The spatial distribution and spectral characteristics of field-induced electron emission sites on broad-area high voltage electrodes

Instrumental techniques are described for displaying (a) the spatial distribution of field emission sites on planar high voltage electrodes and (b) the emission current pattern within an individual site. Typically, emission ‘sites‘ are generally composed of three or more ‘sub-sites’ that become temporally unstable at current > 10^?7 A. The electron energy spectra of sub-sites are characteristically single peaked, whose half-widths (FWHM) initially vary linearly with applied field: from changes in spectral area with field, substantially linear sub-site F-N plots have been obtained having β values in the range 300–500.     

Bulk high-T c magnetometer

We report a study of a bulk YBCO magnetometer based on the detection of harmonics during nonlinear magnetization. For a low-DC field, the amplitude of the even harmonic appearing at the pickup coil depends linearly on the DC field intensity. The performance of the magnetometer depends on the quality of the sample, and the intensity and frequency of the AC field excitation. At higherH _ac, the fourth harmonic becomes more sensitive than the second harmonic. The hysteresis in the magnetometer for DC field measurement has been reported.     

T c trend in high-T c superconductors

In our previous works, we have shown that most existing ceramic superconductors can be considered to be built of superconductor-semiconductor composite and we have estimated the change in phonon spectrum of the intrinsic superconductor unit if a semiconductor unit is attached to it. Moreover, the proximity effect under the size quantization condition has been examined in the superconductor-semiconductor composite. Each of the stated effects by itself could causeT _c enhancement in general as more semiconductor blocks are added to the system. We extend our study in this paper to analyze the combined actions of phonon spectral change and proximity effect without size quantization condition onT _c variation in members of the Tl₁ series of high-T _c superconductors. Our results indicate that an optimumT _c is obtained if the stated effects are included in the idealized unit cells of the superconductors made up of a superconductor-semiconductor array.     

Casting high-T c superconducting BiSCCO

Bi-Sr-Ca-Cu-O (BiSCCO) was melted and cast into molds in order to render the superconductor into fully dense and useful shapes. Special conditions for casting and subsequent heat treatments were required to ensure structural integrity and a high transition temperature,T _c .The history of the castings during heat treatments was studied by thermal analysis, X-ray diffraction, and microscopy.     

Microwave absorption in high-T c superconductors

We have investigated the 9- and 10-GHz microwave properties of bulk samples of strontium and barium substituted La₂CuO₄ and YBa₂Cu₃O₇ and one YBa₂Cu₃O₇ MBE film on SrTiO₃. Anomalous microwave absorption is seen especially for poorly compacted bulk ceramic samples. Transitions were generally broadened nearT _c and the residual losses were high. The real part of the surface impedance could be fit belowT _c to the empirical equation with an added term linear in temperature. The microwave response of the film was masked by paraelectric absorption and acoustic resonances in the substrate.     

Thin-film processing of high-T c superconductors

This review of high-T _c superconducting thin-film processing focuses on the developments in thin-film deposition technologies since 1987. The common deposition processes are described with reference to their effects on superconductor film performance. A comparative evaluation of the potential of the technologies is also given. The development of multilayers and heterostructures is an important requirement for future device applications and is also described. The latest results of the deposition of novel superconducting materials and deposition on uncommon substrates are discussed. The outlook on some imminent topics of future development in process technologies for high-T _c superconducting thin films is discussed.     

t-Model mechanism of high-T c cuprates

A doublon formalism ofd-p model Hubbard Hamiltonian is studied only with transfer integrals, and without the superexchange term. This gives excellent doping-dependent features ofT _c in high-T _c cuprates.     

Spin-glass state in high-T c superconductors

The high-temperature superconductor glass state in the Josephson weak-link model is studied. Reversibility phenomena in weak magnetic fields are outlined.     

Josephson effect in high-T c superconductivity

In this report we present the most important results of our recent analysis (Kupriyanov and Likharev 1990) of the Josephson effect in both the natural (intergrain) and artificial junctions using high-T _c superconductors (HTS). A comparison of the experimental data with the BCS-based theories of the Josephson effect in various tunnel-junction-type and weak-link-type structures has been carried out. The main conclusion is that the data presently available do not enable one to either confirm or reject the theories, and thus to reveal possible deviations of the real microscopic mechanism of the high-T _c superconductivity from the BCS mechanism. We suggest several experiments which would be more fruitful for this purpose, as well as for finding ways of reproducible fabrication of practically useful Josephson junctions. This work was supported by the Soviet Scientific Council on the high-Tc superconductivity problem (Grant No.42). Invited talk at the International Conference on Superconductivity, Bangalore, January 1990.     

