3,384 research outputs found
Enhanced current noise correlations in a Coulomb-Majorana device
Majorana bound states (MBSs) nested in a topological nanowire are predicted
to manifest nonlocal correlations in the presence of a finite energy splitting
between the MBSs. However, the signal of the nonlocal correlations has not yet
been detected in experiments. A possible reason is that the energy splitting is
too weak and seriously affected by many system parameters. Here we investigate
the charging energy induced nonlocal correlations in a hybrid device of MBSs
and quantum dots. The nanowire that hosts the MBSs is assumed in proximity to a
mesoscopic superconducting island with a finite charging energy. Each end of
the nanowire is coupled to one lead via a quantum dot with resonant levels.
With a floating superconducting island, the devices shows a negative
differential conductance and giant super-Poissonian shot noise, due to the
interplay between the nonlocality of the MBSs and dynamical Coulomb blockade
effect. When the island is strongly coupled to a bulk superconductor, the
current cross correlations at small lead chemical potentials are negative by
tuning the dot energy levels. In contrast, the cross correlation is always
positive in a non-Majorana setup. This difference may provide a signature for
the existence of the MBSs.Comment: 11 pages, 10 figure
Finite-temperature conductivity and magnetoconductivity of topological insulators
The electronic transport experiments on topological insulators exhibit a
dilemma. A negative cusp in magnetoconductivity is widely believed as a quantum
transport signature of the topological surface states, which are immune from
localization and exhibit the weak antilocalization. However, the measured
conductivity drops logarithmically when lowering temperature, showing a typical
feature of the weak localization as in ordinary disordered metals. Here, we
present a conductivity formula for massless and massive Dirac fermions as a
function of magnetic field and temperature, by taking into account the
electron-electron interaction and quantum interference simultaneously. The
formula reconciles the dilemma by explicitly clarifying that the temperature
dependence of the conductivity is dominated by the interaction while the
magnetoconductivity is mainly contributed by the quantum interference. The
theory paves the road to quantitatively study the transport in topological
insulators and other two-dimensional Dirac-like systems, such as graphene,
transition metal dichalcogenides, and silicene.Comment: 5 pages, 5 figure
Extrinsic anomalous Hall conductivity of a topologically nontrivial conduction band
A key step towards dissipationless transport devices is the quantum anomalous
Hall effect, which is characterized by an integer quantized Hall conductance in
a ferromagnetic insulator with strong spin-orbit coupling. In this work, the
anomalous Hall effect due to the impurity scattering, namely the extrinsic
anomalous Hall effect, is studied when the Fermi energy crosses with the
topologically nontrivial conduction band of a quantum anomalous Hall system.
Two major extrinsic contributions, the side-jump and skew-scattering Hall
conductivities, are calculated using the diagrammatic techniques in which both
nonmagnetic and magnetic scattering are taken into account simultaneously. The
side-jump Hall conductivity changes its sign at a critical sheet carrier
density for the nontrivial phase, while it remains sign unchanged for the
trivial phase. The critical sheet carrier densities estimated with realistic
parameters lie in an experimentally accessible range. The results imply that a
quantum anomalous Hall system could be identified in the good-metal regime.Comment: 5 pages, 4 figure
Nonlocal noise cross-correlation mediated by entangled Majorana fermions
Due to their nonlocality, qubits nested in Majorana bound states may be the
key to realize decoherence-free quantum computation. Majorana bound states
could be achieved at the ends of a one-dimensional topological superconductor.
However, when the bound states couple directly to electron reservoirs their
nonlocal correlation is quenched by local Andreev reflections. Here we propose
a scheme to generate nonlocal noise cross correlation between two
well-separated quantum dots, mediated by a pair of Majorana bound states. Both
positive and negative cross correlations can be obtained by tuning the gate
voltages applied to the dots. Within a limited range of finite temperatures,
the cross correlation is not suppressed by thermal fluctuations. Furthermore,
we show how the local Andreev reflections suppress the noise cross correlation
when multiple dot energy levels are coupled to the Majorana bound states. The
measurable cross correlation is expected to serve as a sensitive indicator for
the generation of Majorana fermions.Comment: 8 pages, 5 figure
Detecting and Switching Magnetization of Stoner Nanograin in Non-local Spin Valve
The magnetization detection and switching of an ultrasmall Stoner nanograin
in a non-local spin valve (NLSV) device is studied theoretically. With the help
of the rate equations, a unified description can be presented on the same
footing for the NLSV signal that reads out the magnetization, and for the
switching process. The setup can be viewed as that the grain is connected to
two non-magnetic leads via sequential tunneling. In one lead, the chemical
potentials for spin-up and -down electrons are split due to the spin injection
in the NLSV. This splitting (or the spin bias) is crucial to the NLSV signal
and the critical condition to the magnetization switching. By using the
standard spin diffusion equation and parameters from recent NLSV device, the
magnitude of the spin bias is estimated, and found large enough to drive the
magnetization switching of the cobalt nanograin reported in earlier
experiments. A microscopic interpretation of NLSV signal in the sequential
tunneling regime is thereby raised, which show properties due to the ultrasmall
size of the grain. The dynamics at the reversal point shows that there may be a
spin-polarized current instead of the anticipated pure spin current flowing
during the reversal due to the electron accumulation in the floating lead used
for the readout of NLSV signal.Comment: 15 pages, 11 figure
Quantum impurity in the bulk of topological insulator
We investigate physical properties of an Anderson impurity embedded in the
bulk of a topological insulator. The slave-boson mean-field approximation is
used to account for the strong electron correlation at the impurity. Different
from the results of a quantum impurity on the surface of a topological
insulator, we find for the band-inverted case, a Kondo resonant peak and in-gap
bound states can be produced simultaneously. However, only one type of them
appears for the normal case. It is shown that the mixed-valence regime is much
broader in the band-inverted case, while it shrinks to a very narrow regime in
the normal case. Furthermore, a self-screening of the Kondo effect may appear
when the interaction between the bound-state spin and impurity spin is taken
into account.Comment: 11 pages, 8 figure
Quantum Transport in Magnetic Topological Insulator Thin Films
The experimental observation of the long-sought quantum anomalous Hall effect
was recently reported in magnetically doped topological insulator thin films
[Chang et al., Science 340, 167 (2013)]. An intriguing observation is a rapid
decrease from the quantized plateau in the Hall conductance, accompanied by a
peak in the longitudinal conductance as a function of the gate voltage. Here,
we present a quantum transport theory with an effective model for magnetic
topological insulator thin films. The good agreement between theory and
experiment reveals that the measured transport originates from a topologically
nontrivial conduction band which, near its band edge, has concentrated Berry
curvature and a local maximum in group velocity. The indispensable roles of the
broken structure inversion and particle-hole symmetries are also revealed. The
results are instructive for future experiments and transport studies based on
first-principles calculations.Comment: 5 pages, 4 figure
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