1,056 research outputs found
Random Scattering Matrices and the Circuit Theory of Andreev Conductances
The conductance of a normal-metal mesoscopic system in proximity to
superconducting electrode(s) is calculated. The normal-metal part may have a
general geometry, and is described as a ``circuit'' with ``leads'' and
``junctions''. The junctions are each ascribed a scattering matrix which is
averaged over the circular orthogonal ensemble, using recently-developed
techniques. The results for the electrical conductance reproduce and extend
Nazarov's circuit theory, thus bridging between the scattering and the bulk
approaches. The method is also applied to the heat conductance.Comment: 12 pages, RevTeX, including 2 figures with eps
Effective mass and tricritical point for lattice fermions localized by a random mass
This is a numerical study of quasiparticle localization in symmetry class
\textit{BD} (realized, for example, in chiral \textit{p}-wave superconductors),
by means of a staggered-fermion lattice model for two-dimensional Dirac
fermions with a random mass. For sufficiently weak disorder, the system size
dependence of the average (thermal) conductivity is well described by
an effective mass , dependent on the first two moments of the
random mass . The effective mass vanishes linearly when the average
mass , reproducing the known insulator-insulator phase boundary
with a scale invariant dimensionless conductivity and
critical exponent . For strong disorder a transition to a metallic phase
appears, with larger but the same . The intersection of the
metal-insulator and insulator-insulator phase boundaries is identified as a
\textit{repulsive} tricritical point.Comment: 6 pages, 9 figure
Temperature dependent third cumulant of tunneling noise
Poisson statistics predicts that the shot noise in a tunnel junction has a
temperature independent third cumulant e^2\I, determined solely by the mean
current I. Experimental data, however, show a puzzling temperature dependence.
We demonstrate theoretically that the third cumulant becomes strongly
temperature dependent and may even change sign as a result of feedback from the
electromagnetic environment. In the limit of a noninvasive (zero-impedance)
measurement circuit in thermal equilibrium with the junction, we find that the
third cumulant crosses over from e^2/I at low temperatures to -e^2/I at high
temperatures.Comment: 4 pages including 2 figure
Feedback of the electromagnetic environment on current and voltage fluctuations out of equilibrium
A theory is presented for low-frequency current and voltage correlators of a
mesoscopic conductor embedded in a macroscopic electromagnetic environment.
This Keldysh field theory evaluated at its saddle-point provides the
microscopic justification for our earlier phenomenological calculation (using
the cascaded Langevin approach). The nonlinear feedback from the environment
mixes correlators of different orders, which explains the unexpected
temperature dependence of the third moment of tunneling noise observed in a
recent experiment. At non-zero temperature, current and voltage correlators of
order three and higher are no longer linearly related. We show that a Hall bar
measures voltage correlators in the longitudinal voltage and current
correlators in the Hall voltage. We go beyond the saddle-point approximation to
consider the environmental Coulomb blockade. We derive that the leading order
Coulomb blockade correction to the n-th cumulant of current fluctuations is
proportional to the voltage derivative of the (n+1)-th cumulant, generalizing
to any n the earlier results for n=1,2.Comment: 12 pages, 8 figure
Manipulation of photon statistics of highly degenerate chaotic radiation
Highly degenerate chaotic radiation has a Gaussian density matrix and a large
occupation number of modes . If it is passed through a weakly transmitting
barrier, its counting statistics is close to Poissonian. We show that a second
identical barrier, in series with the first, drastically modifies the
statistics. The variance of the photocount is increased above the mean by a
factor times a numerical coefficient. The photocount distribution reaches a
limiting form with a Gaussian body and highly asymmetric tails. These are
general consequences of the combination of weak transmission and multiple
scattering.Comment: 4 pages, 2 figure
Superconductor-proximity effect in chaotic and integrable billiards
We explore the effects of the proximity to a superconductor on the level
density of a billiard for the two extreme cases that the classical motion in
the billiard is chaotic or integrable. In zero magnetic field and for a uniform
