[en] Using the Eliashberg equations for superconductivity in a layered high-temperature superconductor and taking into consideration the phonon-mediated electron-electron interaction and the effect of the disordered magnetic correlations, the author calculated the expressions for the critical temperature. The phonon and the magnetic kernels have a logarithmic divergence in the low-frequency limit and the critical temperature differs from the McMillan form. The effect of various parameters on the decrease of the critical temperature has been analyzed. The renormalization function Z_ph(ω) contains a logarithmic dependence on ω

[en] An overview is given on anharmonic lattice vibrations originating from structural instabilities. The transverse vibrations of the chain oxygens in YBa₂Cu₃O₇ are found to be only moderately anharmonic. Measurements of the phonon linewidths in the Cu-O bond stretching vibrations strongly support recent calculations of the electron-phonon coupling. Results are presented on superconductivity-induced frequency shifts at short wavelengths. 20 refs., 15 figs

[en] The temperature dependence observed in the mid- and near-infrared optical properties of YBa₂Cu₃O₇ is explained in terms of the Drude model for free charge carriers. In the Drude model, the linear temperature dependence of the dc resistivity arises from the free charge carriers having a temperature-dependent mean free path. This temperature dependence results in the plasmon contribution to the dielectric constant having a damping coefficient which also varies linearly with temperature. We find that the temperature dependence which is observed in the absorption and reflection spectra of YBa₂Cu₃O₇ is consistent with this simple model. 15 refs., 2 figs

[en] Silver has been previously added to the melt-textured YBa₂Cu₃O_7-x in order to increase the critical current density (J_c) of these materials. However, the effect of this addition on the J_c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7-x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). The J_c of the MTG YBCO/Ag samples is more than 10⁴ A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, the J_c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper

[en] The superconductivity of metallic iodine is observed at temperatures below 1.2 K under pressures above 28 GPa by magnetization and electrical resistance measurements. The pressure dependence of the superconducting transition temperature is studied up to 74 GPa. This is the first observation of the superconductivity of the molecular-dissociated monatomic metal

[en] The magnetic torque of κ-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field direction θ with respect to the a*-axis under constant magnetic fields, H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found at θ_cl near the H parallel bc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed at θ_c2 and θ_c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component of H giving each cusp is kept constant as H cos θ_cm = const triple-bond H_cpm (n = 1, 2, 3), i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to the bc-plane H_cp1, H_cp2, and H_ep3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors. 6 refs., 2 figs

[en] A simplified technique for synthesizing high-quality millimeter-sized single-crystals of K₃C₆₀ is reported. The preparation method consists of a two-step process where pure C₆₀ crystals are first grown and later doped with potassium vapor. The undoped crystals are grown using an open-tube vapor transport with a flowing helium gas carrier. Doping occurs in a high-vacuum environment inside a specialized quartz tube which allows phase purity to be monitored via magnetization measurements of the superconducting state. Utilizing the aforementioned procedure, one can attain samples with magnetically determined superconducting transition widths (10-90% diamagnetic shielding) of less than 1 K. 8 refs., 4 figs

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

[en] Using the model for layered superstructures with the Josephson bond and additional boundary conditions (frustration), the authors consider the dependence of the critical temperature T_c^(p) on the number P of evaporated layers. The characteristics defined through precise solutions of the Josephson functions functional distinctions on the fluxon lattice T_c^(p) are presented

[en] The Cu oxidation number of YBa₂Cu₃O₇ was measured directly by soft X-ray fluorescent spectroscopy both at room temperature and at liquid N₂ temperature. The measurements are based on a calibration curve from different Ba-O compounds. The effects of changes in oxidation number above and below the transition temperature and its role in high-T_c superconductivity are discussed