can be
taken from outside the terminal at level -2. When exiting the airport terminal, the bus
station is directly ahead, around 50m. There is a bus every hour. Please consult the
schedulat at www.AirportExpress.be.
3) alternatively you can always take a taxi
Restaurants
While the lunch will be provided at the workshop, you will need to find restaurants
for dinner. The choice of restaurants in the center of Antwerp is very wide. Two main
locations with a higher density of restaurants are (see the following map):
1) near the train station,
2) near the cathedral (see the map).
Emergency contact phone numbers
Francois Peeters: +32 473 881 134
Lucian Covaci: +32 483 621 831

Restaurants
Elzenveld Hotel
Prinse Hotel
<strong>Workshop</strong> dinner
Theater Hotel
Conference venue: Klooster van de Grauwzusters, Lange St. Annastraat 7
Conference dinner: Universiteitsclub - Hof van Liere, Prinsstraat 13b
<strong>Workshop</strong> venue
Restaurants: wide choice range near the train station and the cathedral (city center)
Restaurants
Colombus Hotel
Leonardo Hotel
Airport bus
airportexpress.be
Train station
belgianrail.be

First Name Last Name Affiliation Country Email
Mr. Francois Amet Stanford University USA amet@stanford.edu
Prof. Eva Andrei Rutgers University USA eandrei@physics.rutgers.edu
Dr. Zhimin Ao The University of New South Wales Australia zhimin.ao@unsw.edu.au
Prof. Salvador Barraza-Lopez University of Arkansas USA sbarraza@uark.edu
Dr. Golibjon Berdiyorov Universiteit Antwerpen Belgium golibjon.berdiyorov@ua.ac.be
Dr. Bernd Beschoten RWTH Aachen University Germany bernd.beschoten@physik.rwth-aachen.de
Prof. Guido Burkard University of Konstanz Germany guido.burkard@uni-konstanz.de
Mr. Pascal Butti
List of Participants
EMPA,
Swiss Federal Laboratories for Materials Science Switzerland pascal.butti@empa.ch
Prof. Cem Celebi Izmir Institute of Technology, Dept. of Physics Turkey cemcelebi@iyte.edu.tr
Prof. Tapash Chakraborty University of Manitoba Canada tapash@physics.umanitoba.ca
Prof. Andrey Chaves Universidade Federal do Ceara Brazil andrey@fisica.ufc.br
Mr. Paul Clark University of Southampton UK p.clark@soton.ac.uk
Dr. Malcolm Connolly University of Cambridge UK mrc61@cam.ac.uk
Dr. Sebastian Costamagna Instituto de Fisica de Rosario (CONICET) Argentina costagol@gmail.com
Dr. Lucian Covaci Universiteit Antwerpen Belgium lucian@covaci.org
Mr. Christophe De Beule Universiteit Antwerpen Belgium christophe.debeule@ua.ac.be
Mr. Matthias Droth University of Konstanz Germany matthias.droth@uni-konstanz.de
Prof. Reinhold Egger University of Dusseldorf Germany egger@thphy.uni-duesseldorf.de
Ms. Daiara Faria UFF - BR/ Freie Universitat Berlin Brazil daiara.faria@gmail.com
Prof. Josh Folk University of British Columbia Canada jfolk@phas.ubc.ca
Prof. Joaquin Frenandez-Rossier International Iberian Nanotechnology Laboratory Portugal joaquin.fernandez-rossier@inl.int
Prof. Andre Geim University of Manchester UK geim@manchester.ac.uk
Dr. Devrim Guclu Izmir Institute of Technology Turkey devrimguclu@iyte.edu.tr
Prof. Pawel Hawrylak National Research Council of Canada Canada pawel.hawrylak@nrc-cnrc.gc.ca
Mr. Mohsin Jamil Govt. Dyal singh college Pakistan mohsinjamil43@hotmail.com
Prof. Antti-Peka Jauho Center for Nanostructured Graphene, DTU Nanotech Danemark Antti-Pekka.Jauho@nanotech.dtu.dk
Prof. Ahmed Jellal Chouaib Doukkali University Morocco ahmed.jellal@gmail.com
Dr. Andrea Latge Universidade Federal Fluminense Brazil latge@if.uff.br
Dr. Ortwin Leenaerts Universtiteit Antwerpen Belgium Ortwin.Leenaerts@ua.ac.be
Dr. Koen Macken Allied Graphene Belgium koen.macken@mailaps.org
Prof. John McGuire Michigan State University USA mcguire@pa.msu.edu
Dr. mohsen mhadhbi National Institute of Research Tunisia mohsen.mhadhbi@inrap.rnrt.tn
Mr. Slavisa Milovanovic Universiteit Antwerpen Belgium slavisa.milovanovic@gmail.com
Mr. Dean Moldovan Universiteit Antwerpen Belgium dean.moldovan@ua.ac.be
Prof. Markus Morgenstern RWTH Aachen University Germany mmorgens@physik.rwth-aachen.de
Mr. William A. Munõz Universiteit Antwerpen Belgium wallaz85@gmail.com
Mr. Gaurav Nanda Kavli Institute of Nanoscience, TU Delft Netherlands g.nanda@tudelft.nl
Dr. Mehdi Neek-Amal Universiteit Antwerpen Belgium neekamal@srttu.edu
Ms. Isil Ozfidan National Research Council Canada isil.ozfidan@nrc.ca
Prof. Bart Partoens Universiteit Antwerpen Belgium bart.partoens@ua.ac.be
Prof. Francois Peeters Universiteit Antwerpen Belgium francois.peeters@ua.ac.be
Dr. Vitor Pereira National University of Singapore Singapore vpereira@nus.edu.sg
Dr. Pawel Potasz Wroclaw University of Technology Poland pawel.potasz@pwr.wroc.pl

