Abstract - Physics at Lancaster University

Single Cooper pair tunneling in small Josephson junction circuits and superconducting nanowiresA. B. ZorinPhysikalisch-Technische Bundesanstalt, 38116 Braunschweig, GermanyDevelopment of the technology for fabrication of small structures had dramatic impact on the researchof single charge effects in the superconducting circuits. The class of such circuits includes the singleCooper pair transistors, serial arrays of small Josephson junctions, etc. and, recently, superconductingnanowires enabling under certain conditions the quantum slips of superconducting phase. In my talk Iwill review the research on the Bloch oscillations in the Josephson circuits and their phase locking byexternal ac drive and focus on the recent experiments with NbSi nanowires, demonstrating manipulationof individual Cooper pairs leading to quantum interference of voltages. Prospects of the single Cooperpair nanowires and possible application of these circuits in metrology will also be discussed.

Thermoelectric Properties of Graphene –based MolecularJunctionsAmaal AL-backri , Colin LambertLancaster UniversityAbstract:Single-molecule electronics offers new opportunities for tuning the thermoelectricalproperties of organic materials. In the literature, these mainly involve tuning the localdensity of states of the molecule to maximise the slope of the transmission coefficientnear the Fermi energy. We demonstrate a new strategy to enhance the thermoelectricperformance of graphene-based molecular junctions, which involves tuning the localdensity of states of graphene edges. We show that thermoelectric properties areinfluenced by the chemical termination of graphene electrodes and the nature of thebinding of the molecule to the electrodes. Through a suitable combination of theseparameters, large enhancements of the thermopower and figure of merit are possible.

Quantum Communications in the Real WorldAndrew ShieldsToshiba Research Europe Limited208 Cambridge Science Park, Milton Road, Cambridge. CB4 0GZ. U.K.enquires@crl.toshiba.co.ukwww.quantum.toshiba.co.ukApplying quantum theory to information systems allows functionalities beyond thecapabilities of conventional network and computer technology. For example, it provides adirect test of communication secrecy, imaging beyond classical resolution limits andefficient solution of computational problems that are virtually intractable using ordinaryprocessors.In this talk I will discuss recent progress and challenges in realising a practical technologyfor quantum communications. Key to this is the development of semiconductor devicesfor the generation and detection of quantum light states. Quantum cryptography systemsrely upon avalanche photodiodes that can count photons to very high rates, whileentangled light emitting diodes have recently enabled for electrically powered quantumteleporters.

Scanning Gate Microscopy of Quantum Dots:Fabry-Perrot Interferences and Thermally Induced Pattern of RingsAndrii Kleshchonok, Geneviève Fleury, Jean-Louis PichardService de Physique de l'Etat Condensé (CNRS URA 2464),IRAMIS/SPEC, CEA Saclay, 91191 Gif-sur-Yvette, FranceThermally induced rings can be seen in addition to Fabry-Perrot interference fringes spaced by half theFermi wavelength when one studies the conductance of the interferometer formed in a two dimensionalelectron gas (2DEG) between a nanostructured quantum contact and the charged tip of a scanning gatemicroscope. If the contact is made of a quantum dot with odd number of electrons opened in the middleof a Kondo valley, we show how the location of the rings allows to measure by electron interferometrythe magnetic moment of the dot above the Kondo temperature.

Spin manipulation in moving quantum dots via the spin-orbit interactionAnton Ramsak, Ljubljana UniversityAn exact solution is derived for the wavefunction of an electron in a semiconductorquantum wire with spin–orbit interaction and driven by external time-dependentharmonic confining potential. The formalism allows analytical expressions forvarious quantities to be derived, for example spin and pseudo-spin rotations, energyand occupation probabilities for excited states. It is demonstrated how perfect spinand pseudo-spin flips can be achieved at high frequencies of the order of , theconfining potential level spacing. By an appropriately chosen driving term, spinmanipulation can be exactly performed far into the non-adiabatic regime. Theimplications for spin-polarized emission and spin-dependent transport are alsodiscussed.

