58 research outputs found
Effects of electron scattering on the topological properties of nanowires: Majorana fermions from disorder and superlattices
We focus on inducing topological state from regular, or irregular scattering
in (i) p-wave superconducting wires and (ii) Rashba wires proximity coupled to
an s-wave superconductor. We find that contrary to common expectations the
topological properties of both systems are fundamentally different: In p-wave
wires, disorder generally has a detrimental effect on the topological order and
the topological state is destroyed beyond a critical disorder strength. In
contrast, in Rashba wires, which are relevant for recent experiments, disorder
can {\it induce} topological order, reducing the need for quasiballistic
samples to obtain Majorana fermions. Moreover, we find that the total phase
space area of the topological state is conserved for long disordered Rashba
wires, and can even be increased in an appropriately engineered superlattice
potential.Comment: 5 pages, 3 figs, RevTe
Measurement of spin-dependent conductivities in a two-dimensional electron gas
Spin accumulation is generated by injecting an unpolarized charge current
into a channel of GaAs two-dimensional electron gas subject to an in-plane
magnetic field, then measured in a non-local geometry. Unlike previous
measurements that have used spin-polarized nanostructures, here the spin
accumulation arises simply from the difference in bulk conductivities for
spin-up and spin-down carriers. Comparison to a diffusive model that includes
spin subband splitting in magnetic field suggests a significantly enhanced
electron spin susceptibility in the 2D electron gas
Quantal Andreev billiards: Semiclassical approach to mesoscale oscillations in the density of states
Andreev billiards are finite, arbitrarily-shaped, normal-state regions,
surrounded by superconductor. At energies below the superconducting gap,
single-quasiparticle excitations are confined to the normal region and its
vicinity, the essential mechanism for this confinement being Andreev
reflection. This Paper develops and implements a theoretical framework for the
investigation of the short-wave quantal properties of these
single-quasiparticle excitations. The focus is primarily on the relationship
between the quasiparticle energy eigenvalue spectrum and the geometrical shape
of the normal-state region, i.e., the question of spectral geometry in the
novel setting of excitations confined by a superconducting pair-potential.
Among the central results of this investigation are two semiclassical trace
formulas for the density of states. The first, a lower-resolution formula,
corresponds to the well-known quasiclassical approximation, conventionally
invoked in settings involving superconductivity. The second, a
higher-resolution formula, allows the density of states to be expressed in
terms of: (i) An explicit formula for the level density, valid in the
short-wave limit, for billiards of arbitrary shape and dimensionality. This
level density depends on the billiard shape only through the set of
stationary-length chords of the billiard and the curvature of the boundary at
the endpoints of these chords; and (ii) Higher-resolution corrections to the
level density, expressed as a sum over periodic orbits that creep around the
billiard boundary. Owing to the fact that these creeping orbits are much longer
than the stationary chords, one can, inter alia, hear the stationary chords of
Andreev billiards.Comment: 52 pages, 15 figures, 1 table, RevTe
Theory of anomalous magnetic interference pattern in mesoscopic SNS Josephson junctions
The magnetic interference pattern in mesoscopic SNS Josephson junctions is
sensitive to the scattering in the normal part of the system. In this paper we
investigate it, generalizing Ishii's formula for current-phase dependence to
the case of normal scattering at NS boundaries in an SNS junction of finite
width. The resulting flattening of the first diffraction peak is consistent
with experimental data for S-2DEG-S mesoscopic junctions.Comment: 6 pages, 5 figures. Phys. Rev. B 68, 144514 (2003
Low-energy quasiparticle excitations in dirty d-wave superconductors and the Bogoliubov-de Gennes kicked rotator
We investigate the quasiparticle density of states in disordered d-wave
superconductors. By constructing a quantum map describing the quasiparticle
dynamics in such a medium, we explore deviations of the density of states from
its universal form (), and show that additional low-energy
quasiparticle states exist provided (i) the range of the impurity potential is
much larger than the Fermi wavelength [allowing to use recently developed
semiclassical methods]; (ii) classical trajectories exist along which the
pair-potential changes sign; and (iii) the diffractive scattering length is
longer than the superconducting coherence length. In the classically chaotic
regime, universal random matrix theory behavior is restored by quantum
