1,922 research outputs found
Impurity-assisted Andreev reflection at a spin-active half-metal-superconductor interface
The Andreev reflection amplitude at a clean interface between a half-metallic
ferromagnet (H) and a superconductor (S) for which the half metal's
magnetization has a gradient perpendicular to the interface is proportional to
the excitation energy and vanishes at [B\'{e}ri
{\em et al.}, Phys.\ Rev.\ B {\bf 79}, 024517 (2009)]. Here we show that the
presence of impurities at or in the immediate vicinity of the HS interface
leads to a finite Andreev reflection amplitude at . This
impurity-assisted Andreev reflection dominates the low-bias conductance of a HS
junction and the Josephson current of an SHS junction in the long-junction
limit.Comment: 12 pages, 2 figure
Pumped current and voltage for an adiabatic quantum pump
We consider adiabatic pumping of electrons through a quantum dot. There are
two ways to operate the pump: to create a dc current or to create a
dc voltage . We demonstrate that, for very slow pumping,
and are not simply related via the dc conductance as . For the case of a chaotic quantum dot, we consider the statistical
distribution of . Results are presented for the limiting
cases of a dot with single channel and with multichannel point contacts.Comment: 6 pages, 4 figure
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
Distributions of the Conductance and its Parametric Derivatives in Quantum Dots
Full distributions of conductance through quantum dots with single-mode leads
are reported for both broken and unbroken time-reversal symmetry. Distributions
are nongaussian and agree well with random matrix theory calculations that
account for a finite dephasing time, , once broadening due to finite
temperature is also included. Full distributions of the derivatives of
conductance with respect to gate voltage are also investigated.Comment: 4 pages (REVTeX), 4 eps figure
Quantum mechanical time-delay matrix in chaotic scattering
We calculate the probability distribution of the matrix Q = -i \hbar S^{-1}
dS/dE for a chaotic system with scattering matrix S at energy E. The
eigenvalues \tau_j of Q are the so-called proper delay times, introduced by E.
P. Wigner and F. T. Smith to describe the time-dependence of a scattering
process. The distribution of the inverse delay times turns out to be given by
the Laguerre ensemble from random-matrix theory.Comment: 4 pages, RevTeX; to appear in Phys. Rev. Let
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
On hyperovals of polar spaces
We derive lower and upper bounds for the size of a hyperoval of a finite polar space of rank 3. We give a computer-free proof for the uniqueness, up to isomorphism, of the hyperoval of size 126 of H(5, 4) and prove that the near hexagon E-3 has up to isomorphism a unique full embedding into the dual polar space DH(5, 4)
The Thermopower of Quantum Chaos
The thermovoltage of a chaotic quantum dot is measured using a current
heating technique. The fluctuations in the thermopower as a function of
magnetic field and dot shape display a non-Gaussian distribution, in agreement
with simulations using Random Matrix Theory. We observe no contributions from
weak localization or short trajectories in the thermopower.Comment: 4 pages, 3 figures, corrected: accidently omitted author in the
Authors list, here (not in the article
Voltage-probe and imaginary potential models for dephasing in a chaotic quantum dot
We compare two widely used models for dephasing in a chaotic quantum dot: The
introduction of a fictitious voltage probe into the scattering matrix and the
addition of an imaginary potential to the Hamiltonian. We identify the limit in
which the two models are equivalent and compute the distribution of the
conductance in that limit. Our analysis explains why previous treatments of
dephasing gave different results. The distribution remains non-Gaussian for
strong dephasing if the coupling of the quantum dot to the electron reservoirs
is via ballistic single-mode point contacts, but becomes Gaussian if the
coupling is via tunneling contacts.Comment: 9 pages, RevTeX, 6 figures. Mistake in Eq. (35) correcte
Spontaneous Emission in Chaotic Cavities
The spontaneous emission rate \Gamma of a two-level atom inside a chaotic
cavity fluctuates strongly from one point to another because of fluctuations in
the local density of modes. For a cavity with perfectly conducting walls and an
opening containing N wavechannels, the distribution of \Gamma is given by
P(\Gamma) \propto \Gamma^{N/2-1}(\Gamma+\Gamma_0)^{-N-1}, where \Gamma_0 is the
free-space rate. For small N the most probable value of \Gamma is much smaller
than the mean value \Gamma_0.Comment: 4 pages, RevTeX, 1 figur
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