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.     

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.     

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.     

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.     

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.     

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.     

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.     

Relevance of electron-phonon coupling in high-T c superconductors

The experimental observation of phonon renormalization in high-T_c superconductors gives clear evidence for the action of electron-phonon coupling. But in spite of the estimated coupling strength 1–3, these effects do not allow for a unique identification of the coupling mechanism. This situation is contrasted with that for the low-T_c superconductors where tunneling spectroscopy provides a hard proof for electron pairing via phonons.     

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)).     

Structural analysis of high-T c cuprate superconductors

We have shown in this paper that members of the Tl-Ba-Ca-Cu-O and Bi-Sr-Ca-Cu-O superconducting families fabricated so far are built up of superconductor-semiconductor arrays. Four general homologous series are written down for these two families. The basic fundamental block in each series is itself a superconductor. We suggest that the Y-Ba-Cu-O family has a similar characteristic if YBaCu₂O_5-d is a superconductor.     

Interpretation of doping phenomena in high-T c superconductors

On the basis of an indirect-exchange pairing mechanism of superconductivity we present a consistent interpretation of doping phenomena in both hole-doped as well as electron-doped high-T _c superconductors. We argue that in all these materials the unifying feature is the existence of a correlated narrow band of electron states formed due to doping. Numerous experimental evidences for the occurrence of such a band (reflectivity, thermoelectric power, electrical resistivity, X-ray absorption, point-contact tunneling etc.) now exist. Assuming the existence of such a band it was earlier shown that the indirect-exchange (superexchange) coupling between electrons in this band via closed-shell oxygen anions is attractive in the s-wave channel and leads to high-T _c superconductivity. Within the framework of this pairing mechanism, recent doping experiments (for both types of doping) can be given a unified interpretation. In addition, definitive predictions of the doping conditions under which critical temperatures are expected to enhance, are made.     

Magnetization curves and Bean's model in high-T c superconductors

High-T _c superconductors have critical currents that decay sharply with increasing magnetic field. We solve Bean's model forJ _c decaying exponentially withH and obtain qualitative agreement with existing magnetization data. We show thatH _c1 cannot be obtained from the linear part of the magnetization curve; it can only be inferred from a low-field anomaly in the hysteresis curve. Presently quoted values ofH _c1 ,based on the linear part of the magnetization curve, are gross overestimates.     

On dislocation-induced reentrant-like behavior in deoxygenated high-T c superconductors

A possible scenario for dislocation-induced intragrain weak-link generation is proposed. The theory predicts a reentrant-like behavior of the critical current density vs. magnetic field due to the sample deoxygenation. A qualitative agreement with high-T _c single crystal anomalies is also discussed.     

Superconductivity of 2212 phase in Bi-based high-T c superconductors

In the Bi-based high-T _c superconductors, three superconducting transition points were observed above the liquid-N₂ temperature range. Allotropes of the 2212 phase were found. These allotropes were metastable and can interchange with the 2212 phase, and theirT _c's vary from ～85 to ～100 K.     

Effect of interlayer interaction in high-T c cuprate superconductors

We have studied the role of interlayer attractive interaction in a high-T _c system having two layers per unit cell. The single band two-layer tight binding model Hamiltonian is considered and the double time Green's function technique is applied within the mean field approximation. The expressions for the hole density, transition temperature, and intra- and interlayer order parameters are obtained which are found to be dependent on the interlayer interaction and other parameters appearing in the Hamiltonian. The numerical analysis shows that the coupling of the charge carriers (holes) between the layers provides better conditions for the stabilization of long-range order and high superconducting transition temperature in layered superconductors. It is also observed that superconductivity is confined to a narrow region of hole concentration and the single particle tunneling suppresses the transition temperature.     

Specific heat of the high-T c oxide superconductors

We review the specific heat measurements on La₂CuO₄, La_2?xM_xCuO₄ (M = Ca, Sr, and Ba), YBa₂Cu₃O₇, and the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O systems. Tables of properties derived from the data are presented. Results on RBa₂Cu₃O₇ (R=rare earth elements other than Y) are summarized, as are results on YBa₂(Cu_3?xM_x)O₇ (M=Zn, Cr, Fe, or Ni). The difficulties of analyzing the specific heat data, and specifically the separation of the contributions associated with magnetic impurities, are discussed. It is tentatively concluded that the data nearT _c are consistent with BCS theory, although they show evidence of fluctuation effects. It is also concluded that the low-temperature zero-field data on a majority of the high-T _c oxide superconductors provide evidence of anintrinsic term that is proportional toT, a result that is inconsistent with a gap in the electronic density of states.     

A phase diagram of new high-T c superconductors

A phase diagram of new high-T _c superconductors as highly pinning superconductors of the second type is presented. The diagram includes both vortex glass and vortex liquid phases.     

On the characteristic voltage of high-T c superconductors

The critical currents and normal resistances of the small bridges from yttrium-based high-T _c superconducting ceramics have been measured. The characteristic voltage of these bridges was found to be approximately 20 μV. This effect can be explained if, between the ceramic grains, there are contacts of an order of one crystalline cell in size.     

Intrinsic intergranular transport current density in high-T c superconductors

The intrinsic, intergranular, critical current densityJ _c(H=0,T≤T _c) in sintered untextured YBCO specimens of different grain size is self-consistently determined using a recently proposed critical state model [1] on ZFC magnetizationM(H, T) and on pulse transportJ _ct(T) measurements. Flux-creep effects, which are significant in these materials, are treated and theJ _c(T) results are corrected to their intrinsic values.