[en] While nonsuperconducting particles are known to serve as effective pinning centers in conventional superconductors, their effect in high T_c superconductors is still controversial. In this paper, the author gives evidence that nonsuperconducting Y₂BaCuO₅ (21 1) inclusions can act as pinning centers in melt processed YBaCuO superconductors even when their size is orders of magnitude larger than the coherence length. In such a case, the interface provides pinning. Theoretical estimates based on direct summations agree well with the experimental results. The applications of direct summation for obtaining the bulk pinning force is justified by direct observation of the FLL (flux line lattice), where the FLL has no long range order and the fluxoids are pinned by 211 inclusions

[en] Exploration for new non-oxide high temperature superconductors (HTS) had led to the reconsideration of copper chloride as a candidate for superconductivity. In 1975, Chu et al. demonstrated that copper chloride at room temperature exhibits a sharp drop in resistivity when under an imposed pressure of 40 kbar, (delta Ω > 7 orders of magnitude). Later, Brandt et al. showed that rapidly cooling CuCl under a hydrostatic pressure of 5 kbar, induced a transient diamagnetic response of about X = -1/4π at ∼ 100 K. Chu et al, using three different methods of preparation, showed there was a correspondence between the magnetic and resistive anomalies. It was proposed that the diamagnetic response was a manifestation of superconductivity and may originate at the Cu-CuCl interface, where Cu was the result of the disproportionation of 2 CuCl = Cu + CuCl₂ within the sample. Numerous accounts of superconductivity followed, but the superconducting properties observed never matured beyond an anomalous status of some samples. The magnetic anomalies observed in CuCl under pressure, by Chu and others, may be a result of inducing weak antiferromagnetism in those materials. On the face of it these conclusions appear disappointing. However, chemical doping to produce antiferromagnetism and electrical conductivity appears on the road to high temperature superconductors in the copper halides as we have seen in the copper oxides. They believe that although they have not seen any superconductivity here, that perhaps superconductivity will emerge in other chemically substituted copper halides as had been sought in the past. This research continues

[en] Long-time Langevin dynamics simulations of large (N x N,N = 128) 2-dimensional arrays of Josephson junctions in a uniformly frustrating external magnetic field are reported. The results demonstrate: (1) Relaxation from an initially random flux configuration as a universal fit to a glassy stretched-exponential type of relaxation for the intermediate temperatures T(0.3 T_c approx-lt T approx-lt 0.7 T_c), and an activated dynamic behavior for T ∼ T_c; (2) a glassy (multi-time, multi-length scale) voltage response to an applied current. Intrinsic dynamical symmetry breaking induced by boundaries as nucleation sites for flux lattice defects gives rise to transverse and noisy voltage response

[en] An analysis of various methods to obtain the critical fields of the high temperature superconductors from experimental data is undertaken in order to find definitions of these variables that are consistent with the models used to define them. Characteristic critical fields of H_c1, H_c2 and H_c that occur in the Ginsburg-Landau theory are difficult to determine experimentally in the high temperature superconductors because there are additional physical phenomena that obscure the results. The lower critical field is difficult to measure because there are flux pinning and surface barrier effects to flux entry; the upper critical field is difficult because fluctuation effects are large at this phase boundary; the thermodynamic critical field is difficult because fluctuations make it difficult to know the field where the magnetization integral should be terminated. In addition to these critical fields there are at least two other cross-over fields. There is the so called irreversibility line where the vortices transform from a rigid flux line lattice to a fluid lattice and there is a second cross-over field associated with the transition from the fluctuation to the Abrikosov vortex regime. The presence of these new physical effects may require new vocabulary

[en] The large thermal energy available, the strong anisotropy, and short coherence lengths of high temperature superconductors give rise to new phenomena in the mixed state. The author discusses transport and thermodynamic measurements of high-Tc materials and of model systems. In particular, he uses experiments on two dimensional films to compare and isolate two dimensional effects in the cuprates. By using multilayer systems with similar parameters, he identifies decoupling of the superconducting planes in magnetic fields at temperatures much above the irreversibility line. He shows that if the irreversibility line is to be considered a melting transition line, it implies melting of the solid state into a liquid of three dimensional flux lines. He further uses Monte Carlo simulations to study the structure of the vortex state as well as melting

