828 research outputs found
Transport through superconductor/magnetic dot/superconductor structures
The coupling of two s-wave superconductors through a small magnetic dot is
discussed. Assuming that the dot charging energy is small compared to the
superconducting gap, , and that the moment of the dot is
classical, we develop a simple theory of transport through the dot. The
presence of the magnetic dot will position Andreev bound states within the
superconducting gap at energies tunable with the magnetic properties of the
dot. Studying the Josephson coupling it is shown that the constructed junction
can be tuned from a "0" to a ""-junction via a degenerate two-level state
either by changing the magnetic moment of the dot or by changing temperature.
Furthermore, it is shown that details of the magnetic dot can be extracted from
the sub-harmonic structure in the current-voltage characteristics of the
junction.Comment: 5 pages, 4 figures, paper presented at the conference SDP 2001 in
Tokyo on June 2
Full Counting Statistics of Multiple Andreev Reflections in incoherent diffusive superconducting junctions
We present a theory for the full distribution of current fluctuations in
incoherent diffusive superconducting junctions, subjected to a voltage bias.
This theory of full counting statistics of incoherent multiple Andreev
reflections is valid for arbitrary applied voltage. We present a detailed
discussion of the properties of the first four cumulants as well as the low and
high voltage regimes of the full counting statistics. The work is an extension
of the results of Pilgram and the author, Phys. Rev. Lett. 94, 086806 (2005).Comment: Included in special issue Spin Physics of Superconducting
heterostructures of Applied Physics A: Materials Science & Processin
Density-functional study of tilt-angle and temperature-dependent conductance in biphenyl-dithiol single-molecule contacts
Single-channel transmission in gold one-atom contacts and chains
We induce superconductivity by proximity effect in thin layers of gold and
study the number of conduction channels which contribute to the current in
one-atom contacts and atomic wires. The atomic contacts and wires are
fabricated with a Scanning Tunneling Microscope. The set of transmission
probabilities of the conduction channels is obtained from the analysis of the
characteristic curve which is highly non-linear due to multiple Andreev
reflections. In agreement with theoretical calculations we find that there is
only one channel which is almost completely open.Comment: 4 pages, 2 figures. To be published in Phys. Rev. B, Rapid
Communications (2003
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
Conduction channels of superconducting quantum point contacts
Atomic quantum point contacts accommodate a small number of conduction
channels. Their number N and transmission coefficients {T_n} can be determined
by analyzing the subgap structure due to multiple Andreev reflections in the
current-voltage (IV) characteristics in the superconducting state. With the
help of mechanically controllable break-junctions we have produced Al contacts
consisting of a small number of atoms. In the smallest stable contacts, usually
three channels contribute to the transport. We show here that the channel
ensemble {T_n} of few atom contacts remains unchanged up to temperatures and
magnetic fields approaching the critical temperature and the critical field,
respectively, giving experimental evidence for the prediction that the
conduction channels are the same in the normal and in the superconducting
state.Comment: 8 pages, 5 .eps figures. To be published in Physica B 22
Subharmonic gap structure in d-wave superconductors
We present a self-consistent theory of current-voltage characteristics of
d-wave/d-wave contacts at arbitrary transparency. In particular, we address the
open problem of the observation of subharmonic gap structure (SGS) in cuprate
junctions. Our analysis shows that: (i) the SGS is possible in d-wave
superconductors, (ii) the existence of bound states within the gap results in
an even-odd effect in the SGS, (iii) elastic scattering mechanisms, like
impurities or surface roughness, may suppress the SGS, and (iv) in the presence
of a magnetic field the Doppler shift of the Andreev bound states leads to a
very peculiar splitting of the SGS, which is an unambiguous fingerprint of
d-wave superconductivity.Comment: Revtex4, 4 pages, 5 figure
Towards unified understanding of conductance of stretched monatomic contacts
When monatomic contacts are stretched, their conductance behaves in
qualitatively different ways depending on their constituent atomic elements.
Under a single assumption of resonance formation, we show that various
conductance behavior can be understood in a unified way in terms of the
response of the resonance to stretching. This analysis clarifies the crucial
roles played by the number of valence electrons, charge neutrality, and orbital
shapes.Comment: 2 figure
Quantum interference structures in the conductance plateaus of gold nanojunctions
The conductance of breaking metallic nanojunctions shows plateaus alternated
with sudden jumps, corresponding to the stretching of stable atomic
configurations and atomic rearrangements, respectively. We investigate the
structure of the conductance plateaus both by measuring the voltage dependence
of the plateaus' slope on individual junctions and by a detailed statistical
analysis on a large amount of contacts. Though the atomic discreteness of the
junction plays a fundamental role in the evolution of the conductance, we find
that the fine structure of the conductance plateaus is determined by quantum
interference phenomenon to a great extent.Comment: 4 pages, 4 figure
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