Intergranular fluxons in high-T c superconductors

The granular high-T _c superconductors can carry very low transport current, in comparison to that found in the bulk of the material. Magnetization and critical current measurements at very low field indicate that this low transport current behaves as expected from a critical state model. The presence of weak links between the grains in granular aggregates is firmly established, together with the Josephson character of such links. The existence of some kind of magnetic particles, and of a mechanism of pinning for them, is required to explain the critical state regime. In this paper we examine the flux structures which can be present in the granular systems, which can be described by an array of SQUIDs, and we show their similarities to fluxons in a continuous medium and to fluxons in uniform Josephson junctions. A simple model, based on a two-dimensional network of pointlike Josepson junctions, is adequate to demonstrate the existance of the IF (intergranular fluxon). Its characteristics depend on a single parameter, which gives the coupling strength between grains. The discreteness of the system is the cause of an intrinsic pinning of the intergranular fluxons.     

Thermal conductivity of high-T c superconductors

This paper reviews existing data on the thermal conductivity of high-T _c superconductors. Included are discussions of pristine polycrystalline high-T _c ceramics, single crystal specimens, and high-T _c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high-T _c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high-T _c perovskites are discussed and implications for theories of high-T _c superconductivity are noted.     

Analysis of specific heat anomalies of high-T c superconductors based on a two-carrier high-T c superconductor model

We investigate the specific heat properties of a high-T _c superconductor model, the “two-carrier model” which includes an electron-phonon interaction driven by lattice defects such as oxygen deficiencies in the otherwise conventional BCS superconductor. The normal state is identified as a two-carrier model system which is composed of two groups of charge carriers: the normal and the renormalized Bloch carriers. An enhanced normal-state electronic specific heat is obtained, and there is good agreement between theoretical predictions about the specific heat anomalies at and aboveT _c and experimental data.     

The flux-line lattice in high-T c superconductors

The flux-line lattice in type-II superconductors has unusual nonlocal elastic properties which make it soft for short wavelengths of distortion. This softening is particularly pronounced in the highly anisotropic high-T _c superconductors (HTSC) where it leads to large thermal fluctuations and to thermally activated depinning of the Abrikosov vortices. Numerous transitions are predicted for these layered HTSC when the temperatureT, magnetic inductionB, or current densityJ are changed. In particular, the flux lines are now chains of two-dimensional (2D) “pancake vortices” which may “evaporate” by thermal fluctuations or may depin individually. At sufficiently highT, ohmic resistivityρ(T, B) is observed down toJ → 0. This indicates that the flux lines are in a “liquid state” with no shear stiffness and with small depinning energy or that the 2D vortices can move independently. At lowerT, ρ(T, B, J) is nonlinear since the pinning energy of an elastic vortex lattice or “vortex glass” increases with decreasingJ as predicted by theories of collective pinning and by “vortex glass” scaling.     

Vortex structure in high-T c ceramic superconductors

We consider penetration of magnetic flux in the rare earth ceramic superconductors within the framework of a generalized Ginzburg-Landau theory, based on the coexistence of antiferromagnetism and superconductivity. We study vortex structure produced for a magnetic field close to . We make a new prediction that may serve to test the theory experimentally.     

Positron-annihilation lifetime in the high-T c Oxides

The superconducting transition affects the positron-annihilation lifetime in the high-T_c oxides. This effect is due to the unusual properties of the oxides in the normal state and, in particular, to the small Fermi energy. The value of the shift also depends on the interlayer distance.     

Vibrating reed studies on high-T c superconductors

We present vibrating reed (VR) measurements on single crystal and ceramic “1-2-3” and melt-processed polycrystalline Bi-based compounds in a wide range of temperature (4·2–100 K) and magnetic field (B=0–4 T). The “depinning line” (DL) determined by the VR technique is equivalent to the “irreversibility line” determined by magnetization and susceptibility measurements. A comparison of the results on single crystal and polycrystalline 1-2-3 compounds indicates that the VR technique is sensitive to the intragranular properties of the polycrystalline reed. It is found that the DL for 1-2-3 compounds is much steeper than that for Bi-based compounds, reflecting an intrinsically different pinning in both the materials, in agreement with the measured elastic coupling (Labusch constant α(B, T)).