2,539 research outputs found
Ballistic Quantum Transport: Effect of Geometrical Phases
We study the influence of nonuniform magnetic fields on the magneto
conductance of mesoscopic microstructures. We show that the coupling of the
electron spin to the inhomogenous field gives rise to effects of the Berry
phase on ballistic quantum transport and discuss adiabaticity conditions
required to observe such effects. We present numerical results for different
ring geometries showing a splitting of Aharonov-Bohm conductance peaks for
single rings and corresponding signatures of the geometrical phase in weak
localization. The latter features can be qualitatively explained in a
semiclassical approach to quantum transport.Comment: 15 pages, 6 figures. Accepted for publication in Foundations of
Physic
Magnetotransport in disordered two-dimensional topological insulators: signatures of charge puddles
In this numerical study we investigate the influence and interplay of
disorder, spin-orbit coupling and magnetic field on the edge-transport in
HgTe/CdTe quantum wells in the framework of coherent elastic scattering. We
show that the edge states remain unaffected by the combined effect of moderate
disorder and a weak magnetic field at realistic spin-orbit coupling strengths.
Agreement with the experimentally observed linear magnetic field dependence for
the conductance of long samples is obtained when considering the existence of
charge puddles.Comment: 17 pages, 6 figure
Orbital Magnetism of Graphene Nanostructures: Bulk and Confinement Effects
We consider the orbital magnetic properties of non-interacting charge
carriers in graphene-based nanostructures in the low-energy regime. The
magnetic response of such systems results both, frombulk contributions and from
confinement effects that can be particularly strong in ballistic quantum dots.
First we provide a comprehensive study of the magnetic susceptibility of
bulk graphene in a magnetic field for the different regimes arising from the
relative magnitudes of the energy scales involved, i.e. temperature, Landau
level spacing and chemical potential. We show that for finite temperature or
chemical potential, is not divergent although the diamagnetic
contribution from the filled valance band exhibits the well-known
dependence. We further derive oscillatory modulations of ,
corresponding to de Haas-van Alphen oscillations of conventional
two-dimensional electron gases. These oscillations can be large in graphene,
thereby compensating the diamagnetic contribution and yielding a net
paramagnetic susceptibility for certain energy and magnetic field regimes.
Second, we predict and analyze corresponding strong, confinement-induced
susceptibility oscillations in graphene-based quantum dots with amplitudes
distincly exceeding the corresponding bulk susceptibility. Within a
semiclassical approach we derive generic expressions for orbital magnetism of
graphene quantum dots with regular classical dynamics. Graphene-specific
features can be traced back to pseudospin interference along the underlying
periodic orbits. We demonstrate the quality of the semiclassical approximation
by comparison with quantum mechanical results for two exemplary mesoscopic
systems, a graphene disk with infinite mass-type edges and a rectangular
graphene structure with armchair and zigzag edges, using numerical
tight-binding calculations in the latter case.Comment: 21 pages, 16 figure
Statistical description of eigenfunctions in chaotic and weakly disordered systems beyond universality
We present a semiclassical approach to eigenfunction statistics in chaotic
and weakly disordered quantum systems which goes beyond Random Matrix Theory,
supersymmetry techniques, and existing semiclassical methods. The approach is
based on a generalization of Berry's Random Wave Model, combined with a
consistent semiclassical representation of spatial two-point correlations. We
derive closed expressions for arbitrary wavefunction averages in terms of
universal coefficients and sums over classical paths, which contain, besides
the supersymmetry results, novel oscillatory contributions. Their physical
relevance is demonstrated in the context of Coulomb blockade physics
Effects of short-range interactions on transport through quantum point contacts: A numerical approach
We study electronic transport through a quantum point contact, where the
interaction between the electrons is approximated by a contact potential. Our
numerical approach is based on the non-equilibrium Green function technique
which is evaluated at Hartree-Fock level. We show that this approach allows us
to reproduce relevant features of the so-called "0.7 anomaly" observed in the
conductance at low temperatures, including the characteristic features in
recent shot noise measurements. This is consistent with a spin-splitting
interpretation of the process, and indicates that the "0.7 anomaly" should also
be observable in transport experiments with ultracold fermionic atoms.Comment: 12 pages, 10 figure
Two-dimensional topological insulator edge state backscattering by dephasing
To understand the seemingly absent temperature dependence in the conductance
of two-dimensional topological insulator edge states, we perform a numerical
study which identifies the quantitative influence of the combined effect of
dephasing and elastic scattering in charge puddles close to the edges. We show
that this mechanism may be responsible for the experimental signatures in
HgTe/CdTe quantum wells if the puddles in the samples are large and weakly
coupled to the sample edges. We propose experiments on artificial puddles which
allow to verify this hypothesis and to extract the real dephasing time scale
using our predictions. In addition, we present a new method to include the
effect of dephasing in wave-packet-time-evolution algorithms.Comment: 7 pages, 5 figure
Quantum graphs whose spectra mimic the zeros of the Riemann zeta function
One of the most famous problems in mathematics is the Riemann hypothesis:
that the non-trivial zeros of the Riemann zeta function lie on a line in the
complex plane. One way to prove the hypothesis would be to identify the zeros
as eigenvalues of a Hermitian operator, many of whose properties can be derived
through the analogy to quantum chaos. Using this, we construct a set of quantum
graphs that have the same oscillating part of the density of states as the
Riemann zeros, offering an explanation of the overall minus sign. The smooth
part is completely different, and hence also the spectrum, but the graphs pick
out the low-lying zeros.Comment: 8 pages, 8 pdf figure
The role of orbital dynamics in spin relaxation and weak antilocalization in quantum dots
We develop a semiclassical theory for spin-dependent quantum transport to
describe weak (anti)localization in quantum dots with spin-orbit coupling. This
allows us to distinguish different types of spin relaxation in systems with
chaotic, regular, and diffusive orbital classical dynamics. We find, in
particular, that for typical Rashba spin-orbit coupling strengths, integrable
ballistic systems can exhibit weak localization, while corresponding chaotic
systems show weak antilocalization. We further calculate the magnetoconductance
and analyze how the weak antilocalization is suppressed with decreasing quantum
dot size and increasing additional in-plane magnetic field.Comment: 5 page
Spin-polarized Quantum Transport in Mesoscopic Conductors: Computational Concepts and Physical Phenomena
Mesoscopic conductors are electronic systems of sizes in between nano- and
micrometers, and often of reduced dimensionality. In the phase-coherent regime
at low temperatures, the conductance of these devices is governed by quantum
interference effects, such as the Aharonov-Bohm effect and conductance
fluctuations as prominent examples. While first measurements of quantum charge
transport date back to the 1980s, spin phenomena in mesoscopic transport have
moved only recently into the focus of attention, as one branch of the field of
spintronics. The interplay between quantum coherence with confinement-,
disorder- or interaction-effects gives rise to a variety of unexpected spin
phenomena in mesoscopic conductors and allows moreover to control and engineer
the spin of the charge carriers: spin interference is often the basis for
spin-valves, -filters, -switches or -pumps. Their underlying mechanisms may
gain relevance on the way to possible future semiconductor-based spin devices.
A quantitative theoretical understanding of spin-dependent mesoscopic
transport calls for developing efficient and flexible numerical algorithms,
including matrix-reordering techniques within Green function approaches, which
we will explain, review and employ.Comment: To appear in the Encyclopedia of Complexity and System Scienc
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