Scanning gate microscopy: General theory for weak probes and results for aballistic cavityDietmar Weinmann, University of StrasbourgIn scanning gate microscopy (SGM), the conductance change of a device induced bya local perturbation (a charged AFM tip) is measured as a function of the tipposition, yielding spatially resolved informations on the transport properties of thesample.In order to provide an interpretation of what precisely is being measured, wepresent a theory of the conductance changes induced by non-invasive weak SGMtips [1]. Ascattering approach yields expressions for the first and second order conductancechanges due to the perturbation by the tip potential. Only in special cases, such asthat of a centrally symmetric quantum point contact in the conductancequantization regime, the SGM response can be unambiguously related with the localcurrent density. In the non-linear transport regime, the breaking of the left-rightsymmetry of the device by the presence of the tip leads to a noticeable asymmetry ofthe current-voltage characteristic [2].In the regime of strong tips, we have performed numerical calculations for a modelstructure that corresponds to a recent SGM experiment [3] on a large ballistic cavitydefined in a semiconductor heterostructure. When the tip is in a region close to theentry/exit of the cavity, two parallel conduction channels arise, and result in acharacteristic fringe pattern. When the tip is inside the stadium large fluctuations ofthe conductance with the tip position are visible. Quantum calculations using therecursive Green function method agree well with the experimentally measured data.Some of the main features are even captured by a classical approach based onelectron trajectories through the structure.[1] C. Gorini, R.A. Jalabert, W. Szewc, S. Tomsovic, and D. Weinmann,arXiv:1302.1151v1[2] C. Gorini, R.A. Jalabert, and D. Weinmann, in preparation (2013)[3] A.A. Kozikov, D. Weinmann, C. Rössler, T. Ihn, K. Ensslin, C. Reichl, and W.Wegscheider, in preparation (2013)

Quantum State Engineering with QubitsGerd SchönKarlsruhe Institute of Technology, GermanyThe improving level of control of superconducting qubits opens paths to use them for variousquantum state engineering applications. Spectaculars results were obtained forsuperconducting qubits coupled to superconducting resonators. In these circuits conceptsoriginally introduced in the field of quantum optics could be realized with unprecedentedquality. We analyzed several such experiments which demonstrated lasing and cooling of theelectromagnetic modes. They differed in the mechanisms for creating the populationinversion. This includes a conventional driving via a 3 rd level, population inversion in thedressed state basis, but also the Sisyphus mechanism for damping and amplification. Weextended the usual quantum optics description to account for strong qubit-resonator couplingand included the effects of low-frequency noise.Similar scenarios are to be expected in circuits built from semiconducting quantum dots andsuitable high-Q resonators. In that case we observe strong correlations between the lasingstate of the system and its transport properties.As an application of the qubit technology we mention that superconducting qubits have beenapplied to perform spectroscopy on material specific 2-level fluctuators.

Lambda Systems for population transfer in superconductingnanocircuitsGiuseppe FalciDipartimento di Fisica e Astronomia Università di CataniaThe implementation of a Lambda scheme in superconducting artificial atomscould allow detection of stimulated Raman adiabatic passage (STIRAP) andother quantum manipulations in the microwave regime. However symmetrieswhich on one hand protect the system against decoherence, yield selectionrules which may cancel coupling to the pump external drive. The trade-offbetween efficient coupling and decoherence due to broad-band colored Noise(BBCN), which is often the main source of decoherence is addressed, in theclass of nanodevices based on the Cooper pair box (CPB) design. We studytransfer efficiency by STIRAP, showing that substantial efficiency is achievedfor off-symmetric bias only in the charge-phase regime. We find a number ofresults uniquely due to non-Markovianity of BBCN, namely: (a) the efficiencyfor STIRAP depends essentially on noise channels in the trapped subspace;(b) low-frequency fluctuations can be analyzed and represented as fictitiouscorrelated fluctuations of the detunings of the external drives; (c) a simplefigure of merit for design and operating prescriptions allowing theobservation of STIRAP is proposed. The emerging physical picture alsoapplies to other classes of coherent nanodevices subject to BBCN.Next we introduce a three-photon scheme allowing to obtain a Lambdaconfiguration at the symmetry point. Couplings and noise now both increasewith the Josephson energy, this effect being compensated by leakage from thethree-level subspace due to the spectrum becoming more and more harmonic.Finally we present a protocol where coherent population transfer is obtainedwith an always on coupling switched by a suitable detuning. The absence of anexact dark state makes the phenomenon of disruptive interference non-trivial.This configuration may allow to couple the device with electrical ormechanical modes of solid-state resonators.

