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, $E_c\ll \Delta$, 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 "$\pi$"-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

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 $I(V)$ 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