250 research outputs found
Bott periodicity for the topological classification of gapped states of matter with reflection symmetry
Using a dimensional reduction scheme based on scattering theory, we show that
the classification tables for topological insulators and superconductors with
reflection symmetry can be organized in two period-two and four period-eight
cycles, similar to the Bott periodicity found for topological insulators and
superconductors without spatial symmetries. With the help of the dimensional
reduction scheme the classification in arbitrary dimensions can be
obtained from the classification in one dimension, for which we present a
derivation based on relative homotopy groups and exact sequences to classify
one-dimensional insulators and superconductors with reflection symmetry. The
resulting classification is fully consistent with a comprehensive
classification obtained recently by Shiozaki and Sato [Phys.\ Rev.\ B {\bf 90},
165114 (2014)]. The use of a scattering-matrix inspired method allows us to
address the second descendant \bZ_2 phase, for which the topological
nontrivial phase was previously reported to be vulnerable to perturbations that
break translation symmetry.Comment: 18 pages, 7 figure
Tunable Magnetic Relaxation In Magnetic Nanoparticles
We investigate the magnetization dynamics of a conducting magnetic
nanoparticle weakly coupled to source and drain electrodes, under the
assumption that all relaxation comes from exchange of electrons with the
electrodes. The magnetization dynamics is characterized by a relaxation time
, which strongly depends on temperature, bias voltage, and gate voltage.
While a direct measure of a nanoparticle magnetization might be difficult, we
find that can be determined through a time resolved transport
measurement. For a suitable choice of gate voltage and bias voltage, the
magnetization performs a bias-driven Brownian motion regardless of the presence
of anisotropy.Comment: 4 pages, 2 eps figure
Mesoscopic effects in adiabatic spin pumping
We show that temporal shape modulations (pumping) of a quantum dot in the
presence of spin-orbital coupling lead to a finite dc spin current. Depending
on the strength of the spin-orbit coupling, the spin current is polarized
perpendicular to the plane of the two-dimensional electron gas, or has an
arbitrary direction subject to mesoscopic fluctuations. We analyze the
statistics of the spin and charge currents in the adiabatic limit for the full
cross-over from weak to strong spin-orbit coupling.Comment: 4 pages, 1 figure same as version 1. Added a comma to separate the
two author name
Semiclassical theory of persistent current fluctuations in ballistic chaotic rings
The persistent current in a mesoscopic ring has a Gaussian distribution with
small non-Gaussian corrections. Here we report a semiclassical calculation of
the leading non-Gaussian correction, which is described by the three-point
correlation function. The semiclassical approach is applicable to systems in
which the electron dynamics is ballistic and chaotic, and includes the
dependence on the Ehrenfest time. At small but finite Ehrenfest times, the
non-Gaussian fluctuations are enhanced with respect to the limit of zero
Ehrenfest time.Comment: 9 pages, 3 figures; submitted as invited contribution to a special
issue in Physica E in memory of Markus Buettike
Semiclassical theory of speckle correlations
Coherent wave propagation in random media results in a characteristic speckle
pattern, with spatial intensity correlations with short-range and long-range
behavior. Here, we show how the speckle correlation function can be obtained
from a ray picture for two representative geometries: A chaotic cavity and a
random waveguide. Our calculation allows us to study the crossover between a
"ray limit" and a "wave limit", in which the Ehrenfest time is larger
or smaller than the typical transmission time , respectively.
Remarkably, long-range speckle correlations persist in the ray limit .Comment: 13 pages, 7 figure
Density of states as a probe of electrostatic confinement in graphene
We theoretically analyze the possibility to confine electrons in single-layer
graphene with the help of metallic gates, via the evaluation of the density of
states of such a gate-defined quantum dot in the presence of a ring-shaped
metallic contact. The possibility to electrostatically confine electrons in a
gate-defined ``quantum dot'' with finite-carrier density, surrounded by an
undoped graphene sheet, strongly depends on the integrability of the electron
dynamics in the quantum dot. With the present calculations we can
quantitatively compare confinement in dots with integrable and chaotic
dynamics, and verify the prediction that the Berry phase associated with the
pseudospin leads to partial confinement in situations where no confinement is
expected according to the arguments relying on the classical dynamics only.Comment: 9 pages, 7 figure
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