Comparative photoemission study of Bi2(Sr,Ca)3Cu2O8+y and Bi1.7Pb0.3Sr2Ca2Cu3O10+y

The electronic structure of Bi-based 2223 single phase superconductor has been studied by ultraviolet and X-ray photoemission. By comparison with that of 2212 phase superconductor, we find a higher density of states nearE _F for 2223 phase. From analysis of the Cu 2p core-level spectroscopy, we obtain a relatively smaller charge transfer energy between copper and oxygen as well as the Coulomb repulsion energy on Cu site for 2223 phase. We relate these changes to the increase of hole concentration from 2212 to 2223 phase. The experimental results support the viewpoint that the transition temperature should be correlated with the density of states atE _F .     

Quantum cascade phenomenon in Bi2Sr2CaCu2O(8+delta) single crystals

We study interlayer transport in Bi2Sr2CaCu2O(8+delta) cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in nonequilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly nonequilibrium state in natural atomic superlattices, bearing certain resemblance with operation of a quantum cascade laser. Conclusions are supported by in situ generation-detection experiments and by numerical simulations.     

Elastic properties of high-temperature superconductors Pb2Sr2ACu3O8 (A=Ho,Y,Y0.5Ca0.5) derived from high-pressure X-ray diffraction

The crystal structure of the superconducting compounds Pb₂Sr₂ACu₃O₈, where A=Ho, Y and Y_0.5Ca_0.5, has been studied at room temperature in the pressure range up to 50 GPa. A transition from orthorhombic to tetragonal structure is observed at about 10 GPa for all these components. The transition is reversible and without any discontinuous volume change. The value of the bulk modulus for the two compounds with A=Ho and Y is in the range 160–164 GPa, whereas for A=Y_0.5 Ca_0.5 a larger value of 195 GPa has been measured.     

Stepwise behavior of vortex-lattice melting transition in tilted magnetic fields in single crystals of Bi(2)Sr(2)CaCu(2)O(8 + delta)

The vortex-lattice melting transition in Bi(2)Sr(2)CaCu(2)O(8 + delta) single crystals was studied using in-plane resistivity measurements in magnetic fields tilted away from the c axis to the ab plane. In order to avoid the surface barrier effect which hinders the melting transition in the conventional transport measurements, we used the Corbino geometry of electric contacts. The complete H(c) - H(ab) phase diagram of the melting transition in Bi(2)Sr(2)CaCu(2)O(8 + delta) is obtained for the first time. The c-axis melting field component H(c)(melt) exhibits the novel, stepwise dependence on the in-plane magnetic fields H(ab) which is discussed on the basis of the crossing vortex-lattice structure. The peculiar resistance behavior observed near the ab plane suggests the change of phase transition character from first to second order.     

Absorption of Dy3+ and Nd3+ ions in BaR 2F8 single crystals

The Dy³⁺ absorption and excitation spectra of BaY₂F₈ and BaYb₂F₈ single crystals are investigated in the ultraviolet, vacuum ultraviolet, and visible ranges at a temperature of 300 K. These crystals exhibit intense broad absorption bands due to the spin-allowed 4f-5d transitions in the range (56–78) × 10^?3 cm^?1 and less intense absorption bands that correspond to the spin-forbidden transitions in the range (50–56) × 10^?3 cm^?1. The Nd³⁺ absorption spectra of BaY₂F₈ single crystals are studied in the range (34–82) × 10^?3 cm^?1 at 300 K for different crystal orientations.