Ultrathin silicon membranes to study supercurrent transport in crystalline semiconductors

We have developed a two-step anisotropic etching process to fabricate thin silicon membranes, used to study supercurrent transport in semiconductor coupled weak links. The process uses a shallow BF2+ implantation, and permits easy control of membrane thickness less-than-or-equal-to 100 nm. Preliminary measurements on membrane-based Nb-Si-Nb junctions reveal the simultaneous occurrence of tunnel behavior and Josephson coupling

Subgap anomaly and above-energy-gap structure in chains of diffusive SNS junctions

We present the results of low-temperature transport measurements on chains of superconductor--normal-constriction--superconductor (SNS) junctions fabricated on the basis of superconducting PtSi film. A comparative study of the properties of the chains, consisting of 3 and 20 SNS junctions in series, and single SNS junctions reveals essential distinctions in the behavior of the current-voltage characteristics of the systems: (i) the gradual decrease of the effective suppression voltage for the excess conductivity observed at zero bias as the quantity of the SNS junctions increases, (ii) a rich fine structure on the dependences dV/dI-V at dc bias voltages higher than the superconducting gap and corresponding to some multiples of 2\Delta/e. A model to explain this above-energy-gap structure based on energy relaxation of electron via Cooper-pair-breaking in superconducting island connecting normal metal electrods is proposed.Comment: RevTex, 5 pages, 4 figure

Two-dimensional array of diffusive SNS junctions with high-transparent interfaces

We report the first comparative study of the properties of two-dimensional arrays and single superconducting film - normal wire - superconducting film (SNS) junctions. The NS interfaces of our SNS junctions are really high transparent, for superconducting and normal metal parts are made from the same material (superconducting polycrystalline PtSi film). We have found that the two-dimensional arrays reveal some novel features: (i) the significant narrowing of the zero bias anomaly (ZBA) in comparison with single SNS junctions, (ii) the appearance of subharmonic energy gap structure (SGS), with up to n=16 (eV=\pm 2\Delta/n), with some numbers being lost, (iii) the transition from 2D logarithmic weak localization behavior to metallic one. Our experiments show that coherent phenomena governed by the Andreev reflection are not only maintained over the macroscopic scale but manifest novel pronounced effects as well. The behavior of the ZBA and SGS in 2D array of SNS junctions strongly suggests that the development of a novel theoretical approach is needed which would self-consistently take into account the distribution of the currents, the potentials, and the superconducting order parameter.Comment: RevTex, 5 pages, 5 figure

Circuit theory of multiple Andreev reflections in diffusive SNS junctions: the incoherent case

The incoherent regime of Multiple Andreev Reflections (MAR) is studied in long diffusive SNS junctions at applied voltages larger than the Thouless energy. Incoherent MAR is treated as a transport problem in energy space by means of a circuit theory for an equivalent electrical network. The current through NS interfaces is explained in terms of diffusion flows of electrons and holes through tunnel and Andreev resistors. These resistors in diffusive junctions play roles analogous to the normal and Andreev reflection coefficients in OTBK theory for ballistic junctions. The theory is applied to the subharmonic gap structure (SGS); simple analytical results are obtained for the distribution function and current spectral density for the limiting cases of resistive and transparent NS interfaces. In the general case, the exact solution is found in terms of chain-fractions, and the current is calculated numerically. SGS shows qualitatively different behavior for even and odd subharmonic numbers, and the maximum slopes of the differential resistance correspond to the gap subharmonics. The influence of inelastic scattering on the subgap anomalies of the differential resistance is analyzed.Comment: 14 pages, 9 figures, title and text revised, to appear in PR

Electronic transport across niobium-silicon interfaces

A critical factor for current flow from a superconductor into a semiconductor is the transparency of the Schottky barrier at the interface. Theoretically, significant penetration of superconducting carriers into the semiconductor is not expected for any doping concentration. This is confirmed by measurements of the absence of a depression of the critical temperature of thin Nb films on doped Si. I - V characteristics of coplanar Nb - Si - Nb junctions are also presented. Results can be understood as back-to-back super-Schottky diode behaviour. At very high doping concentrations the onset of Josephson coupling is observed

Regional Brain Glucose Metabolism in Patients with Complex Partial Seizures Investigated by Intracranial Eeg

We performed interictal 18F-2-fluoro-2-deoxy-D-glucose positron emission tomography (18FDG-PET) studies in 57 patients with complex partial epilepsy (CPE), not controlled by medical treatment and considered for surgical resection of their epileptic focus. A precise localization of the epileptic focus was obtained in 37 of these patients with a combination of subdural and depth electrodes. We visually inspected the metabolic images; we also measured glucose consumption in a number of brain regions and compared the values with those obtained in 17 normal controls. Eighty-two percent of the 57 patients had an area of glucose hypometabolism on the 18FDG-PET images. Six patients had a frontal epileptic focus, 3 of them had a frontal lobe hypometabolism. Twenty-six patients had a unilateral temporal lobe focus and all of them displayed a temporal lobe hypometabolism. The asymmetry was more pronounced in the lateral temporal cortex (-20%) than in the mesial part of the temporal lobe (-9.6%). In each cortical brain region on the side of the epileptic focus (except the sensorimotor cortex), glucose consumption rate was lower than in the contralateral region or than in controls. No differences could be found between patients with a seizure onset restricted to the hippocampus and patients with a seizure onset involving the hippocampus and the adjacent neocortex. Divergent metabolic patterns were obtained in 5 patients with bilateral temporal seizure foci. Combined with other non invasive techniques (EEG, neuroradiology), PET contributes increasingly to the selection of patients with CPE who could benefit from surgical treatment.(ABSTRACT TRUNCATED AT 250 WORDS