Proximity and Josephson effects in superconductor - two dimensional electron gas planar junctions

The DC Josephson effect is theoretically studied in a planar junction in which a two dimensional electron gas (2DEG) infinite in lateral directions is in contact with two superconducting electrodes placed on top of the 2DEG. An energy gap in the excitation spectrum is created in the 2DEG due to the proximity effect. It is shown that under certain conditions, the region of the 2DEG underneath the superconductors is analogous to a superconducting region with an order parameter $\varepsilon_g\exp(i\phi)$\/, where $\varepsilon_g~(\varepsilon_g<\Delta)$\/ depends on the interface transmittance and the Fermi velocity mismatch between the superconductors and the 2DEG.Comment: 9 pages REVTeX, 5 figures available on reques

Experimental determination of the quasi-particle decay length ξSm\xi_{\text{Sm}} in a superconducting quantum well

We have investigated experimentally the electronic transport properties of a two-dimensional electron gas (2DEG) present in an AlSb/InAs/AlSb quantum well, where part of the toplayer has been replaced by a superconducting Nb strip, with an energy gap $\Delta_0$. By measuring the lateral electronic transport underneath the superconductor, and comparing the experimental results with a model based on the Bogoliubov-de Gennes equation and the Landauer-B\"uttiker formalism, we obtain a decay length $\xi_{\text{Sm}} \approx 100~\text{nm}$ for electrons. This decay length corresponds to an interface transparency $T_{\text{SIN}}=0.7$ between the Nb and InAs. Using this value, we infer an energy gap in the excitation spectrum of the SQW of $\Delta_{\text{eff}} = 0.97 \Delta_0 = 0.83~\text{meV}$.Comment: Revtex, 3 PostScript figure

Extraordinary Temperature Dependence of the Resonant Andreev Reflection

An extraordinary temperature dependence of the resonant Andreev reflection via discrete energy level in a normal-metal / quantum-dot / superconductor (N-QD-S) system is predicted theoretically by using Green function technique. The width of zero bias conductance peak in N-QD-S is about $\sqrt{\Gamma _L^2+\Gamma_R^2}$ and does not exhibit thermal broadening, where $\Gamma_L$ and $\Gamma_R$ are the coupling strength between QD and leads. Considering the intra-dot Coulomb interaction, the Coulomb blockade oscillations conducted by Andreev reflection differs dramatically from that in N-QD-N. Instead of thermal broadening, finite temperature induces more resonant peaks around the oscillation peaks of zero temperature. This effect can be applied to determine the coupling strength and QD level spacing in N-QD-S.Comment: 11 pages, 3 figures, LaTe

Nonlinearity in NS transport: scattering matrix approach

A general formula for the current through a disordered normal--superconducting junction is derived, which is valid at finite temperature and includes the full voltage dependence. The result depends on a multichannel scattering matrix, which describes elastic scattering in the normal region, and accounts for the Andreev scattering at the NS interface. The symmetry of the current with respect to sign reversal in the subgap regime is discussed. The Andreev approximation is used to derive a spectral conductance formula, which applies to voltages both below and above the gap. In a case study the spectral conductance formula is applied to the problem of an NINIS double barrier junction.Comment: 26 pages, 4 Postscript figures, Latex, to be published in Phys. Rev.

Mesoscopic proximity effect in double barrier Superconductor/Normal Metal junctions

We report transport measurements down to T=60mK of SININ and SNIN structures in the diffusive limit. We fabricated Al-AlOx/Cu/AlOx/Cu (SININ) and Al/Cu/AlOx/Cu (SNIN) vertical junctions. For the first time, a zero bias anomaly was observed in a metallic SININ structure. We attribute this peak of conductance to coherent multi-reflections of electrons between the two tunnel barriers. This conductance maximum is quantitatively fitted by the relevant theory of mesoscopic SININ structures. When the barrier at the SN interface is removed (SNIN structure), we observe a peak of conductance at finite voltage accompagnied by an excess of sub-gap conductance.Comment: 4 pages, 4 figures, editorially approved for publication in Phys. Rev. B Rapid Com

Evidence of two-electron tunneling interference in Nb/InAs junctions

The impact of junction transparency in driving phase-coherent charge transfer across diffusive semiconductor-superconductor junctions is demonstrated. We present conductivity data for a set of Nb-InAs junctions differing only in interface transparency. Our experimental findings are analyzed within the quasi-classical Green-function approach and unambiguously show the physical processes giving rise to the observed excess zero-bias conductivity.Comment: 10 pages (RevTex), 4 figures (PostScript), accepted for pubblication in Physical Review

Effect of transport-induced charge inhomogeneity on point-contact Andreev reflection spectra at ferromagnet-superconductor interfaces

We investigate the transport properties of a ferromagnet-superconductor interface within the framework of a modified three-dimensional Blonder-Tinkham-Klapwijk formalism. In particular, we propose that charge inhomogeneity forms via two unique transport mechanisms, namely, evanescent Andreev reflection and evanescent quasiparticle transmission. Furthermore, we take into account the influence of charge inhomogeneity on the interfacial barrier potential and calculate the conductance as a function of bias voltage. Point-contact Andreev reflection (PCAR) spectra often show dip structures, large zero-bias conductance enhancement, and additional zero-bias conductance peak. Our results indicate that transport-induced charge inhomogeneity could be a source of all these anomalous characteristics of the PCAR spectra.Comment: 9 pages, 6 figure