dynamical diffraction which shifts the low energy states away from zero energy,
and the quasiparticle density of states exhibits a linear pseudogap below an
energy threshold .Comment: 4 pages, 3 figures, RevTe
Topological information device operating at the Landauer limit
We propose and theoretically investigate a novel Maxwell's demon implementation based on the spin-momentum locking property of topological matter. We use nuclear spins as a memory resource which provides the advantage of scalability. We show that this topological information device can ideally operate at the Landauer limit; the heat dissipation required to erase one bit of information stored in the demon's memory approaches kBTln2. Furthermore, we demonstrate that all available energy, kBTln2 per one bit of information, can be extracted in the form of electrical work. Finally, we find that the current-voltage characteristic of topological information device satisfy the conditions of an ideal memristor.</p
Scattering Theory of Current-Induced Spin Polarization
We construct a novel scattering theory to investigate magnetoelectrically
induced spin polarizations. Local spin polarizations generated by electric
currents passing through a spin-orbit coupled mesoscopic system are measured by
an external probe. The electrochemical and spin-dependent chemical potentials
on the probe are controllable and tuned to values ensuring that neither charge
nor spin current flow between the system and the probe, on time-average. For
the relevant case of a single-channel probe, we find that the resulting
potentials are exactly independent of the transparency of the contact between
the probe and the system. Assuming that spin relaxation processes are absent in
the probe, we therefore identify the local spin-dependent potentials in the
sample at the probe position, and hence the local current-induced spin
polarization, with the spin-dependent potentials in the probe itself. The
statistics of these local chemical potentials is calculated within random
matrix theory. While they vanish on spatial and mesoscopic average, they
exhibit large fluctuations, and we show that single systems typically have spin
polarizations exceeding all known current-induced spin polarizations by a
parametrically large factor. Our theory allows to calculate quantum
correlations between spin polarizations inside the sample and spin currents
flowing out of it. We show that these large polarizations correlate only weakly
with spin currents in external leads, and that only a fraction of them can be
converted into a spin current in the linear regime of transport, which is
consistent with the mesoscopic universality of spin conductance fluctuations.
We numerically confirm the theory.Comment: Final version; a tunnel barrier between the probe and the dot is
considered. To appear in 'Nanotechnology' in the special issue on "Quantum
Science and Technology at the Nanoscale
Probing d-wave pairing correlations in the pseudogap regime of the cuprate superconductors via low-energy states near impurities
The issue of probing the pseudogap regime of the cuprate superconductors, specifically with regard to the existence and nature of superconducting pairing correlations of d-wave symmetry, is explored theoretically. It is shown that if the d-wave correlations believed to describe the superconducting state persist into the pseudogap regime, but with pair-potential phase fluctuations that destroy their long-range nature, then the low-energy quasiparticle states observed near extended impurities in the truly superconducting state should also persist as resonances in the pseudogap regime. The scattering of quasiparticles by these phase-fluctuations broadens what was (in the superconducting state) a sharp peak in the single-particle spectral function at low energy, as we demonstrate within the context of a simple model. This peak and its broadening are, in principle, accessible via scanning tunneling spectroscopy near extended impurities in the pseudogap regime. If so, such experiments would provide a probe of the extent to which d-wave superconducting correlations persist upon entering the pseudogap regime, thus providing a stringent diagnostic of the phase-fluctuation scenario
Density of states in d-wave superconductors disordered by extended impurities
The low-energy quasiparticle states of a disordered d-wave superconductor are
investigated theoretically. A class of such states, formed via tunneling
between the Andreev bound states that are localized around extended impurities
(and result from scattering between pair-potential lobes that differ in sign)
is identified. Its (divergent) contribution to the total density of states is
determined by taking advantage of connections with certain one-dimensional
random tight-binding models. The states under discussion should be
distinguished from those associated with nodes in the pair potential.Comment: 5 pages, 1 figur
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