Electrical Transport in Sculpturene-Based Systems for Sensing ApplicationsHatef Sadeghi, Lancaster UniversityAs discussed in previous talk, sculptured carbon based structures calledsculpturenes are novel allotropes of carbon, with new topologies and geometries.They could be formed by sculpting selected shapes from stacked graphene layersand allowing the shapes to reconstruct. Here we discuss the electrical properties ofsome of these structures. In particular, results are presented for the electricalconductance of sculpturene tori connected to external electrodes. These structurescontain a hydrophobic pocket, which can encapsulate molecules from a surroundingenvironment. We demonstrate that the electrical conductance is sensitive to suchencapsulation, thereby providing a new mechanism for environmental sensing.

Explorations in adiabatic quantum computingJoachim Wabnig, Nokia Research Center, Cambridge, UK.The big promise of quantum computing is a huge speedup compared to its classicalcounterpart. Twenty years after the discovery of Shor's factoring algorithm a wholezoo of quantum algorithms exists with speedups ranging from polynomial tosuperpolynomial, but universal quantum computers able to run these algorithmswill remain out of reach for years to come. A more feasible approach to quantumcomputing that emerged in recent years is to abandon universality and concentratethe efforts on hardware that can natively solve quadratic binary optimisationproblems. I will review an implementation using superconducting qubits anddiscuss what kind of problems can be solved on the hardware. A connection will bemade between binary optimisation problems and machine learning. Finally I willpoint out problems of interest and encourage the audience to scratch their heads.

Statistical Topological InsulatorsJonathan Edge, Leiden UniversityWe define a class of insulators with gapless surface states protected fromlocalization due to the statistical properties of a disordered ensemble, namely due tothe ensemble's invariance under a certain symmetry. We show that these insulatorsare topological, and are protected by a Z2 invariant. Finally, we prove that everytopological insulator gives rise to an infinite number of classes of statisticaltopological insulators in higher dimensions. Our conclusions are confirmed bynumerical simulations.

Spin lifetime measurements in a Si/SiGe double quantum dot.J. R. Prance 1 , Zhan Shi 1 , C. B. Simmons 1 , X. Wu 1 , D. E. Savage 1 , M. G. Lagally 1 , L. R. Schreiber 2 , L. M. K.Vandersypen 2 , Mark Friesen 1 , Robert Joynt 1 , S. N. Coppersmith 1 , and M. A. Eriksson 1(1) University of Wisconsin-Madison(2) Kavli Institute of Nanoscience, TU DelftThe two-electron singlet and triplet spin states in a double quantum dot form the basis for a logicalqubit that combines fast manipulation and a spin readout mechanism. Here we present lifetimemeasurements of these states in a Si/SiGe double quantum dot. The states are measured by usingPauli spin blockade to perform spin-to-charge conversion, and by detecting the charge state of thedouble dot in real time using a nearby quantum-point-contact charge sensor. At zero magnetic fieldthe state lifetimes are found to be ~10ms. As the magnetic field increases, the lifetime of the spinpolarizedT- triplet increases, reaching 3 seconds at 1T. This work was supported by ARO and LPS(W911NF-08-1-0482) and by the United States Department of Defense. The views and conclusionscontained in this document are those of the authors and should not be interpreted as representingthe official policies, either expressly or implied, of the US Government.

