1,363 research outputs found
Diagrammatic method of integration over the unitary group, with applications to quantum transport in mesoscopic systems
A diagrammatic method is presented for averaging over the circular ensemble
of random-matrix theory. The method is applied to phase-coherent conduction
through a chaotic cavity (a ``quantum dot'') and through the interface between
a normal metal and a superconductor.Comment: 37 pages RevTeX, 21 postscript figures include
Spin and Conductance-Peak-Spacing Distributions in Large Quantum Dots: A Density Functional Theory Study
We use spin-density-functional theory to study the spacing between
conductance peaks and the ground-state spin of 2D model quantum dots with up to
200 electrons. Distributions for different ranges of electron number are
obtained in both symmetric and asymmetric potentials. The even/odd effect is
pronounced for small symmetric dots but vanishes for large asymmetric ones,
suggesting substantially stronger interaction effects than expected. The
fraction of high-spin ground states is remarkably large.Comment: 4 pages, 3 figure
How to realize a robust practical Majorana chain in a quantum dot-superconductor linear array
Semiconducting nanowires in proximity to superconductors are promising
experimental systems for Majorana fermions, which may ultimately be used as
building blocks for topological quantum computers. A serious challenge in the
experimental realization of the Majorana fermions is the supression of
topological superconductivity by disorder. We show that Majorana fermions
protected by a robust topological gap can occur at the ends of a chain of
quantum dots connected by s-wave superconductors. In the appropriate parameter
regime, we establish that the quantum dot/superconductor system is equivalent
to a 1D Kitaev chain, which can be tuned to be in a robust topological phase
with Majorana end modes even in the case where the quantum dots and
superconductors are both strongly disordered. Such a spin-orbit coupled quantum
dot - s-wave superconductor array provides an ideal experimental platform for
the observation of non-Abelian Majorana modes.Comment: 8 pages; 3 figures; version 2: Supplementary material updated to
include more general proof for localized Majorana fermion
Interactions and Disorder in Quantum Dots: Instabilities and Phase Transitions
Using a fermionic renormalization group approach we analyse a model where the
electrons diffusing on a quantum dot interact via Fermi-liquid interactions.
Describing the single-particle states by Random Matrix Theory, we find that
interactions can induce phase transitions (or crossovers for finite systems) to
regimes where fluctuations and collective effects dominate at low energies.
Implications for experiments and numerical work on quantum dots are discussed.Comment: 4 pages, 1 figure; version to appear in Phys Rev Letter
Acoustoelectric current and pumping in a ballistic quantum point contact
The acoustoelectric current induced by a surface acoustic wave (SAW) in a
ballistic quantum point contact is considered using a quantum approach. We find
that the current is of the "pumping" type and is not related to drag, i.e. to
the momentum transfer from the wave to the electron gas. At gate voltages
corresponding to the plateaus of the quantized conductance the current is
small. It is peaked at the conductance step voltages. The peak current
oscillates and decays with increasing SAW wavenumber for short wavelengths.
These results contradict previous calculations, based on the classical
Boltzmann equation.Comment: 4 pages, 1 figur
Andreev Conductance of Chaotic and Integrable Quantum Dots
We examine the voltage V and magnetic field B dependent Andreev conductance
of a chaotic quantum dot coupled via point contacts to a normal metal and a
superconductor. In the case where the contact to the superconductor dominates,
we find that the conductance is consistent with the dot itself behaving as a
superconductor-- it appears as though Andreev reflections are occurring locally
at the interface between the normal lead and the dot. This is contrasted
against the behaviour of an integrable dot, where for a similar strong coupling
to the superconductor, no such effect is seen. The voltage dependence of the
Andreev conductance thus provides an extremely pronounced quantum signature of
the nature of the dot's classical dynamics. For the chaotic dot, we also study
non-monotonic re-entrance effects which occur in both V and B.Comment: 13 pages, 9 figure
Griffiths effects and quantum critical points in dirty superconductors without spin-rotation invariance: One-dimensional examples
We introduce a strong-disorder renormalization group (RG) approach suitable
for investigating the quasiparticle excitations of disordered superconductors
in which the quasiparticle spin is not conserved. We analyze one-dimensional
models with this RG and with elementary transfer matrix methods. We find that
such models with broken spin rotation invariance {\it generically} lie in one
of two topologically distinct localized phases. Close enough to the critical
point separating the two phases, the system has a power-law divergent
low-energy density of states (with a non-universal continuously varying
power-law) in either phase, due to quantum Griffiths singularities. This
critical point belongs to the same infinite-disorder universality class as the
one dimensional particle-hole symmetric Anderson localization problem, while
the Griffiths phases in the vicinity of the transition are controlled by lines
of strong (but not infinite) disorder fixed points terminating in the critical
point.Comment: 14 pages (two-column PRB format), 9 eps figure
Floquet scattering in parametric electron pumps
A Floquet scattering approach to parametric electron pumps is presented and
compared with Brouwer's adiabatic scattering approach [Phys. Rev. B 58, R10135
(1998)] for a simple scattering model with two harmonically oscillating
delta-function barriers. For small strength of oscillating potentials these two
approaches give exactly equivalent results while for large strength, these
clearly deviate from each other. The validity of the adiabatic theory is also
discussed by using the Wigner delay time obtained from the Floquet scattering
matrix.Comment: 10 pages, 7 figure
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