Proximity effect in planar Superconductor/Semiconductor junction

We have measured the very low temperature (down to 30 mK) subgap resistance of Titanium Nitride (Superconductor, Tc = 4.6 K)/highly doped Silicon (Semiconductor) SIN junction (the insulating layer stands for the Schottky barrier). As the temperature is lowered, the resistance increases as expected in SIN junction. Around 300 mK, the resistance shows a maximum and decreases at lower temperature. This observed behavior is due to coherent backscattering towards the interface by disorder in Silicon ("Reflectionless tunneling"). This effect is also observed in the voltage dependence of the resistance (Zero Bias Anomaly) at low temperature (T<300 mK). The overall resistance behavior (in both its temperature and voltage dependence) is compared to existing theories and values for the depairing rate, the barrier resistance and the effective carrier temperature are extracted.Comment: Submitted to LT22, Helsinki - August 1999, phbauth.cls include

Reflectionless Tunneling Through a Double-Barrier NS Junction

The resistance is computed of an ${\rm NI}_{1}{\rm NI}_{2}{\rm S}$ junction, where N = normal metal, S = superconductor, and ${\rm I}_{i}$ = insulator or tunnel barrier (transmission probability per mode $\Gamma_{i}$). The ballistic case is considered, as well as the case that the region between the two barriers contains disorder (mean free path $l$, barrier separation $L$). It is found that the resistance at fixed $\Gamma_{2}$ shows a {\em minimum} as a function of $\Gamma_{1}$, when $\Gamma_{1}\approx\sqrt{2}\Gamma_{2}$, provided $l\gtrsim\Gamma_2 L$. The minimum is explained in terms of the appearance of transmission eigenvalues close to one, analogous to the ``reflectionless tunneling'' through a NIS junction with a disordered normal region. The theory is supported by numerical simulations. ***Submitted to Physica B.***Comment: 10 pages, REVTeX-3.0, 6 postscript figures appended as self-extracting archive, INLO-PUB-940607

Interference of two electrons entering a superconductor

The subgap conductivity of a normal-superconductor (NS) tunnel junction is thought to be due to tunneling of two electrons. There is a strong interference between these two electrons, originating from the spatial phase coherence in the normal metal at a mesoscopic length scale and the intrinsic coherence of the superconductor. We evaluated the interference effect on the transport through an NS junction. We propose the layouts to observe drastic Aharonov-Bohm and Josephson effects.Comment: 8 pages REVTex, [PostScript] figures upon reques

Theory of Andreev reflection in a junction with a strongly disordered semiconductor

We study the conduction of a {\sl N~-~Sm~-~S} junction, where {\sl Sm} is a strongly disordered semiconductor. The differential conductance $dI/dV$ of this {\sl N~-~Sm~-~S} structure is predicted to have a sharp peak at $V=0$. Unlike the case of a weakly disordered system, this feature persists even in the absence of an additional (Schottky) barrier on the boundary. The zero-bias conductance of such a junction $G_{NS}$ is smaller only by a numerical factor than the conductance in the normal state $G_N$. Implications for experiments on gated heterostructures with superconducting leads are discussed.Comment: 4 pages, 2 figures, to appear in Rapid Communication section of Phys. Rev.

Back gating of a two-dimensional hole gas in a SiGe quantum well

A device comprising a low-resistivity, n-type, Si substrate as a back gate to a p-type (boron), remote-doped, SiGe quantum well has been fabricated and characterized. Reverse and forward voltage biasing of the gate with respect to the two-dimensional hole gas in the quantum well allows the density of holes to be varied from 8 × 1011 cm–2 down to a measurement-limited value of 4 × 1011 cm–2. This device is used to demonstrate the evolution with decreasing carrier density of a re-entrant insulator state between the integer quantum Hall effect states with filling factors 1 and 3

Is the `Finite Bias Anomaly' in planar GaAs-Superconductor junctons caused by point-contact like structures?