Engineering the Thermopower of C 60 Molecular JunctionsCharalambos Evangeli 1 , Katalin Gillemot 2 , Edmund Leary 1,3 , M. Teresa González 3 , GabinoRubio-Bollinger 1,4 , Colin J. Lambert 2 and Nicolás Agraït 1,3,41 Departamento de Física de la Materia Condensada,Universidad Autónoma de Madrid, Madrid, Spain2 Department of Physics, Lancaster University, Lancaster, United Kingdom3 Instituto Madrileño de Estudios Avanzados en Nanociencia IMDEA-Nanociencia, Madrid, Spain4 Instituto Universitario de Ciencia de Materiales ‘‘Nicolás Cabrera’’, Universidad Autónoma de Madrid,Madrid, SpainThe development of new higher-efficiency and low-cost thermoelectric devices is a desirabletechnology that would allow direct heat-to-electrical energy conversion from otherwisewasted low-level heat sources and would have an enormous impact on global energyconsumption. Nanoscale systems and especially organic junctions are very promising in thisrespect, due to the fact that transport takes place through discrete energy levels 1 . A number oftechniques for enhancing thermoelectric performance have been suggested, but to date nonehas led to sufficiently-high values of figure of merit (ZT). We propose a new strategy 2 forenhancing the thermoelectric performance of single-molecule devices, based on controllingthe coupling between C 60 molecules. The strategy is demonstrated using a combination of anew experimental technique and first principles theory. In contrast to all previousmeasurements, the newly-developed measurement technique allows us to measuresimultaneously the electrical conductance and thermopower of single and coupled molecules,and unambiguously characterize these at the single molecule level. We demonstrate thatthermopower can be almost doubled by tuning the interaction between two neighbouring C 60molecules under ambient conditions and further doubled by coupling three C 60 s. This trend isaccompanied by an unprecedented increase in the thermoelectric figure of merit ZT.[1] Malen, J.A.; Yee, S.K.; Majumdar, A.; Segalman, R.A. Fundamentals of energy transport, energy conversion,and thermal properties in organic–inorganic heterojunctions, Chem. Phys. Lett. 2010, 491, 109-122.[2] Evangeli, C.; Gillemot, K.; Leary, E.; González, M.T.; Rubio-Bollinger, G.; Lambert, C.J.; Agraït, N.Engineering the Thermopower of C 60 Molecular Junctions, Nano Lett. 2013, Article ASAP.

TitleSculpturenes: A New Family of sp 2 -Bonded Carbon-Based StructuresLaith Algharagholy, Steven W. D. Bailey, Thomas Pope, and Colin J. LambertLancaster University/UKAl-Qadisiyah University/IraqAbstractWe demonstrate a technique for creating unique forms of pure sp 2 -bondedcarbon and unprecedented heteromolecules. These new structures, which werefer to as sculpturenes, are formed by sculpting selected shapes from bilayergraphene, heterobilayers, or multilayered materials and allowing the shapes tospontaneously reconstruct. The simplest sculpturene is topologically equivalentto a torus, with dimensions comparable to those of fullerenes. The topology ofthese new molecular structures is stable against atomic-scale defects. Wedemonstrate that sculpturenes can form the basic building blocks of hollow,multiconnected structures.

Statistics of single electron emittersMarkus Buttiker, University of GenevaWe develop a novel approach to address the statistical properties of periodicallydriven systems such as quantum pumps and mesoscopic capacitors working in thesingle electron emission regime. Our approach focuses on the distribution functionof the waiting time between the arrival of charge carriers. Such short time statisticspermits to ascertain that only one electron is emitted during each driving cycle. Forperiodically driven systems the waiting time distribution describes the crossoverfrom poissonian electron transfer far from optimal conditions to electron emissionwhich is synchronized to the pump signal up to the uncertainty limit required by theescape-time [1]. To extend our approach to fully quantum mechanical systems wehave investigated the waiting time distribution of a quantum point contact under dcbias. In the zero-temperature limit this corresponds to an incident state in whichelectrons are as closely space as required by the Pauli principle. We find that thewaiting time distribution exhibits a Wigner-Dyson distribution for perfecttransmission and exhibits a crossover to a Poisson distribution close to pinch-off ofthe quantum point contact. We discuss several analogies with energy level statisticsand random matrix theory [2]. Additional results for the waiting time distributionfor dynamical quantum mechanical scatterers are presented for oscillating barrierand for Lorentzian pulses.[1 ] M. Albert, C. Flindt, M. Büttiker, Phys. Rev. Lett. 107, 086805 (2011).[2 ] M. Albert, G. Haack, C. Flindt, M. Büttiker, Phys. Rev. Lett. 108, 186806 (2012).[3] D. Dasenbrook, C. Flindt and M. Buttiker, (unpublished).

