Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
With the recent advances in nanofabrication techniques it has become
possible to manipulate and explore the electronic transport through
individual molecules. This has posed an exciting theoretical challenge,
namely the understanding of the conduction mechanisms at the molecular
scale. So far, the effort has been concentrated in the analysis of the
role of the electronic structure, but little has been done on the role of
the internal degree of freedom of molecules. In this talk I will present our
efforts to understand what are the effects of the vibrational modes in the
molecular conduction. Making use of the density functional theory, we
have developed an ab inito approach to describe the influence in the electrical
current of the inelastic electron-phonon processes. This approach
allows us to address many different questions such as (i) what makes
that in some experimental situations the vibrational modes enhance the
current and some others they reduce it? (ii) What are the possible signatures
of these modes in the current-voltage characteristics? (iii) What
are the selection rules that explain why some vibrational modes do not
show up in the transport experiments?
TT 27.8 Do 11:45 H19
Controlled contacting of single-walled carbon nanotubes
— •Daniel Secker 1 , Ralph Krupke 1 , Heiko B. Weber 1 ,
and Hilbert v. Löhneysen 2,3 — 1 Forschungszentrum Karlsruhe,
Institut für Nanotechnologie, PO-Box 3640, D-76021 Karlsruhe —
2 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, PO-Box
3640, D-76021 Karlsruhe — 3 Physikalisches Institut, Universität
Karlsruhe, D-76128 Karlsruhe
We present a method to encapsulate the ends of single-walled carbon
nanotubes (SWNT) in metallic leads. For this purpose we employed an
inorganic shadow mask of Si3N4 patterned by e-beam lithography on a
PMMA/Si3N4/SiO2/Si substrate [1]. The SWNTs are deposited between
the contacts after a first evaporation step by applying an ac electric field
making use of the dielectrophoretic force [2]. A second evaporation step
encapsulates the ends of the SWNTs by the metallic leads. Therefore
in contrast to marker-assisted e-beam lithography any exposure of the
SWNTs to organic substances like PMMA is avoided. We present lowtemperature
electrical transport measurements with samples thus prepared.
[1] T. Hoss et al., Microel. Engin. 46 (1999) 149-152
[2] R. Krupke et al., Appl. Phys. A 76 (2003) 397-400
TT 27.9 Do 12:00 H19
Parallel and Perpendicular Magnetoresistance of Multiwall
Carbon Nanotubes — •Bernhard Stojetz 1 , Christoph
Hendlmeier 1 , Christian Hagen 1 , Edina Ljubović 2 , Lazlo
Forró 2 , and Christoph Strunk 1 — 1 Institut für Experimentelle und
Angewandte Physik, Universität Regensburg — 2 Institute of Physics of
Complex Matter, FBS Swiss Federal Institute of Technology (EPFL),
Lausanne, Switzerland
We report resistance measurements for single multiwall carbon nanotubes
in magnetic fields both parallel and perpendicular to the tube axis.
The tubes were trapped onto pre-patterned Al gate electrodes by means
of an ac electric field. The strong electrostatic coupling of the nanotube
to the gate allows a considerable variation of the doping level of the
tube. Magnetoresistance traces were measured for many values of the
gate voltage, which allows an ensemble averaging of the conductance
fluctuations induced by quantum interference. The ensemble averaging
reduces the conductance fluctuations while leaving the weak localization
contribution to the resistance unchanged. The data can be consistently
interpreted in terms of quantum transport in presence of weak disorder.
