Transport through a 2DEG channel with superconducting boundaries

Journal of Physics: Conference Series, 2010

We present results on the microscopic dynamics of electrons in nanoscale systems coupled to superconducting leads. By solving the time-dependent Bogoliubov-deGennes equations for the Nambu-Gorkov Keldysh Green's function we are able to calculate the current and the charge density when the system is perturbed by bias voltages. For scattering of electrons across a normal-superconducting surface we provide a time-dependent picture of the Andreev reflections. When the nanoscale system is contacted to two DC biased superconducting leads the amplitude of the current oscillations at even multiple of the bias can be extracted by Fourier transforming the time-dependent results. In the transient regime the dwelling time is inversely proportional to the bias in agreement with the occurrence of multiple Andreev reflections.

Physica C: Superconductivity, 2001

Experimental and theoretical results of quantum transport in superconductor±semiconductor junctions are summarized. We will ®rst show the experimental results of reentrant behavior of the conductance as well as of giant Andreev backscattering. Then Andreev re¯ection in the quantum Hall regime is described. The charging eect on the proximity correction is ®nally discussed.

Physical review letters, 1996

We have investigated transport in a cross-shaped two-dimensional electron gas with superconducting electrodes coupled to two opposite arms. Multiterminal resistances, measured as a function of the superconducting phase difference and the magnetic flux, are analyzed in terms of an extended Landauer-Büttiker transport formalism. We show that extended reciprocity relations hold. Correlations between transport coefficients are obtained from, e.g., (negative) three-terminal and nonlocal resistances. Energy spectroscopy reveals a reentrant behavior of the transport coefficients around the Thouless energy.

Physical Review B

We present numerical results for the phase-periodic conductance of an Andreev interferometer and predict the existence of a voltage-induced crossover from a zero-phase minimum to a zero-phase maximum. This contrasts with a recent analysis of Stoof and Nazarov and Volkov, Allsopp, and Lambert, which predicts a vanishing amplitude of oscillation at zero temperature and voltage, respectively, and demonstrates that such behavior is nonuniversal. ͓S0163-1829͑97͒03526-1͔

Physica B: Condensed …, 2009

In this work we analyze the Tomasch effect in double barrier insulating superconducting N 1 ISIN 2 (N: normal metal, I: insulator and S: superconductor) junctions. From the solution of the Bogoliubov-de Gennes equations we find that the differential conductance presents resonances when the applied voltage changes. These resonances are originated by the formation of quasibound states in the superconducting region and depend on the symmetry of the pair potential. We develop an analytical model in order to find the quasibound states energies and its lifetimes. This model allows us to calculate the voltage at which each resonance appears and the resonance widths. We calculate and analyze the dependence of the transmission coefficients with the thickness of the superconducting layer.

Nature Physics, 2012

Conventional superconductivity is incompatible with ferromagnetism, because the magnetic exchange field tends to spinpolarize electrons and breaks apart the opposite-spin singlet Cooper pairs 1 . Yet, the possibility of a long-range penetration of superconducting correlations into strong ferromagnets has been evinced by experiments that found Josephson coupling between superconducting electrodes separated afar by a ferromagnetic spacer 2-7 . This is considered a proof of the emergence at the superconductor/ferromagnetic (S/F) interfaces of equalspin triplet pairing, which is immune to the exchange field and can therefore propagate over long distances into the F (ref. 8). This effect bears much fundamental interest and potential for spintronic applications 9 . However, a spectroscopic signature of the underlying microscopic mechanisms has remained elusive. Here we do show this type of evidence, notably in a S/F system for which the possible appearance of equal-spin triplet pairing is controversial 10-12 : heterostructures that combine a half-metallic F (La 0.7 Ca 0.3 MnO 3 ) with a d-wave S (YBa 2 Cu 3 O 7 ). We found quasiparticle and electron interference effects in the conductance across the S/F interfaces that directly demonstrate the long-range propagation across La 0.7 Ca 0.3 MnO 3 of superconducting correlations, and imply the occurrence of unconventional equal-spin Andreev reflection. This allows for an understanding of the unusual proximity behaviour observed in this type of heterostructures 12,13 .

Superlattices and Microstructures, 1999

Physica B: Condensed Matter, 2000

Atomic quantum point contacts accommodate a small number of conduction channels. Their number N and transmission coefficients {Tn} 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 {Tn} 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.

Physical Review B, 1998

Peaks associated to multiple Andreev reflections have been used to identify the number of conduction channels, and their transmitivity, in mesoscopic superconducting junctions. We analyze the influence of the detailed shape of each channel, and the number of weak links within it, on the final results. Connexions with the statistical interpretation of conductance histograms is also made.

EPL (Europhysics Letters), 2010

PACS 72.10.-d-Theory of electronic transport; scattering mechanisms PACS 73.23.-b-Electronic transport in mesoscopic systems PACS 73.63.-b-Electronic transport in nanoscale materials and structures PACS 74.50.+r-Tunneling phenomena; point contacts, weak links, Josephson effects Abstract.-The conductance in two-dimensional (2D) normal-superconducting (NS) systems is analyzed in the limit of strong magnetic fields when the transport is mediated by the electronhole states bound to the sample edges and NS interface, i.e., in the Integer Quantum Hall Effect regime. The Andreev-type process of the conversion of the quasiparticle current into the superflow is shown to be strongly affected by the mixing of the edge states localized at the NS and insulating boundaries. The magnetoconductance in 2D NS structures is calculated for both quadratic and Dirac-like normal state spectra. Assuming a random scattering of the edge modes we analyze both the average value and fluctuations of conductance for an arbitrary number of conducting channels.

EPL (Europhysics Letters), 2009

PACS 73.23.-b -Electronic transport in mesoscopic systems PACS 74.45.+c -Proximity effects; Andreev effect; SN and SNS junctions PACS 72.25.Ba -Spin polarized transport in metals

Microelectronics Journal, 2008

Magneto-fluctuations of the normal resistance R N have been reproducibly observed in YBa 2 Cu 3 O 7-d biepitaxial grain boundary junctions at low temperatures. We attribute them to mesoscopic transport in narrow channels across the grain boundary line, occurring even in the presence of large voltage drops. The Thouless energy appears to be the relevant energy scale. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed.

Nanoscale Science and Technology, 1998

The charging energy is important for a mesoscopic body with small capacitance to the environment. Connecting the small island via low capacitance, high resistance tunnel junctions, the current through the resulting structure will depend upon the charge of the island -charge that can be applied via a gate electrode. For a normal metal island, the current varies periodically with the charge, the periodicity is the electron charge e. If the island is superconducting, the period, e or 2e, depends upon whether the superconducting energy gap is less than or larger than the charging energy. This parity effect can be used to study possible nodes in the gap function of a high-T c superconductor.

Phys Rev B, 1997

We present experiments revealing a singularity in the coherent current across a superconductor/ semiconductor/superconductor ͑SSmS͒ junction at the bias voltage corresponding to the superconducting energy gap Vϭ⌬/e. The SSmS structure consists of highly doped GaAs with superconducting electrodes of aluminum configured as an interferometer. The phase-coherent component of the current is probed as the amplitude of h/2e vs magnetic-field oscillations in the differential resistance of the interferometer.