K Wu

YBaCuO superconducting thin films with zero resistance at 84 K by multilayer deposition

YBaCuO superconducting thin films were prepared by multilayer deposition on different kinds of substrates. High Tc with an onset of 94 K and a zero resistance at 84 K has been obtained by deposition on pure ZrO2 substrates. The nature of zero resistance as well as the dR/dT characteristic above Tc and the influence of substrates are discussed.

HETEROJUNCTIONS OF SOLID C70 AND CRYSTALLINE SILICON : RECTIFYING PROPERTIES AND BARRIER HEIGHTS

Heterojunctions of solid C70 and n‐ or p‐type crystalline Si have been made. Current–voltage measurements show that both C70/n‐Si and C70/p‐Si contacts are rectifying but their directions of rectification are opposite to each other. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.23 eV for C70/n‐Si and 0.27 eV for C70/p‐Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high‐frequency capacitance–voltage characteristics for Ti/C70/p‐Si structures.

Heterojunctions of solid C60 and crystalline silicon: rectifying properties and energy-band models

Heterojunctions of undoped solid C60 and n- or p-type-doped crystalline Si have been obtained. Current-voltage measurements show that both C60/n-Si and C60/p-Si contacts are rectifying but their directions of rectification are opposite. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.30 eV for C60/n-Si and 0.48 eV for C60/p-Si. Using energy-band models for heterojunctions we assign values to the positions of the conduction and valence bands of the solid C60 relative to those of crystalline Si and derive the electron affinity and band gap of solid C60 film as 3.92 eV and <1.72 eV, respectively.

Electroluminescence from Au/(nanoscale Ge/nanoscale SiO2) superlattices/p-Si

Nanoscale Ge/nanoscale SiO2 superlattices (SLs) with four periods have been grown using the two-electron-beam alternation evaporating technique. Visible electroluminescence (EL) from the semitransparent Au film/(nanoscale Ge/nanoscale SiO2) SL/p-Si structures was observed when the forward bias exceeded 5 V, and their EL power efficiencies were significantly higher than that of a semitransparent Au film/nanoscale Ge particles embedded SiO2 film/p-Si structure. The effects of thicknesses of nanoscale Ge layers in the SLs and of annealing temperatures on the EL were studied. It is found that the intensity and position of the major EL peak being located in a range of 640-680 nm vary synchronously, while the EL shoulder around 520 nm remains unchanged in wavelength with increasing Ge layer thickness. The results strongly support the viewpoint that EL originates from the luminescence centers in the SiO2 layers

Surface enhanced electronic transport. A new method to probe the possible interactions between C60 and non-alkali metals

Abstract By measuring the resistance in situ during the deposition of C 60 on ultrathin metal layers in a UHV system, we find the resistance decreases sharply in most cases. This enhanced electronic transport in metal—C 60 multilayers (bilayers), may be related to charge transfer from the metal to C 60 and surface bonding effects, and hence can be used as a new possible approach for probing the interfacial interactions between C 60 and metals.

The bias-temperature effect in a rectifying Nb/C60/p-Si structure: evidence for mobile negative charges in the solid C60 film

Solid C60 film was grown on a p-type Si substrate and a rectifying Nb/C60/p-Si structure was prepared. Capacitance-voltage (C-V) measurements showed that for temperatures above 260 K the C-V curve of the Nb/Co60/p-Si structure shifted along the voltage axis depending on biasing conditions. We analysed this effect to reveal the existence of mobile negative charges in the C60 layer and determine the density of the mobile charges.

The scaling law of the pinning force and anomalous Hall effect in the high-Tc superconductors

A novel approach to evaluating the pinning force in superconductors is developed on the basis of the extended model of thermally activated depinning. Our method expresses in a unified manner the influence of temperature, magnetic field and current density upon the pinning force acting on the flux. It explains the resistive behaviour, the scaling law of pinning force and the anomalous Hall effect in the mixed state of the superconductors.

In situ electronic transport measurement as a tool for investigating the 2D doping in metal-C60 interfacial systems

The two-dimensional doping by charge transfer and/or diffusion of metal atoms into C60 has been studied by in situ electronic transport measurements during the deposition of metal-C60 ultrathin bilayers. The results show that the transport properties of these interfacial systems are significantly altered by such doping processes. Some useful information about the charge transfer from metal to C60 and the electronic transport properties of the metal-doped-monolayer C60 can be obtained after careful analysis.

Rubidium-doped epitaxial C60 thin films: synthesis and electronic transport

Epitaxial thin films of C60 have been synthesized and doped with rubidium. Above 80 K, the sample resistivity shows classic metallic temperature dependence just like that of K3C60 single crystals. However, the temperature coefficient changes sign at about 80 K, and the resistivity increases by 5% before the superconducting state near 30 K is reached. As for different samples, the zero-resistance temperature varies sharply with the sample lowest resistivity and can be fitted to a functional form of Tc0=a+b rho -5. The critical current density Jc is 103-104 A cm2 at approximately= 5 K, 0 T and it is found that Jc is proportional to (1-T/Tc)alpha , where alpha is in the range 1.3-2.0. Furthermore, the voltage-current characteristic of the films is very similar to that of high-Tc superconductors and remains a problem for further research on this new kind of material.