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Search Results: 1 - 10 of 461784 matches for " A. Kormanyos "
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Graphene Andreev Billiards
J. Cserti,I. Hagymasi,A. Kormanyos
Physics , 2009, DOI: 10.1103/PhysRevB.80.073404
Abstract: We studied the energy levels of graphene based Andreev billiards consisting of a superconductor region on top of a monolayer graphene sheet. For the case of Andreev retro-reflection we show that the graphene based Andreev billiard can be mapped to the normal metal-superconducting billiards with the same geometry. We also derived a semiclassical quantization rule in graphene based Andreev billiards. The exact and the semiclassically obtained spectrum agree very well both for the case of Andreev retro-reflection and specular Andreev reflection.
Andreev reflection through Fano resonances in molecular wires
A. Kormanyos,I. Grace,C. J. Lambert
Physics , 2008, DOI: 10.1103/PhysRevB.79.075119
Abstract: We study Andreev reflection in a normal conductor-molecule-superconductor junction using a first principles approach. In particular, we focus on a family of molecules consisting of a molecular backbone and a weakly coupled side group. We show that the presence of the side group can lead to a Fano resonance in the Andreev reflection. We use a simple theoretical model to explain the results of the numerical calculations and to make predictions about the possible sub-gap resonance structures in the Andreev reflection coefficient.
Bound states in inhomogeneous magnetic field in graphene: a semiclassical approach
A. Kormanyos,P. Rakyta,L. Oroszlany,J. Cserti
Physics , 2008, DOI: 10.1103/PhysRevB.78.045430
Abstract: We derive semiclassical quantization equations for graphene mono- and bilayer systems where the excitations are confined by the applied inhomogeneous magnetic field. The importance of a semiclassical phase, a consequence of the spinor nature of the excitations, is pointed out. The semiclassical eigenenergies show good agreement with the results of quantum mechanical calculations based on the Dirac equation of graphene and with numerical tight binding calculations.
Bound states in Andreev billiards with soft walls
F. Libisch,S. Rotter,J. Burgdoerfer,A. Kormanyos,J. Cserti
Physics , 2005, DOI: 10.1103/PhysRevB.72.075304
Abstract: The energy spectrum and the eigenstates of a rectangular quantum dot containing soft potential walls in contact with a superconductor are calculated by solving the Bogoliubov-de Gennes (BdG) equation. We compare the quantum mechanical solutions with a semiclassical analysis using a Bohr--Sommerfeld (BS) quantization of periodic orbits. We propose a simple extension of the BS approximation which is well suited to describe Andreev billiards with parabolic potential walls. The underlying classical periodic electron-hole orbits are directly identified in terms of ``scar'' like features engraved in the quantum wavefunctions of Andreev states determined here for the first time.
Quantum-classical correspondence in the wavefunctions of Andreev billiards
A. Kormanyos,Z. Kaufmann,J. Cserti,C. J. Lambert
Physics , 2005, DOI: 10.1103/PhysRevLett.96.237002
Abstract: We present a classical and quantum mechanical study of an Andreev billiard with a chaotic normal dot. We demonstrate that in general the classical dynamics of these normal-superconductor hybrid systems is mixed, thereby indicating the limitations of a widely used retracing approximation. We show that the mixed classical dynamics gives rise to a wealth of wavefunction phenomena, including periodic orbit scarring and localization of the wavefunction onto other classical phase space objects such as intermittent regions and quantized tori.
Magnetic focusing in normal-superconductor hybrid systems: a semiclassical analysis
P. Rakyta,A. Kormanyos,Z. Kaufmann,J. Cserti
Physics , 2007, DOI: 10.1103/PhysRevB.76.064516
Abstract: We study a transverse electron-hole focusing effect in a normal-superconductor system. The spectrum of the quasiparticles is calculated both quantum mechanically and in semiclassical approximation, showing an excellent agreement. A semiclassical conductance formula is derived which takes into account the effect of electron-like as well as hole-like quasiparticles. At low magnetic field the semiclassical conductance shows characteristic oscillations due to the Andreev reflection, while at higher fields it goes to zero. These findigs are in line with the results of previous quantum calculations and with the expectations based on the classical dynamics of the quasiparticles.