Spinful Kitaev model for spin-singlet topological superconductors and–periodic Josephson transportPanagiotis Kotetes 1 , Alexander Shnirman 2 , Gerd Schön 11 Institut für Theoretische Festkörperphysik, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany2 Institut für Theorie der Kondensierten Materie, Karlsruhe Institute of Technology, 76128 Karlsruhe, GermanyCurrent trends in the field of quantum information relate to topological quantum computing based onMajorana fermions. Recent experiments involving devices consisting of a Rashba spin-orbit coupledsemiconducting wire in proximity to an s-wave superconductor, have provided striking results when aZeeman field is applied, that could be associated with the presence of Majorana fermions. From atheoretical point of view, in the infinite magnetic field limit the above system is equivalent to a spinlessp-wave superconductor and can be effectively described by Kitaev’s celebrated lattice model. Thismodel owes the distinctive property of yielding Majorana fermions which are localized at the edges. Sofar, a vast number of previous studies were based on this mapping to Kitaev’s model, neglecting in thismanner the possibility of actively manipulating the spin degree of freedom. To tackle this problem, weconstruct an effective spinful Kitaev model that can be used for finite magnetic fields. By employingthe particular model, we study the magnetically controllable 4-periodic Josephson transport thatoriginates from two coupled neighbouring Majorana fermions that reside on two different wires.Although fundamental, the latter effects lack of a complete investigation for this type of devices. Ourresults are also in agreement with previous related Majorana-fermion-mediated magneto-Josephsoneffects [1]. Finally, this model can crucially facilitate the development of new spin-based quantumcomputing applications.[1] P. Kotetes, A. Shnirman and G. Schön, arXiv:1207.2691 (to appear in JKPS)

Spin thermoelectrics in a disordered two-dimensional Fermi gasJ. Borge 1 , Cosimo Gorini 2 , and Roberto Raimondi 1Dipartimento di Matematica e Fisica, Università Roma Tre,Via della Vasca Navale 84, 00146 Roma, Italy and2 Institut für Physik, Universität Augsburg, 86135 Augsburg, GermanyWe study the connection between the spin-heat and spin-charge response in a disordered Fermigas with spin-orbit coupling. It is shown that the ratio between the above responses can beexpressed as the thermopower S = −(πk B) 2 Tσ ′ /3eσ times a number R s which depends on thestrength and type of the spin-orbit couplings considered. The general results are illustratedby examining different two-dimensional electron or hole systems with different and competingspin-orbit mechanisms, and we conclude that a metallic system could prove much more efficient asa heat-to-spin than as a heat-to-charge converter.J. Borge, C. Gorini, R. Raimondi, Physical Review 87, 085309 (2013).

