Semiconductor-metal-semiconductor like transitions in amorphous Ba1-xKxBiO3-δ oxides

In this paper we report the dc conductivities of the Ba_1-xK_xBiO_3-δ (x = 0,0.2,0.4,0.625 and 0.75) glasses at different temperatures (80 to 450 K). Distinct anomalies in the temperature dependence of the dc conductivities, indicating semiconductor-metal-semiconductor (SMS) like transitions, are observed for all the glasses at temperatures ranging between 297 K and 325 K (depending on x). A similar anomaly is also observed from the dielectric constant measurements around the same temperature region. Such a SMS transition is very uncommon in oxide glasses. The orientational motion of the BiO₃ or BiO₆ structural units present in the glasses (as observed from the infrared spectra) is considered to be responsible for such anomalous behavior. A model Hamiltonian considering strong electron-phonon interaction giving rise to a lattice instability is suggested to describe such a SMS type transition observed in the present glassy systems.     

ON THE SUPERCONDUCTING MECHANISM OF K3C60 AND Ba1-xKxBiO3

A superconducting mechanism for K₃C₆₀ and Ba_1-xK_xBiO₃ has been suggested. This is the combining picture of the Cooper pair and the Ogg pair. Numerical calculations within the framework of this picture have been done for K₃C₆₀ and Ba_1-xK_xBiO₃, and the overall consistency with the experimental data of superconducting properties is good.     

Dimensional crossover in cuprate superconductors

A central concern in understanding the mechanism for the occurrence of superconductivity in cuprates is the interaction driving the phase transition and their dimensionality. As physical systems near a phase transition have a marked dependence on dimensionality, this can be explored with symples where one of the physical dimensions is reduced and becomes comparable to the correlation length. Recently, it became possible to fabricate sufficiently thin cuprate slabs, revealing a fall ofT _c with reduced thickness, becoming pronounced for slabs a few unit cells thick. Related effects have been observed in the YBCO bulk compounds 123, 124 and 247. We analyze the experimental data by invoking finite size scaling and a Ginzburg-Landau treatment. The main conclusions include the following: the fall ofT _c with decreasing thickness corresponds to a dimensional crossover, revealing the three-dimensional nature of the interaction mediating superconductivity; there is a predominance of two-dimensional fluctuations and boundaries with reduced thickness; there are crossover phenomena reminiscent of⁴He films and thin slabs of conventional super-conductors.     

Spin-lattice relaxation in cuprate superconductors

This presentation gives a personal review of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spin-lattice relaxation studies in cuprate superconductors mainly dealing with the YBa₂Cu₄O₈ compound with many examples from the Zürich laboratory. The studies were performed in both the normal and the superconducting state with various NMR isotopes (e.g.,¹⁷O,^63,65Cu,^135,137Ba). The relatively broad signals were mostly obtained by a phase-alternating add-subtract spin-echo technique. We will discuss the general behavior of spin-lattice relaxation in the normal state and the calculation of the dynamic spin including an approach (on the basis of thet-J model) to calculate the relaxation for plane copper, oxygen, and yttrium. An application of the Luttingerliquid model to the relaxation of chain copper in YBa₂Cu₃O₇ and YBa₂Cu₄O₈ is also given. We then will deal with characteristic features of the YBa₂Cu₄O₈ structure: the spin gap, an electronic crossover in the normal state, the single-spin fluid model, and the d-wave pairing.