phase in the superconductor, a chaotic billiard has an excitation gap equal to
the Thouless energy. In contrast, an integrable (rectangular or circular)
billiard has a reduced density of states near the Fermi level, but no gap. We
present numerical calculations for both cases in support of our analytical
results. For the chaotic case, we calculate how the gap closes as a function of
magnetic field or phase difference.Comment: 4 pages, RevTeX, 2 Encapsulated Postscript figures. To be published
by Physica Scripta in the proceedings of the "17th Nordic Semiconductor
Meeting", held in Trondheim, June 199
Behavior of quantum entropies in polaronic systems
Quantum entropies and state distances are analyzed in polaronic systems with
short range (Holstein model) and long range (Frhlich model)
electron-phonon coupling. These quantities are extracted by a variational wave
function which describes very accurately polaron systems with arbitrary size in
all the relevant parameter regimes. With the use of quantum information tools,
the crossover region from weak to strong coupling regime can be characterized
with high precision. Then, the linear entropy is found to be very sensitive to
the range of the electron-phonon coupling and the adiabatic ratio. Finally, the
entanglement entropy is studied as a function of the system size pointing out
that it not bounded, but scales as the logarithm of the size either for weak
electron-phonon coupling or for short range interaction. This behavior is
ascribed to the peculiar coupling induced by the single electron itinerant
dynamics on the phonon subsystem.Comment: 4 figures, to be published in Phys. Rev.
Correspondence between Andreev reflection and Klein tunneling in bipolar graphene
Andreev reflection at a superconductor and Klein tunneling through an n-p
junction in graphene are two processes that couple electrons to holes -- the
former through the superconducting pair potential Delta and the latter through
the electrostatic potential U. We derive that the energy spectra in the two
systems are identical, at low energies E<<Delta and for an antisymmetric
potential profile U(-x,y)=-U(x,y). This correspondence implies that bipolar
junctions in graphene may have zero density of states at the Fermi level and
carry a current in equilibrium, analogously to superconducting Josephson
junctions. It also implies that nonelectronic systems with the same band
structure as graphene, such as honeycomb-lattice photonic crystals, can exhibit
pseudo-superconducting behavior.Comment: 7 pages, 7 figures; much expanded version, with a revised title, test
of the analytics by computer simulation, temperature dependence of the
persistent current, and an appendix with details of the calculatio
Medium/high field magnetoconductance in chaotic quantum dots
The magnetoconductance G in chaotic quantum dots at medium/high magnetic
fluxes Phi is calculated by means of a tight binding Hamiltonian on a square
lattice. Chaotic dots are simulated by introducing diagonal disorder on surface
sites of L x L clusters. It is shown that when the ratio W/L is sufficiently
large, W being the leads width, G increases steadily showing a maximum at a
flux Phi_max ~ W. Bulk disordered ballistic cavities (with an amount of
impurities proportional to L) does not show this effect. On the other hand, for
magnetic fluxes larger than that for which the cyclotron radius is of the order
of L/2, the average magnetoconductance inceases almost linearly with the flux
with a slope proportional to W^2, shows a maximum and then decreases stepwise.
These results closely follow a theory proposed by Beenakker and van Houten to
explain the magnetoconductance of two point contacts in series.Comment: RevTeX including six postscript figure
Bell's inequality test with time-delayed two-particle correlations
Adopting the frame of mesoscopic physics, we describe a Bell type experiment
involving time-delayed two-particle correlation measurements. The
indistinguishability of quantum particles results in a specific interference
between different trajectories. We show how the non-locality in the
time-delayed correlations due to the indistinguishability of the quantum
particles manifests itself in the violation of a Bell inequality, where the
degree of violation is related to the accuracy of the measurement. We
demonstrate how the interrelation between the orbital- and the spin exchange
symmetry can by exploited to infer knowledge on spin-entanglement from a
measurement of orbital entanglement.Comment: 8 pages, 4 figure
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