Leandro R. Tessler

Preparation and patterning of YBaCuO thin films obtained by sequential deposition of CuOx/Y2O3/BaF2

Superconducting thin films of YBaCuO were prepared by stacking layers of CuOx , Y2O3, and BaF2 onto substrates held at room temperature. The CuOx layer was obtained by oxidizing in air a previously deposited metallic Cu film. After deposition of the subsequent layers, the films were annealed at 880 °C in flowing oxygen. Samples 1 μm thick on sapphire showed an onset at Tonsetc =85 K and zero resistance at T0c =70 K; on yttria‐stabilized zirconia (YSZ), Tonsetc =90 K and T0c =80 K. Patterned films are easily obtained by applying conventional photolithographic methods to the Cu film.

Flux‐creep‐limited critical currents in YBa2Cu3O7−δ ceramics

Critical currents of YBa2Cu3O7−δ bulk‐sintered samples were measured at and above 77 K in the presence of magnetic fields. In moderate magnetic fields a H−n dependence with n<1 was observed. Transport measurements give n≂0.5, indicating that critical currents are limited by flux creep rather than by field quenching of Josephson junctions.

Low cost data acquisition system for transport characterization of high‐Tc superconductors

A low cost microcomputer‐based data‐acquisition system designed specifically to perform transport characterization of high‐Tc superconductors is presented. It can measure the temperature dependence of both the resistance and the critical current. It utilizes the four‐point ac in‐phase method to eliminate the effects of lead and contact resistance and thermal voltages. Some of the advantages of this method over dc methods are pointed out. The system is implemented on a PC (IBM or compatible) through the IEEE‐488 interface. The temperature is controlled by the system with resolution and accuracy of 0.1 and ±0.5 K, respectively, which are the limits of the platinum thermometer used. Voltages in the sample can be measured with a resolution better than 5 nV. The system can be easily modified to measure the critical current dependence on an applied magnetic field.

Contacts of very low resistivity to YBaCuO thin films

The authors report a method for the preparation of contacts of very low surface resistivity (contact resistance multiplied by contact area) to polycrystalline YBaCuO thin films. The method provides contacts with resistivities smaller than 3*10-7 Omega cm2, which are obtained by the application of silver paint dots onto the film surface prior to annealing. Only one annealing procedure is required for the preparation of samples and contacts. Above the critical current both the four-terminal and two-terminal resistances obeyed Ohms law for several current decades.

Hall voltage measurements using ac lock‐in detection

The measurement of the Hall constant in insulating highly resistive films is a difficult and challenging task. Owing to the small currents that may be passed through the insulating films, the Hall voltages are often very small in magnitude. A simple electronic circuit using commercial lock‐in amplifiers is described which allows measuring these minute Hall voltages. Other practical measuring suggestions are also presented.

Temperature dependent Hall coefficient in two-dimensional heavily doped p-type germanium

Abstract We performed accurate measurements of the Hall coefficient at low temperatures of two-dimensional polycrystalline thin films of heavily doped p-type germanium. We found a logarithmic dependence on the temperature, that can be associated with interaction effects in the particle-hole channel. Comparison with the variation of the resistance per square indicated that interaction is more important than localisation for the determination of the electronic properties of this material. The screening factor F was found to be 0.55±0.15, taking into account the strong spin-orbit coupling in p-type germanium. This value is lower than the one predicted by the free electron gas model.

Flux creep limited current in YBa2Cu3O7-† ceramics under applied fields

Abstract We present measurements of the field dependence of critical currents in YBa 2 Cu 2 O 7-† ceramics prepared by the sintering and by the sol-gel reaction methods. For high quality samples we find a region in magnetic fields where the transport critical current decreases like H −n with n=0.52±0.06. This behavior is qualitatively similar to that observed in single grains, interpreted as indicative of flux creep rather than Josephson limited critical current.

Experimental investigation of two-dimensional In Ge thin films

We report low temperature measurements of the resistivity of two-dimensional inhomogeneous In Ge thin films, prepared by electron beam co-deposition of the constituents onto a cold (77 K) substrate. Samples with a wide range of resistances (up to MΩ/□) were obtained. For In concentrations between 20 and 50 vol% a double superconducting transition is observed. Although the dependence of the resistivity on In concentration hints percolative behavior, the temperature dependence of the resistivity and the I-V curves can only be described by considering the vortex flow resistance and the Kosterlitz-Thouless transition

Hole Localization and Interaction in 2-D Heavily Doped p-Type Germanium

We have measured conductivity and magnetoconductivity as a function of temperature and magnetic field (T from 1.3 to 4.2 K, H from 0 to 1.25 T) on two-dimensional heavily doped p-type germanium thin films,. The films were semicontinuous, aluminum doped with carrier concentrations up to around 1020 holes/cm and sheet resistances from 350 to 1200 ohm/square at 4.2 K. The resistance depends on temperature as a In T in the range studied, with a between 0.81 and 1.50 e2/2π2\hbar. The magnetoconductivity at low fields and low temperatures is mainly due to localization, positive in the most heavily doped samples and negative in the others. At fields higher than approximately 0.4 T interaction effects become dominant and the magnetoconductivity is negative.