114 research outputs found
Peculiar Nature of Snake States in Graphene
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.Comment: 4 pages, 5 figure
Emergence of bound states in ballistic magnetotransport of graphene antidots
An experimental method for detection of bound states around an antidot formed
from a hole in a graphene sheet is proposed by measuring the ballistic two
terminal conductances. In particularly, we consider the effect of bound states
formed by magnetic field on the two terminal conductance and show that one can
observe Breit-Wigner like resonances in the conductance as a function of the
Fermi level close to the energies of the bound states. In addition, we develop
a new numerical method in which the computational effort is proportional to the
linear dimensions, instead of the area of the scattering region beeing typical
for the existing numerical recursive Green's function method.Comment: 7 pages, 6 figure
Non-retracing orbits in Andreev billiards
The validity of the retracing approximation in the semiclassical quantization
of Andreev billiards is investigated. The exact energy spectrum and the
eigenstates of normal-conducting, ballistic quantum dots in contact with a
superconductor are calculated by solving the Bogoliubov-de Gennes equation and
compared with the semiclassical Bohr-Sommerfeld quantization for periodic
orbits which result from Andreev reflections. We find deviations that are due
to the assumption of exact retracing electron-hole orbits rather than the
semiclassical approximation, as a concurrently performed
Einstein-Brillouin-Keller quantization demonstrates. We identify three
different mechanisms producing non-retracing orbits which are directly
identified through differences between electron and hole wave functions.Comment: 9 pages, 12 figures, Phys. Rev. B (in print), high resolution images
available upon reques
Theory of resistor networks: The two-point resistance
The resistance between arbitrary two nodes in a resistor network is obtained
in terms of the eigenvalues and eigenfunctions of the Laplacian matrix
associated with the network. Explicit formulas for two-point resistances are
deduced for regular lattices in one, two, and three dimensions under various
boundary conditions including that of a Moebius strip and a Klein bottle. The
emphasis is on lattices of finite sizes. We also deduce summation and product
identities which can be used to analyze large-size expansions of two-and-higher
dimensional lattices.Comment: 30 pages, 5 figures now included; typos in Example 1 correcte
Effect of the band structure topology on the minimal conductivity for bilayer graphene with symmetry breaking
Using the Kubo formula we develop a general and simple
expression for the minimal conductivity in systems described by
a 2Ă—2 Hamiltonian. As an application we derive an analytical
expression for the minimal conductivity tensor of bilayer
graphene as a function of a complex parameter w related to
recently proposed symmetry breaking mechanisms resulting from
electron-electron interaction or strain applied to the sample.
The number of Dirac points changes with varying parameter w, and
this directly affects the minimal conductivity. Our analytic
expression is confirmed using an independent calculation based
on the Landauer approach, and we find remarkably good agreement
between the two methods. We demonstrate that the minimal
conductivity is very sensitive to the change of the parameter w
and the orientation of the electrodes with respect to the
sample. Our results show that the minimal conductivity is
closely related to the topology of the low-energy band
structure
Human brain distinctiveness based on EEG spectral coherence connectivity
The use of EEG biometrics, for the purpose of automatic people recognition,
has received increasing attention in the recent years. Most of current analysis
rely on the extraction of features characterizing the activity of single brain
regions, like power-spectrum estimates, thus neglecting possible temporal
dependencies between the generated EEG signals. However, important
physiological information can be extracted from the way different brain regions
are functionally coupled. In this study, we propose a novel approach that fuses
spectral coherencebased connectivity between different brain regions as a
possibly viable biometric feature. The proposed approach is tested on a large
dataset of subjects (N=108) during eyes-closed (EC) and eyes-open (EO) resting
state conditions. The obtained recognition performances show that using brain
connectivity leads to higher distinctiveness with respect to power-spectrum
measurements, in both the experimental conditions. Notably, a 100% recognition
accuracy is obtained in EC and EO when integrating functional connectivity
between regions in the frontal lobe, while a lower 97.41% is obtained in EC
(96.26% in EO) when fusing power spectrum information from centro-parietal
regions. Taken together, these results suggest that functional connectivity
patterns represent effective features for improving EEG-based biometric
systems.Comment: Key words: EEG, Resting state, Biometrics, Spectral coherence, Match
score fusio
Electronic standing waves on the surface of the topological insulator Bi2Te3
A line defect on a metallic surface induces standing waves in the electronic
local density of states (LDOS). Asymptotically far from the defect, the wave
number of the LDOS oscillations at the Fermi energy is usually equal to the
distance between nesting segments of the Fermi contour, and the envelope of the
LDOS oscillations shows a power-law decay as moving away from the line defect.
Here, we theoretically analyze the LDOS oscillations close to a line defect on
the surface of the topological insulator Bi2Te3, and identify an important
pre-asymptotic contribution with wave number and decay characteristics markedly
different from the asymptotic contributions. Wave numbers characterizing the
pre-asymptotic LDOS oscillations are in good agreement with recent data from
scanning tunneling microscopy experiments [Phys. Rev. Lett. 104, 016401
(2010)].Comment: 8 pages, 5 figures; published versio
A simple model for the vibrational modes in honeycomb lattices
The classical lattice dynamics of honeycomb lattices is studied in the
harmonic approximation. Interactions between nearest neighbors are represented
by springs connecting them. A short and necessary introduction of the lattice
structure is presented. The dynamical matrix of the vibrational modes is then
derived, and its eigenvalue problem is solved analytically. The solution may
provide deeper insight into the nature of the vibrational modes. Numerical
results for the vibrational frequencies are presented. To show that how
effective our method used for the case of honeycomb lattice is, we also apply
it to triangular and square lattice structures. A few suggested problems are
listed in the concluding section.Comment: 9 pages, 12 figures, submitted to American Journal of Physic
Negative length orbits in normal-superconductor billiard systems
The Path-Length Spectra of mesoscopic systems including diffractive
scatterers and connected to superconductor is studied theoretically. We show
that the spectra differs fundamentally from that of normal systems due to the
presence of Andreev reflection. It is shown that negative path-lengths should
arise in the spectra as opposed to normal system. To highlight this effect we
carried out both quantum mechanical and semiclassical calculations for the
simplest possible diffractive scatterer. The most pronounced peaks in the
Path-Length Spectra of the reflection amplitude are identified by the routes
that the electron and/or hole travels.Comment: 4 pages, 4 figures include
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