Static and Dynamic Magnetic Domain Walls in Small MagnetsR. AllenspachIBM Research – Zurich, 8803 Rueschlikon, SwitzerlandDevices taking advantage of spin offer operation at high speeds, low power, and densescaling, as well as giving the opportunity of entirely new functionality, such as non-volatiledata storage or dynamically reconfigurable logic. The basic magnetic nanostructure for manyof these devices is the magnetic domain wall, the transition zone between regions of differentmagnetization direction. Domain walls in small elements exhibit complex spin arrangementsthat strongly deviate from the wall types commonly encountered in magnetic thin filmsystems or bulk ferromagnets. They can be modified by changing the geometry of the element[1], and they can be excited and propagated by magnetic field or electrical current.We use spin-polarized scanning electron microscopy [2] and magneto-optical Kerr effect toimage the domain walls and study their propagation in nanowires with high resolution. Wefind persistent modifications of the wall structure upon propagation by current: Vortex wallstransform to transverse configurations upon subsequent pulse injections [3], and transversewalls can exhibit polarity reversals [4]. When propagated by magnetic field, domain wallsalso undergo drastic changes above a threshold field: Wall motion becomes turbulent atWalker breakdown [5]. In a study combining experiment and micromagnetic simulations, weinvestigated vortex walls in permalloy wires with widths ranging from 300 to 900 nm,fabricated by electron-beam lithography and by a nanostencil tool [6]. In wide wires, we findthe dynamics of vortex walls to depart significantly from the current description of a compactentity evolving along the wire [7]. Instead, the wall is composed of several substructurespropagating in different dynamic regimes with very different velocities. Wire edges cruciallyaffect this dynamics and can be influenced by variation of growth parameters. Micromagneticsimulations reveal the unusual wall structure and complement the experimental findings.Possibilities how to overcome the limits imposed by the Walker breakdown will be discussed.[1] 1. P.-O. Jubert, R. Allenspach, and A. Bischof, Phys. Rev. B 69, 220410(R) (2004).[2] R. Allenspach, IBM J. Res. Develop. 44, 553 (2000).[3] M. Kläui, P.-O. Jubert, R. Allenspach, A. Bischof, J.A.C. Bland, G. Faini, U. Rüdiger,C.A.F. Vaz, L. Vila, and C. Vouille, Phys. Rev. Lett. 95, 026601 (2005).[4] A. Vanhaverbeke, A. Bischof, and R. Allenspach, Phys. Rev. Lett. 101, 107202 (2008).[5] R. Cowburn and D. Petit, Nature Mater. 4, 721 (2005).[6] L. Gross, R.R. Schlittler, G. Meyer, and R. Allenspach, Nanotechnology 21, 325301(2010).[7] C. Zinoni, A. Vanhaverbeke, P. Eib, G. Salis, and R. Allenspach, Phys. Rev. Lett. 107,207204 (2011).

Minimal self-contained quadridot quantum refrigeration machineRosario Fazio, Scuola Normale Superiore, 56126 Pisa, ItalyWe present a theoretical study of an electronic quantum refrigerator based onfour quantum dots arranged in a square configuration, in contact with asmany thermal reservoirs. We show that the system implements the basicminimal mechanism for acting as a self-contained quantum refrigerator, bydemonstrating heat extraction from the coldest reservoir and the cooling ofthe nearby quantum-dot.Work done in collaboration with Davide Venturelli and Vittorio Giovannetti.

Entanglement and the Kondo effect in a triangular cluster of quantumdotsS. B. Tooski 1, 2,3 ,A. Ramsak 1,2 , R. Zitko 2 , and 31 Faculty of Physics and Mathematics, University of Ljubljana, Jadranska 19, Ljubljana,Slovenia2 Jozef Stefan Institute, Ljubljana, Slovenia3 Institute of Molecular Physics, Polish Academy of Sciences, ul. M. Smoluchowskiego 17,Poznan, PolandAbstractWe consider a triple quantum dot system in a triangular geometry 1 with one of the dotsconnected to metallic leads. The numerical renormalization group 2,3 is employed to studyquantum entanglement and its relation to the thermodynamic and transport properties, in theregime where each of the dots is singly occupied on average, but with non-negligible chargefluctuations. It is shown that even in the regime of significant charge fluctuations the formationof the Kondo singlets induces switching between separable and perfectly entangled states. Thequantum phase transition between unentangled and entangled states is analyzed quantitativelyand the corresponding phase diagram is explained by exactly solvable spin model.References[1] S. B. Tooski, A. Ramsak, R. Zitko, and B. R. Bulka, arXiv:1303.6577v1.[2] H. R. Krishna-murthy, J. W. Wilkins and K. G. Wilson, Phys. Rev. B 21, 1003 (1980).[3] R. Bulla, T. Costi, and T. Pruschke, Rev. Mod. Phys. 80, 395 (2008).