[en] Fluctuation effects in conventional superconductors such as broadening of phase transitions and flux creep tend to be very small primarily because of the large coherence lengths. Thus mean field theory, with only small fluctuation corrections, usually provides an adequate description of these systems. Regimes in which fluctuation effects cause qualitatively different physics are very difficult to study as they typically occur in very small regions of the phase diagram or, in transport, require measuring extremely small voltages. In striking contrast, in the high temperature cuprate superconductors a combination of factors - short coherence lengths, anisotropy and higher temperatures - make fluctuation effects many orders of magnitude larger. The current understanding of transport and phase transitions in the cuprate superconductors-particularly YBCO and BSCCO-is reviewed. New results are presented on the two-dimensional regimes and 2D-3D crossover in the strongly anisotropic case of BSCCO. The emphasis is on pinning and vortex glass behavior

[en] The author discusses various experiments conducted in the micro- and millimeter wave spectral range on thin film and single crystal specimens of the high temperature oxide superconductors. For high quality film the surface resistance R_s is, except at low temperatures, due to thermally excited carriers, with extrinsic effects playing only a secondary role. Because of the low loss various passive microwave components, such as resonators, delay lines and filters, with performance far superior to those made of normal metals can be fabricated. The conductivity measured at millimeter wave frequencies displays a peak below T_c. Whether this is due to coherence factors or due to the change of the relaxation rate when the materials enter the superconducting state remains to be seen

[en] Static imaging has been used to complement transport I-V measurements in inferring the phase diagram and structure of the mixed state of the high T_c superconductors YBCO(123) and BSCCO (2212). High resolution Bitter patterns reveal the static structure of the flux lattice in the low field mixed state of BSCCO (2212). For B parallel c, the in-plane effective mass anisotropy is found to distort the lattice. Rotation of the field away from the c axis results in the appearance of a unique structure: a striking array of vortex chains. At low fields, the pattern is otherwise amorphous. Increasing the field above ∼ 20G results in an abrupt transition into an hexatic phase. At high fields, I-V curves with picovolt resolution imply critical behavior within the mixed state, consistent in both 123 and 2212 with a three-dimensional vortex glass transition well below T_c. In 2212, a field and temperature dependent crossover in the effective dimensionality of the vortex lattice is also found

[en] The authors present estimates for the bare Sommerfeld constant (γ_B) and the strength of the electron-phonon coupling based on specific heat measurements of YBa₂Cu₃O_7-δ. γ_B is determined from measurements made at high temperatures (300 to 700K). The strength of the electron-phonon coupling is determined by the ratio of the low- to the high-temperature values of the Sommerfeld constant. The high temperature specific heat is determined from measurements of the enthalpy of YBa₂Cu₃O_7-δ between 300 and 700K. Here, the authors will present only the data; full details of the analysis are given. As seen in the paper, the high temperature specific heat agrees well with the lower temperature measurements

[en] Research on high-T_c superconductivity covers most of the fields of materials science, and therefore, interdisciplinary investigations are necessary by scientists with diverse backgrounds in physics, chemistry, ceramics, metallurgy and so on. At present, after much research on the physical properties of materials, the creation of a theory of high-T_c superconductivity is extremely urgent. If a theory can be successfully established, its effects must be very wide and deep. solid state physics may be transformed, and the search for new superconducting materials will be accelerated. Furthermore, many applications will be greatly advanced by understanding the phenomena of high-T_c materials, and especially concepts for new electronic devices may be forthcoming. In the past, interactions between science and technology have been very clear. They sometimes resonate with each other and exhibit rapid progress in a very short period and give a big impact on society. The research and developments of high-T_c superconductivity will hopefully retrace the brilliant history of the great success of the science and technology of semiconductors in the near future. The author is very optimistic about this