Electronic structure of the high Tc superconductor Ba2YCu3O6.9

The electronic structure of Ba₂YCu₃O_6.9 has been calculated using the linear augmented-plane-wave method. The results show strongly 2-dimensional and 1-dimensional features in the energy bands. These give rise to competing ch density wave instabilities. We argue that the proximity of these instabilities and the large electron-phonon matrix elements of the Fermi-level electrons make important contributions to the high T_c of this material.     

Analysis of temperature and enhanced diffusion effects in sputtering of CrSi2

Previously reported experimental results on sputtering and enhanced diffusion processes in CrSi₂ during 100 keVXe⁺ bombardment at different temperatures have been quantitatively analyzed.The framework for the analysis is a simple theoretical model in which the Si atoms are considered mobile in a matrix of Cr atoms whose density is assumed constant and diffusivity is considered zero everywhere. Erosion velocity of the matrix (due to the sputtering of Cr atoms), sputtering and enhanced diffusion processes of Si atoms are taken into account in the mathematical model.In our analysis we show that the time evolution of the total number of sputtered atoms in binary solids cannot yield an unambigous conclusion as to the existence of preferential sputtering.Further, it is found that in the case of CrSi₂ the preferential sputtering of Si atoms depends on the suicide temperature.     

Influence of oxygen on diffusion in the Cu/Mo/Au system

The annealing behaviour of the Cu/Mo/Au metallization system is investigated. Backscattering spectrometry reveals a rapid diffusion of Cu and Au across the Mo film and the formation of AuCu after annealing at 600°C for 30 min in vacuum, but only when no impurity is detected in the as-deposited polycrystalline Mo layer. The Au-Cu interaction is impeded when 5.5 at% of oxygen is introduced in the as-deposited Mo layer. The thin film microstructures are analyzed using X-ray diffraction and transmission electron microscopy. The thermal stability difference between the samples with a pure Mo layer and a contaminated Mo layer is discussed in terms of fast diffusion process inhibited by a grain boundary decoration with oxygen atoms.     

Ambipolar diffusion phenomena in BaTiO3 and SrTiO3

The electrical conductivity of resistive oxygen sensors based on semiconducting titanates (BaTiO₃, SrTiO₃) is determined by the amount and the ratio of the different concentrations of atomic defects in the sensor material. At sufficiently high temperatures the sensor exchanges oxygen with the surrounding gas atmosphere resulting in variations of the concentration of defects. The kinetics of these oxygen exchange processes are determined by the diffusion of the defects in the solid. In this paper the diffusion coefficients of the defects, which are decisive for the electrical conductivity, are determined by measurement of the conductivity during the diffusion processes. The evaluation of these results by an analytical model which considers the interaction between all simultaneously diffusing types of defects, allows a survey of the effects influencing the diffusion rate. With this knowledge, it is possible to estimate the influence of grain boundaries, acceptors, temperature and oxygen pressure on the response time of the sensor.     

Relaxation of the magnetic structures in high temperature superconducting ceramics Tl2Ca2Ba2Cu3Ox. Investigation of the microwave response

A new method to study relaxation of magnetic structures is suggested, based on the observation of the relaxation of microwave response. A new explanation of these processes is proposed, using the theory of “polychromatic” kinetics in the solid state.     

Preparation of Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba2Ca2Cu3Ox films

Preparation of superconducting Tl-Ba-Ca-Cu-O thin films by diffusion of Tl into laser evaporated Ba₂Ca₂Cu₃O_x thin films is reported. From a sintered Ba₂Ca₂Cu₃O_x bulk sample we prepared using a pulsed Nd:YAG laser, Ba-Ca-Cu-O thin films on sapphire and SrTiO₃ single-crystal substrates. Subsequently the films were loaded with Tl by simultaneously annealing the films together with a sintered Tl₂Ba₂Ca₂Cu₃O₁₀ sample both enclosed in a small stainless steel box; in our procedure Tl contamination was reduced to a minimum. T_c values near 100 K and critical currents of 5·10³ A/cm² at 77 K were obtained.     

Determination of the copper diffusion coefficient in silicon from transient ion-drift

We use the transient ion drift in a depletion region of a Schottky barrier to determine ion diffusivities at moderate temperatures. The pulsed reverse bias leads to temperature dependent capacitance transients similar to deep level carrier emission transients. A simple theoretical model together with classical transient signal analysis provide the means to extract the ion diffusion constant. When applied to copper in silicon, diffusion data are obtained in a not yet investigated temperature range (280–400 K) which agree well with both low and high temperature diffusion data.     

Volume dependence of the raman frequencies in Y Ba2Cu3Ox single crystals with different oxygen content

Abstract

The Raman spectra of Y B _a2C_u3O_x, single crystals (x=6.25; 6.75; 7.0) were measured at pressures up to 22 GPa (35 GPa for x=6.25) at room temperature in nearly hydrostatic conditions. The frequency-volume curves for most of the Raman-active fundamental vibrations were derived from the present data making use of a previous high-pressure study of the equations of state of Y Ba₂ Cu ₃ O _x compounds.     

Low temperature diffusion in thin-film couples of polycristalline silver and gold

Limited diffusion spaces which may be easily traversed by atoms diffusing along grain boundaries generally cause thin-film diffusion to differ from common bulk diffusion. These peculiarities were studied in Ag−Au thin-film couples by means of electrical resistance measurements. Diffusion coefficients were found to decrease with annealing time mainly as a consequence of recrystallisation and recovery in the films during the diffusion anneal. It is shown that the rate of homogenisation is fairly independent of the film thickness thus giving evidence that diffusion into the crystallites occurs out of the grain boundary network rather than directly through the couple interface. Effective diffusion coefficients determined between 150 and 250° C ranged from 10⁻¹⁴ to 10⁻¹⁶ cm² s⁻¹ revealing an activation energy of 25 kcal mol⁻¹.     

The temperature dependent thermal and electrical resistivity of high purity silver from 2 to 20 K

The low temperature (T), temperature dependent electrical resistivity (?), and thermal resistivity (ω), have been measured in a sample having a residual resistivity ratio of 10,000: 1. Below approximately 10 K ? ∝ T⁵ and ω ∝ T² — in excellent agreement with theory. Both resistivities are considerably smaller than those obtained by previous workers on less pure samples. Above 10 K the resistivities rise more rapidly with temperature — particularly for ω. The experimentally determined temperature dependent Lorenz number, $\text{L}{}_{\mathrm{i}}\text{=}\text{?}\text{ωT}$ is examined and suggests that the augmentation of the horizontal scattering of electrons by umklapp processes is greater for ω than for ?.     

Remanent fields and critical currents in Tl2 Ba2Ca2Cu3Ox ceramic

The temperature and magnetic-field dependences of the remanent magnetization have been determined from far-field measurements for thallium ceramic samples. The current flow that gives rise to the observed magnetization is intragranular. The results are consistent with flux-creep-controlled transport with a distance between pinning centers of - 1/2/m. Using near-field measurements, the superconducting properties are found to be heterogeneous on a scale of 1 mm. Magnetic domains have been induced by pplying spatial variation in the applied field.     

X-ray irradiation enhanced critical current density and strong pinning sites created in Gd1Ba2Cu3O7−x thin films

An enhancement of the critical current density (to more than 10⁶ A/cm² below 65 K) of Gd₁Ba₂Cu₃O_7–x superconducting thin films was achieved by X-ray irradiation with oxygen annealing. This process of X-ray irradiation with oxygen annealing introduced stable pinning centers into the crystal and also increased the activation energy from 0.1 eV to 0.25 eV.