Xiong∗Physics Department, University of Ioannina,Ioannina 45110, Greece∗National Laboratory of Solid StateMicrostructures andDepartment of Physics, Nanjing University, Nanjing210093, China n n

I-V characteristics of atomic and molecular wireswith defect-induced quasibound statesTomasz KostyrkoFaculty of Physics, A. Mickiewicz University, Poznań, PolandThe results of our computations [1-4] concerning the electronic conductanceof junctions including atomic and molecular chains perturbed by defects or localexternal electrostatic potential will be summarized. Using a combination of asimple tight-binding modelling and a DFT treatment we showed various effectsof the perturbations on the transport. In the long molecular chains the defectsare detrimental for the conductance, often leading to quasibound states thatreduce the conductance by one quantum unit for a particular energy value. In theshort chains the scattering of electrons by the defects interfere with scatteringby the molecule-metal boundary, leading to oscillations of the conductance withthe position of the defects. Moreover, the quasibound state induced by theperturbations in the short chains may sometimes enhance the conductance, bypromoting the quasibound state to a neighbourhood of the Fermi level of theleads. Under influence of the bias voltage the position of the quasibound peakin the transmission may change in a way that depends on the position of thedefect in the chain. This effect may be interesting for two reasons: (1) it may beexploited to produce the molecular junction with an asymmetric current-voltagecharacteristics that can work as a molecular diode (2) it can be used to identifythe defect and determine its position in the molecular chain from a careful studyof the differential conductance of the junction. The above mentioned effectswill be exemplified with the cases of defected carbon nanotubes, hydrocarbonchains under influence of local external fields and chains of gold atoms withsubstitutional impurities.Bibliography1. T. Kostyrko, M. Bartkowiak, G.D. Mahan: “Localization in carbon nanotubeswithin a tight-binding model”, Phys. Rev. B 60, 10735 (1999).2. T. Kostyrko, V.M. García-Suárez, C.J. Lambert i B.R. Bu̷lka: “Currentrectification in molecular junctions produced by local potential fields”, Phys.Rev. B 81, 085308 (2010).3. M. Wawrzyniak-Adamczewska and T. Kostyrko: “Defect-induced conductanceoscillations in short atomic chains” J. Phys.: Condens. Matter 24,185305 (2012).4. M. Wawrzyniak-Adamczewska and T. Kostyrko: “Quasibound states andtransport characteristics of Au chains with a substitutional S impurity”, J.Phys.: Condens. Matter 25, 085504 (2013).

Symmetry-dependent thermoelectric properties of atomic junctionsVíctor M. García Suárez, University of Oviedo / CINN (CSIC)We calculate general expressions for the low-temperature thermoelectriccoefficients of atomic junctions. These results are used to analyze general trends inthe thermoelectric properties of atomic and molecular junctions. In particular, westudy the Seebeck coefficient and figure of merit of a series of molecular wires madeof porphyrin units with different metallic atoms in the core. We show how thesecoefficients depend upon the specific electronic structure of the metallic atom andits coupling to the molecular wire.

Band Structure and Optical Transitions in Atomic Layers of HexagonalGallium ChalcogenidesViktor Zolyomi, Lancaster UniversityWe report density functional theory calculations of the electronic band structures andoptical absorption spectra of two-dimensional crystals of Ga2X2 (X=S, Se, and Te). Ourcalculations show that all three two-dimensional materials are dynamically stableindirect-band-gap semiconductors with a Mexican-hat dispersion of holes near the top ofthe valence band. We predict that they undergo a Lifshitz transition upon hole doping andthat their optical properties make them ideal active elements in ultraviolet photondetectors.

Accelerated quasi-particle recombination in clean,anisotropic superconductorsAlexander Kozorezov 1 , Colin Lambert 1 , Csaba Peterfalvi 1 ,Douglas Bennett 2 , James Hays-Wehle 2 , and Robert Horansky 2 , Joel Ullom 21 Department of Physics, Lancaster University, Lancaster, UK; 2 NIST, Boulder, USWhile superconducting absorbers can provide high stopping power with little heatcapacity, the slow thermalization of deposited energy has limited their applicability.Pulse decays are described with at least two exponential time constants, the longer ofwhich can be 10s of milliseconds or more. Of the many superconductors, only Sn hasfound common application because of its unusually fast and efficient thermalization.We have recently demonstrated that the use of glue to attach absorbers introduces anadditional decay constant that gives the appearance of slow thermalization. Whenthe glue is removed, thermalization in Sn is observed to be extremely efficient, thusraising the question why is the expected "quasiparticle bottleneck" absent in Sn?We have developed a theory of quasiparticle recombination in clean superconductorswhich shows that gap anisotropy can enhance recombination by many orders ofmagnitude. This enhancement is due to the concentration of quasiparticles at smallareas of the Fermi surface where the superconducting gap is close to a localminimum. This concentration also results in faster escape of the recombinationphonons. We find that the enhancement of quasiparticle recombination in hexagonaltin may be especially strong due to the special shape of its Fermi surface.

