324 research outputs found
The phase-dependent linear conductance of a superconducting quantum point contact
The exact expression for the phase-dependent linear conductance of a weakly
damped superconducting quantum point contact is obtained. The calculation is
performed by summing up the complete perturbative series in the coupling
between the electrodes. The failure of any finite order perturbative expansion
in the limit of small voltage and small quasi-particle damping is analyzed in
detail. In the low transmission regime this nonperturbative calculation yields
a result which is at variance with standard tunnel theory. Our result predicts
the correct sign of the quasi-particle pair interference term and exhibits an
unusual phase-dependence at low temperatures in qualitative agreement with the
available experimental data.Comment: 12 pages (revtex) + 1 postscript figure. Submitted to Phys. Rev. Let
Non-equilibrium dynamics of Andreev states in the Kondo regime
The transport properties of a quantum dot coupled to superconducting leads
are analyzed. It is shown that the quasiparticle current in the Kondo regime is
determined by the non-equilibrium dynamics of subgap states (Andreev states)
under an applied voltage. The current at low bias is suppressed exponentially
for decreasing Kondo temperature in agreement with recent experiments. We also
predict novel interference effects due to multiple Landau-Zener transitions
between Andreev states.Comment: Revtex4, 4 pages, 4 figure
Conductance properties of nanotubes coupled to superconducting leads: signatures of Andreev states dynamics
We present a combined experimental and theoretical analysis of the low bias
conductance properties of carbon nanotubes coupled to superconducting leads. In
the Kondo regime the conductance exhibits a zero bias peak which can be several
times larger than the unitary limit in the normal case. This zero bias peak can
be understood by analyzing the dynamics of the subgap Andreev states under an
applied bias voltage. It is shown that the existence of a linear regime is
linked to the presence of a finite relaxation rate within the system. The
theory provides a good fitting of the experimental results.Comment: 6 revtex4 pages, 6 figures, to appear in SS
An embedding potential definition of channel functions
We show that the imaginary part of the embedding potential, a generalised
logarithmic derivative, defined over the interface between an electrical lead
and some conductor, has orthogonal eigenfunctions which define conduction
channels into and out of the lead. In the case of an infinitely extended
interface we establish the relationship between these eigenfunctions and the
Bloch states evaluated over the interface. Using the new channel functions, a
well-known result for the total transmission through the conductor system is
simply derived.Comment: 14 pages, 2 figure
Controlled dephasing of Andreev states in superconducting quantum point-contacts
We have studied the relaxation and dephasing processes in a superconducting
quantum point contact induced by the interaction with an electromagnetic
environment. Based on a density matrix approach we obtain the rates for the
dissipative dynamics as function of the transmission, the phase difference on
the contact and the external impedance. Our calculation allows to determine the
appropriate range of parameters for the observation of coherent oscillations in
the current through the contact.Comment: 8 pages, 2 figures. To appear in Physical Review
Non-local electron transport and cross-resistance peak in NSN heterostructures
We develop a microscopic theory describing the peak in the temperature
dependence of the non-local resistance of three-terminal NSN devices. This peak
emerges at sufficiently high temperatures as a result of a competition between
quasiparticle/charge imbalance and subgap (Andreev) contributions to the
conductance matrix. Both the height and the shape of this peak demonstrate the
power law dependence on the superconductor thickness in contrast to the
zero-temperature non-local resistance which decays (roughly) exponentially with
increasing . A similar behavior was observed in recent experiments.Comment: 4 pages, 3 figure
Proximity induced time-reversal topological superconductivity in Bi2Se3 films without phase tuning
Many proposals to generate a time-reversal invariant topological superconducting phase are based on imposing a π phase difference between the superconducting leads proximitizing a nanostructure. We show that this phase can be induced on a thin film of a topological insulator like Bi2Se3 in proximity to a single s-wave superconductor. In our analysis we take into account the parity degree of freedom of the electronic states which is not included in effective Dirac-like surface theories. We find that the topological phase can be reached when the induced interparity pairing dominates over the intraparity one. Application of an electric field perpendicular to the film extends the range of parameters where the topological phase occurs.Fil: Casas, Oscar E.. Universidad Nacional de Colombia; Colombia. Universidad Autónoma de Madrid; EspañaFil: Arrachea, Liliana del Carmen. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín. Escuela de Ciencia y Tecnología; ArgentinaFil: Herrera, William J.. Universidad Nacional de Colombia; ColombiaFil: Levy Yeyati, Alfredo. Universidad Autónoma de Madrid; Españ
Microscopic origin of the conducting channels in metallic atomic-size contacts
We present a theoretical approach which allows to determine the number and
orbital character of the conducting channels in metallic atomic contacts. We
show how the conducting channels arise from the atomic orbitals having a
significant contribution to the bands around the Fermi level. Our theory
predicts that the number of conducting channels with non negligible
transmission is 3 for Al and 5 for Nb one-atom contacts, in agreement with
recent experiments. These results are shown to be robust with respect to
disorder. The experimental values of the channels transmissions lie within the
calculated distributions.Comment: 11 pages, 4 ps-figures. Submitted to Phys. Rev. Let
Real space finite difference method for conductance calculations
We present a general method for calculating coherent electronic transport in
quantum wires and tunnel junctions. It is based upon a real space high order
finite difference representation of the single particle Hamiltonian and wave
functions. Landauer's formula is used to express the conductance as a
scattering problem. Dividing space into a scattering region and left and right
ideal electrode regions, this problem is solved by wave function matching (WFM)
in the boundary zones connecting these regions. The method is tested on a model
tunnel junction and applied to sodium atomic wires. In particular, we show that
using a high order finite difference approximation of the kinetic energy
operator leads to a high accuracy at moderate computational costs.Comment: 13 pages, 10 figure
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