We correlate transmission electron microscope (TEM) pictures of superconducting In contacts to an AlGaAs/GaAs heterojunction with differential conductance spectroscopy performed on the same heterojunction. Metals deposited onto a (100) AlGaAs/GaAs heterostructure do not form planar contacts but, during thermal annealing, grow down into the heterostructure along crystallographic planes in pyramid-like `point contacts'. Random surface nucleation and growth gives rise to a different interface transmission for each superconducting point contact. Samples annealed for different times, and therefore having different contact geometry, show variations in $dI/dV$ characteristic of ballistic transport of Cooper pairs, wave interference between different point emitters, and different types of weak localization corrections to Giaever tunneling. We give a possible mechanism whereby the `finite bias anomaly' of Poirier et al. (Phys. Rev. Lett., {\bf 79}, 2105 (1997)), also observed in these samples, can arise by adding the conductance of independent superconducting point emitters in parallel

Redetermination of bis­(O,O′-diethyl dithio­phosphato-κ2 S,S′)nickel(II)

The centrosymmetric title complex, [Ni{S2P(OC2H5)2}2], has been redetermined using area-detector data. The central Ni(S2P)2 core is essentially planar and confirms the early results of McConnell & Kastalsky [Acta Cryst. (1967), 22, 853–859] based on multiple film technique data. In the title structure, the standard uncertainty values are approximately seven times lower and all H-atom positions are calculated. A pair of short symmetry-related H⋯H contacts with distances of 2.33 Å is observed in the crystal structure

Semiconductor High-Energy Radiation Scintillation Detector

We propose a new scintillation-type detector in which high-energy radiation produces electron-hole pairs in a direct-gap semiconductor material that subsequently recombine producing infrared light to be registered by a photo-detector. The key issue is how to make the semiconductor essentially transparent to its own infrared light, so that photons generated deep inside the semiconductor could reach its surface without tangible attenuation. We discuss two ways to accomplish this, one based on doping the semiconductor with shallow impurities of one polarity type, preferably donors, the other by heterostructure bandgap engineering. The proposed semiconductor scintillator combines the best properties of currently existing radiation detectors and can be used for both simple radiation monitoring, like a Geiger counter, and for high-resolution spectrography of the high-energy radiation. The most important advantage of the proposed detector is its fast response time, about 1 ns, essentially limited only by the recombination time of minority carriers. Notably, the fast response comes without any degradation in brightness. When the scintillator is implemented in a qualified semiconductor material (such as InP or GaAs), the photo-detector and associated circuits can be epitaxially integrated on the scintillator slab and the structure can be stacked-up to achieve virtually any desired absorption capability

Subgap conductivity in SIN-junctions of high barrier transparency

We investigate the current-voltage characteristics of high-transparency superconductor-insulator-normal metal (SIN) junctions with the specific tunnel resistance below 30 kOhm per square micron. The junctions were fabricated from different superconducting and normal conducting materials, including Nb, Al, AuPd and Cu. The subgap leakage currents were found to be appreciably larger than those given by the standard tunnelling model. We explain our results using the model of two-electron tunnelling in the coherent diffusive transport regime. We demonstrate that even in the high-transparency SIN-junctions, a noticeable reduction of the subgap current can be achieved by splitting a junction into several submicron sub-junctions. These structures can be used as nonlinear low-noise shunts in Rapid-Single-Flux-Quantum (RSFQ) circuitry for controlling Josephson qubits.Comment: 6 pages, 5 figures, 1 tabl

Suppression and enhancement of the critical current in multiterminal S/N/S mesoscopic structures

We analyse the measured critical current $I_{m\text{}}$ in a mesoscopic 4-terminal S/N/S structure. The current through the S/N interface is shown to consist not only of the Josephson component $I_{c}\sin \phi ,$ but also a phase-coherent part $I_{sg}\cos \phi$ of the subgap current. The current $I_{m}$ is determined by the both components $I_{c}$ and $I_{sg},$ and depends in a nonmonotonic way on the voltage $V$ between superconductors and normal reservoirs reaching a maximum at $V\cong \Delta /e$. The obtained theoretical resultas are in qualitative agreement with recent experimental data.Comment: 4 page, 3 figures. To be puplished in PRB Rapid co