Simulating the effect of organic solvents on OPEs in metalmolecule-metaljunctionsDávid Visontai, Colin LambertLancaster UniversityWe conducted a theoretical study on the electronic transport through metalmolecule-metaljunctions based on solvent tuned π-π stacking interactionsusing classical molecular dynamics (CMD), density functional theory (DFT)and non-equilibrium Greens function techniques (NEGF). There isexperimental evidence that the solvent has effect on aromatic interactions inmetal-molecule-metal junctions formed with monothiol terminatedoligophenyleneethynylenes (OPEs) In five different environments (decane,mesitylene (TMB), 1,4-dioxane,1,2,4-trichlorobenzene (TCB) and argon) theconductance showed well defined features in all solvents with differences injunction formation probabilities, characteristic lengths and junctionconductances. With the molecules measured in the present study, we showedthat the π-π stacking interactions and junction conductance values increasewith the number of benzene rings in the molecule. Clear evidence of quantuminterference is found in theoretical conductances, which in the presence ofsolvents, correlate well with experimental values.

Cooperative Fluorescence from a Strongly Driven Dilute Cloud of AtomsJohan Ott, University of GenevaWe investigate cooperative fluorescence in a dilute cloud of strongly driven twolevelemitters. Starting from the Heisenberg equations of motion, we compute thefirst-order scattering corrections to the saturation of the excited-state populationand to the resonance-fluorescence spectrum, which both require going beyond thestate-of-the-art linear-optics approach to describe collective phenomena. A dipoleblockade is observed due to long range dipole-dipole coupling that vanishes atstronger driving fields. Furthermore, we compute the inelastic component of thelight scattered by a cloud of many atoms and find that the Mollow triplet is affectedby cooperativity. In a lobe around the forward direction, the inelastic Mollow tripletdevelops a spectral asymmetry, observable under experimental conditions.

Classification of engineered topological superconductorsPanagiotis Kotetes 1 , Jens Michelsen 1 , Alexander Shnirman 2 , Gerd Schön 11 Institut für Theoretische Festkörperphysik, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany2 Institut für Theorie der Kondensierten Materie, Karlsruhe Institute of Technology, 76128 Karlsruhe, GermanyWe perform a complete classification of 2d, quasi-1d and 1d topological superconductors whichsupport Majorana fermions and originate from the suitable combination of inhomogeneous Rashbaspin-orbit coupling, magnetism and superconductivity. Our analysis reveals alternative topologicalsuperconducting platforms which are experimentally accessible. Specifically, we observe that for aquasi-1d Rashba semiconductor in proximity to a conventional superconductor, the flow of asupercurrent can lead to a class D topological superconductor without the demand for any kind ofmagnetic order. Furthermore, we also consider situations where additional symmetries emerge thatconstitute a product of space group and complex conjugation operations, with significant impact on thetopological properties of the system. For the latter cases, we put forward some new directions intopological quantum computing.

Geometry relaxation and transmission properties of Isocyano compoundsRubén R. Ferradás , University of Oviedo / CINN (CSIC)We show first-principles calculations of a series of molecules (isocyano compounds)with binding groups showing a strong dipole moment. We initially placed themolecules between two Au(111) surfaces terminated with a four atoms pyramidand displaced both tips in opposite direction along the z-axis to increase/decreasethe distance between the electrodes. In each step, we relaxed the coordinates of themolecule, and calculated the conductance and the transmission coefficients. Weshow how these transmission properties depend upon the specific binding group.