1,101 research outputs found
Helical liquids and Majorana bound states in quantum wires
We show that the combination of spin-orbit coupling with a Zeeman field or
strong interactions may lead to the formation of a helical liquid in
single-channel quantum wires. In a helical liquid, electrons with opposite
velocities have opposite spin precession. We argue that zero-energy Majorana
bound states are formed in various situations when the wire is situated in
proximity to a conventional s-wave superconductor. This occurs when the
external magnetic field, the superconducting gap, or, in particular, the
chemical potential vary along the wire. We discuss experimental consequences of
the formation of the helical liquid and the Majorana bound states.Comment: 4+epsilon page
Quantum Transport through Nanostructures with Orbital Degeneracies
Geometric symmetries cause orbital degeneracies in a molecule's spectrum. In
a single-molecule junction, these degeneracies are lifted by various
symmetry-breaking effects. We study quantum transport through such
nanostructures with an almost degenerate spectrum. We show that the master
equation for the reduced density matrix must be derived within the
singular-coupling limit as opposed to the conventional weak-coupling limit.
This results in signatures of the density matrix's off-diagonal elements in the
transport characteristics
Striped quantum Hall phases
Recent experiments seem to confirm predictions that interactions lead to
charge density wave ground states in higher Landau levels. These new
``correlated'' ground states of the quantum Hall system manifest themselves for
example in a strongly anisotropic resistivity tensor. We give a brief
introduction and overview of this new and emerging field.Comment: 10 pages, 1 figure, updated reference to experimental wor
Fractional Shot Noise in the Kondo Regime
Low temperature transport through a quantum dot in the Kondo regime proceeds
by a universal combination of elastic and inelastic processes, as dictated by
the low-energy Fermi-liquid fixed point. We show that as a result of inelastic
processes, the charge detected by a shot-noise experiment is enhanced relative
to the noninteracting situation to a universal fractional value, .
Thus, shot noise reveals that the Kondo effect involves many-body features even
at low energies, despite its Fermi-liquid nature. We discuss the influence of
symmetry breaking perturbations.Comment: 4 pages, 2 figure
Topological superconducting phases in disordered quantum wires with strong spin-orbit coupling
Zeeman fields can drive semiconductor quantum wires with strong spin-orbit
coupling and in proximity to s-wave superconductors into a topological phase
which supports end Majorana fermions and offers an attractive platform for
realizing topological quantum information processing. Here, we investigate how
potential disorder affects the topological phase by a combination of analytical
and numerical approaches. Most prominently, we find that the robustness of the
topological phase against disorder depends sensitively and non-monotonously on
the Zeeman field applied to the wire.Comment: 6 pages, 3 figures; published versio
Level Statistics and Localization for Two Interacting Particles in a Random Potential
We consider two particles with a local interaction in a random potential
at a scale (the one particle localization length). A simplified
description is provided by a Gaussian matrix ensemble with a preferential
basis. We define the symmetry breaking parameter
associated to the statistical invariance under change of basis. We show that
the Wigner-Dyson rigidity of the energy levels is maintained up to an energy
. We find that when (the
inverse lifetime of the states of the preferential basis) is smaller than
(the level spacing), and when . This implies that the two-particle localization length first
increases as before eventually behaving as .Comment: 4 pages REVTEX, 4 Figures EPS, UUENCODE
Thermopower of Single-Molecule Devices
We investigate the thermopower of single molecules weakly coupled to metallic
leads. We model the molecule in terms of the relevant electronic orbitals
coupled to phonons corresponding to both internal vibrations and to
oscillations of the molecule as a whole. The thermopower is computed by means
of rate equations including both sequential-tunneling and cotunneling
processes. Under certain conditions, the thermopower allows one to access the
electronic and phononic excitation spectrum of the molecule in a
linear-response measurement. In particular, we find that the phonon features
are more pronounced for weak lead-molecule coupling. This way of measuring the
excitation spectrum is less invasive than the more conventional current-voltage
characteristic, which, by contrast, probes the system far from equilibrium.Comment: 13 pages, 7 figures included; minor changes, version published in PR
Theory of the Franck-Condon blockade regime
Strong coupling of electronic and vibrational degrees of freedom entails a
low-bias suppression of the current through single-molecule devices, termed
Franck-Condon blockade. In the limit of slow vibrational relaxation, transport
in the Franck-Condon-blockade regime proceeds via avalanches of large numbers
of electrons, which are interrupted by long waiting times without electron
transfer. The avalanches consist of smaller avalanches, leading to a
self-similar hierarchy which terminates once the number of transferred
electrons per avalanche becomes of the order of unity. Experimental signatures
of self-similar avalanche transport are strongly enhanced current (shot) noise,
as expressed by giant Fano factors, and a power-law noise spectrum. We develop
a theory of the Franck-Condon-blockade regime with particular emphasis on
effects of electron cotunneling through highly excited vibrational states. As
opposed to the exponential suppression of sequential tunneling rates for
low-lying vibrational states, cotunneling rates suffer only a power-law
suppression. This leads to a regime where cotunneling dominates the current for
any gate voltage. Including cotunneling within a rate-equation approach to
transport, we find that both the Franck-Condon blockade and self-similar
avalanche transport remain intact in this regime. We predict that cotunneling
leads to absorption-induced vibrational sidebands in the Coulomb-blockaded
regime as well as intrinsic telegraph noise near the charge degeneracy point.Comment: 20 pages, 10 figures; minor changes, version published in Phys. Rev.
- …