No abstract

Solid solution systems of (La1−xSrx)2CuO4−δ and (La1−xCax)2CuO4−δ with x=0.05, 0.075, and 0.10 were found to exhibit the superconductivity through a.c. magnetic susceptibility and electrical resistivity measurements. The onset critical temperature for (La0.90Sr0.10)2CuO4−δ was 37 K, the highest among all the superconducting materials ever known, while that for (La0.95Ca0.05)2CuO4−δ was 18 K.

In the scheme of a quantum nondemolition (QND) measurement, an observable is measured without perturbing its evolution. In the context of studies of decoherence in quantum computing, we examine the 'open' quantum system of a two-level atom or, equivalently, a spin-1/2 system, in interaction with quantum reservoirs of either oscillators or spins, under the QND condition of the Hamiltonian of the system commuting with the system-reservoir interaction. For completeness, we also examine the well-known non-QND spin-Bose problem. For all these many-body systems, we use the methods of functional integration to work out the propagators. The propagators for the QND Hamiltonians are shown to be analogous to the squeezing and rotation operators, respectively, for the two kinds of baths considered. Squeezing and rotation being both phase-space area-preserving canonical transformations, this brings out an interesting connection between the energy-preserving QND Hamiltonians and the homogeneous linear canonical transformations.

The system (La1−x(Ba,Sr,Ca)x)2CuO4−δ with x=0.1 has been found to be a high temperature superconducting phase in the entire compositional range of the three alkali earth cations. (La0.9Sr0.1)2CuO4−δ, displayed the highest critical temperature and the shortest distance for Cu–O bonds within the basal layer plane of the K2NiF4-type crystal structure, suggesting the two-dimensional nature of the conduction mechanism.

A reproducible superconducting transition between 83 and 95 K has been observed both resistively and magnetically in the new Yb-Ba-Cu oxide system at ambient pressure. The superconducting phase has been attributed to (Yb1-x Ba x )3 Cu2O7-δ which has a quite similar structure to that of Sr3Ti2O7 but with a large oxygen deficiency.