107 research outputs found
Interpolative method for transport properties of quantum dots in the Kondo regime
We present an interpolative method for describing coherent transport through
an interacting quantum dot. The idea of the method is to construct an
approximate electron self-energy which becomes exact both in the limits of weak
and strong coupling to the leads. The validity of the approximation is first
checked for the case of a single (spin-degenerate) dot level. A generalization
to the multilevel case is then discussed. We present results both for the
density of states and the temperature dependent linear conductance showing the
transition from the Kondo to the Coulomb blockade regime.Comment: 8 pages, 3 figures, includes lamuphys.sty, submitted to the
Proceedings of the XVI Sitges Conference on Statistical Mechanic
Influence of spin filtering and spin mixing on the subgap structure of I-V characteristics in superconducting quantum point contact
The effect of spin filtering and spin mixing on the dc electric current for
voltage biased magnetic quantum point contact with superconducting leads is
theoretically studied. The I-V characteristics are calculated for the whole
range of spin filtering and spin mixing parameters. It is found that with
increasing of spin filtering the subharmonic step structure of the dc electric
current, typical for low-transparency junction and junction without
considerable spin filtering qualitatively changes. In the lower voltage region
and for small enough spin mixing the peak structure arises. When spin mixing
increases the peak subgap structure evolves to the step structure. The voltages
where subharmonic gap features are located are found to be sensitive to the
value of spin filtering. The positions of peaks and steps are calculated
analytically and the evolution of the subgap structure from well-known tunnel
limit to the large spin filtering case is explained in terms of multiple
Andreev reflection (MAR) processes. In particular, it is found that for large
spin filtering the subgap feature at arises from and
order MAR processes, while in the tunnel limit the step at
is known to result from order MAR process.Comment: 9 pages, 3 figure
General transport properties of superconducting quantum point contacts: a Green functions approach
We discuss the general transport properties of superconducting quantum point
contacts. We show how these properties can be obtained from a microscopic model
using nonequilibrium Green function techniques. For the case of a one-channel
contact we analyze the response under different biasing conditions: constant
applied voltage, current bias and microwave-induced transport. Current
fluctuations are also analyzed with particular emphasis on thermal and
shot-noise. Finally, the case of superconducting transport through a resonant
level is discussed. The calculated properties show a remarkable agreement with
the available experimental data from atomic-size contacts measurements. We
suggest the possibility of extending this comparison to several other
predictions of the theory.Comment: 10 pages, revtex, 8 figures, submitted to a special issue of
Superlattices and Microstructure
Temperature dependence of Andreev spectra in a superconducting carbon nanotube quantum dot
Tunneling spectroscopy of a Nb coupled carbon nanotube quantum dot reveals
the formation of pairs of Andreev bound states (ABS) within the superconducting
gap. A weak replica of the lower ABS is found, which is generated by
quasi-particle tunnelling from the ABS to the Al tunnel probe. An inversion of
the ABS-dispersion is observed at elevated temperatures, which signals the
thermal occupation of the upper ABS. Our experimental findings are well
supported by model calculations based on the superconducting Anderson model.Comment: 6 pages, 7 figure
Long range crossed Andreev reflections in high Tc superconductors
We analyze the non-local transport properties of a d-wave superconductor
coupled to metallic electrodes at nanoscale distances. We show that the
non-local conductance exhibits an algebraical decay with distance rather than
the exponential behavior which is found in conventional superconductors.
Crossed Andreev processes, associated with electronic entanglement, are favored
for certain orientations of the symmetry axes of the superconductor with
respect to the leads. These properties would allow its experimental detection
using present technologies.Comment: 5 pages, 5 figure
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