Semiclassical study of edge states and transverse electron focusing for strong spin-orbit coupling
Andor Kormanyos
Physics , 2010, DOI: 10.1103/PhysRevB.82.155316
Abstract: We studied the edge states and transverse electron focusing in the presence of spin-orbit interaction in a two dimensional electron gas. Assuming strong spin-orbit coupling we derived semiclassical quantization conditions to describe the dispersion of the edge states. Using the dispersion relation we then make predictions about certain properties of the focusing spectrum. Comparison of our analytical results with quantum mechanical transport calculations reveals that certain features of the focusing spectrum can be quite well understood in terms of the interference of the edge states while the explanation of other features seems to require a different approach.
Peculiar Nature of Snake States in Graphene
P. Rakyta,L. Oroszlany,A. Kormanyos,C. J. Lambert,J. Cserti
Physics , 2007, DOI: 10.1103/PhysRevB.77.081403
Abstract: We study the dynamics of the electrons in a non-uniform magnetic field applied perpendicular to a graphene sheet in the low energy limit when the excitation states can be described by a Dirac type Hamiltonian. We show that as compared to the two-dimensional electron gas (2DEG) snake states in graphene exibit peculiar properties related to the underlying dynamics of the Dirac fermions. The current carried by snake states is locally uncompensated even if the Fermi energy lies between the first non-zero energy Landau levels of the conduction and valence bands. The nature of these states is studied by calculating the current density distribution. It is shown that besides the snake states in finite samples surface states also exist.
Non-universal suppression of the excitation gap in chaotic Andreev billiards: Superconducting terminals as sensitive probes for scarred states
Andor Kormanyos,Henning Schomerus
Physics , 2006, DOI: 10.1103/PhysRevLett.97.124102
Abstract: When a quantum-chaotic normal conductor is coupled to a superconductor, random-matrix theory predicts that a gap opens up in the excitation spectrum which is of universal size $E_g^{\rm RMT}\approx 0.3 \hbar/t_D$, where $t_D$ is the mean scattering time between Andreev reflections. We show that a scarred state of long lifetime $t_S\gg t_D$ suppresses the excitation gap over a window $\Delta E\approx 2 E_g^{\rm RMT}$ which can be much larger than the narrow resonance width $\Gamma_S=\hbar/t_S$ of the scar in the normal system. The minimal value of the excitation gap within this window is given by $\Gamma_S/2\ll E_g^{\rm RMT}$. Hence the scarred state can be detected over a much larger energy range than it is the case when the superconducting terminal is replaced by a normal one.
Intrinsic and substrate induced spin-orbit interaction in chirally stacked trilayer graphene
Andor Kormanyos,Guido Burkard
Physics , 2012, DOI: 10.1103/PhysRevB.87.045419
Abstract: We present a combined group-theoretical and tight-binding approach to calculate the intrinsic spin-orbit coupling (SOC) in ABC stacked trilayer graphene. We find that compared to monolayer graphene, a larger set of d orbitals (in particular the d_{z^2} orbital) needs to be taken into account. We also consider the intrinsic SOC in bilayer graphene, because the comparison between our tight-binding bilayer results and the density functional computations of Ref.[40] allows us to estimate the values of the trilayer SOC parameters as well. We also discuss the situation when a substrate or adatoms induce strong SOC in only one of the layers of bilayer or ABC trilayer graphene. Both for the case of intrinsic and externally induced SOC we derive effective Hamiltonians which describe the low-energy spin-orbit physics. We find that at the K point of the Brillouin zone the effect of Bychkov-Rashba type SOC is suppressed in bilayer and ABC trilayer graphene compared to monolayer graphene.
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