TT 27.10 Do 12:15 H19
Scaling of the Performance of Schottky barrier Carbon Nanotube
Transistors — •Stefan Heinze 1 , Marko Radosavljević 2 ,
Jerry Tersoff 3 , and Phaedon Avouris 3 — 1 Institute of Applied
Physics, University of Hamburg, Jungiusstr. 11, 20355 Hamburg, Germany
— 2 Novel Device Group, Intel Corporation, Hillsboro, OR 97124,
USA — 3 IBM Research Division, T. J. Watson Research Center, Yorktown
Heights, NY 10598, USA
Today, the performance of carbon nanotube field-effect transistors
(CNFETs) is already competitive with that of state-of-the-art silicon
transistors [1]. However, a metal-semiconductor junction as in CNFETs
fabricated to date necessitates a Schottky type contact which may present
a significant barrier for transport. Most CNFETs thus behave as Schottky
barrier transistors [2]. This leads to an unexpected scaling of CNFET
performance as the device size is reduced [3]. Using an analytic model, we
derive explicit scaling laws in the turn-on regime and for the transistor
OFF state – in excellent agreement with experimental data [3]. An important
consequence of the scaling behavior is an exponential increase of
the OFF current with drain voltage for ultra-thin oxide CNFETs which
limits the usable drain voltage and thus the achievable ON currents [4].
[1] S. J. Wind et al., Appl. Phys. Lett. 80, 3817 (2002).
[2] S. Heinze et al., Phys. Rev. Lett. 89, 106801 (2002).
[3] S. Heinze et al., cond-mat/0302175 and Phys. Rev. B (in press).
[4] M. Radosavljević et al., Appl. Phys. Lett. 83, 2435 (2003).
TT 27.11 Do 12:30 H19
Transport through a Carbon Nanotube with Superconducting
Contacts — •Wolfgang Belzig, Mark Buitelaar, Thomas
Nussbaumer, Christoph Bruder, and Christian Schöneberger
— Universität Basel, Klingelbergstr. 82, 4056 Basel, Schweiz
We report on experimental and theoretical studies of transport through
multi-walled carbon nanotubes strongly coupled to superconducting
leads. In the normal state of the leads the conductance displays a Kondo
behaviour for odd numbers of electrons on the nanotube dot. If superconductivity
is turned on, the transport characteristic changes dramatically.
A large zero bias conductance (with G ≫ 2e 2 /h) and a strongly nonlinear
differential conductance are observed. Both effects depend on the
gate voltage. We explain the experimental observations theoretically by a
subtle interplay between Kondo physics and proximity effect on the dot.
Multiple Andreev reflections play a key-role to explain the features seen
in the differential conductance.
TT 28 FV-internes Symposium ”Theoretical Modeling of Materials with Correlated
Electrons”
Zeit: Donnerstag 14:00–18:10 Raum: H20
Hauptvortrag TT 28.1 Do 14:00 H20
Opportunities and challenges from electron spectroscopy for
realistic correlated electron theory — •J. W. Allen — Randall
Laboratory of Physics, University of Michigan, Ann Arbor, MI 48109,
USA
In the past 25 years electron spectroscopy has developed steadily to
take its place with optical and Raman spectroscopy and with neutron
scattering as an established general technique for studying the electronic
structure of condensed matter systems. Already the current quality
of available data on strongly correlated electron systems calls for theory
beyond that of the model Hamiltonian and the need is expected to increase
with further improvements in the experimental technique. I will
give current examples in the context of the elucidation of Landau Fermi
liquid theory quasi-particles and their absence in photoemission spectra.
Hauptvortrag TT 28.2 Do 14:25 H20
Dynamical Mean Field Theory (DMFT) and Electronic Structure
Calculation — •G. Kotliar — Department of Physics and
Astronomy, Rutgers University, Piscataway, NJ 08854-8019, USA
We will discuss how a many body technique, DMFT is interfaced with
band structure methods so as to obtain quantitative system specific information
about correlated materials. Our talk will cover, computation
of total energies, photoemission spectra, thermodynamical quantities
transport properties phonon spectra, and optical conductivity. We will
illustrate the method drawing examples from a simple system d electron
system undergoing a density driven Mott transition (LaSrTiO3) and from
lanthanide and actinide materials.
Fachvortrag TT 28.3 Do 14:50 H20
How Chemistry Controls Electron Localization in 3d 1 Perovskites
— •O. K. Andersen — Max-Planck Institute for Solid State
Research, D-70506 Stuttgart