Dr. Stephen Power Center for Nanostructured Graphene, DTU Nanotech Denmark spow@nanotech.dtu.dk
Dr. Massoud Ramezani Masir Universiteit Antwerpen Belgium mrmphys@gmail.com
Mr. Sven Reichardt RWTH Aachen University Germany sven.reichardt@rwth-aachen.de
Dr. Hasan Sahin Universiteit Antwerpen Belgium hasan.sahin@ua.ac.be
Dr. Seyda Sahin Universiteit Antwerpen Belgium seyda.sahin@ua.ac.be
Prof. Nancy Sandler Ohio University/Freie Universitat USA/Germanysandler@ohio.edu
Dr. Koen Schouteden KU Leuven Belgium koen.schouteden@fys.kuleuven.be
Mr. Ankit Sharma Central University of Rajasthan India ankitphy@gmail.com
Dr. Artsem Shylau Technical University of Denmark Denmark arts@nanotech.dtu.dk
Mr. Sandeep Singh Universiteit Antwerpen belgium san2007iit@gmail.com
Mr. Jozef Sivek Universiteit Antwerpen Belgium jozef.sivek@ua.ac.be
Mr. Hajo Söde
EMPA,
Swiss Federal Laboratories for Materials Science Switzerland hajo.soede@empa.ch
Prof. Christoph Stampfer RWTH Aachen Germany stampfer@physik.rwth-aachen.de
Mr. Endre Tovari Technical University of Budapest Hungary tovariendre@gmail.com
Prof. Björn Trauzettel University of Wuerzburg Germany bjoern.trauzettel@physik.uni-wuerzburg.de
Prof. Sergio Ulloa Ohio University & Freie Universitat Berlin Germany ulloa@ohio.edu
Prof. Lieven Vandersypen Delft University of Technology Netherlands l.m.k.vandersypen@tudelft.nl
Mr. Ben Van Duppen Universiteit Antwerpen Belgium ben.vanduppen@ua.ac.be
Dr. Hengyi Xu Heinrich-Heine-Universitat Dusseldorf Germany hengyi.xu@hhu.de
Dr. Shengjun Yuan Radboud University of Nijmegen Netherlands s.yuan@science.ru.nl
Mr. MohammadZarenia Universiteit Antwerpen Belgium m.zarenia@gmail.com