J. V. Waszczak

Thermal Conductivity of Single-Crystal and Sintered RBa2Cu3O7 at low Temperatures

The thermal conductivity k of micro-twinned single crystals of YBa 2 Cu 3 O 7 and HoBa 2 Cu 3 O 7 and a sintered sample of YBa 2 Cu 3 O 7 has been measured for temperatures 0.03 1.8 - 1.9 This behavior resembles k for glassy insulators except for the lack of a plateau above IK. It is concluded that the thermal carriers are phonons with their mean free path limited by resonant scattering from tunneling entities, as in glasses. Suggestions for the location of tunneling systems are given. For the sinter, k is still smaller but does not follow a power law T-dependence. It is similar to other sintered ceramics with the same particle size, where the phonon mean free path is dominated by Rayleigh scattering from the particles. This strong scattering from the microstructure presumably masks the scattering from TS within each particle.

Vibrational Raman Scattering from high T C single Crystals

Polarized Raman spectra have been measured on single crystals of the high T c materials A 2 BCu 3 O x with A=Ba and Sr, B=Y and La and 6 zz )on chemical composition is in agreement with its assignment to the O(1) axial stretching vibration. The intensity of the 600 cm 1 band, which has been ascribed to a defect induced Cu-O mode, strongly depends on the crystal preparation and resulting purity.

Electronic structure of Pb2Sr2PrCu3O8 as studied by resonant photoemission spectroscopy.

Resonant photoemission spectroscopy has been used to study the electronic structure of the recently discovered high-temperature superconductor Pb{sub 2}Sr{sub 2}PrCu{sub 3}O{sub 8}. The valence-band features are compared with those from other high-temperature superconductors and with band-structure calculations. A strong Cu 3{ital d}{sup 8} satellite arises at approximately the same energy position as in the other high-{ital T}{sub {ital c}} superconductors, implying that to first order the model parameter {ital U}{sub {ital dd}} will be quite similar. A 0.7-eV shift to higher binding energy of the band-theoretical density of states is required to obtain the best agreement with the photoemission spectra. The strong Cu 3{ital d}{sup 8} satellite and the discrepancy between the band theory and the photoemission spectra show the importance of correlations in this material. The valency of Pr in the material was observed to be 3+, as it had been in PrBa{sub 2}Cu{sub 3}O{sub 7}. The extracted Pr 4{ital f} spectral weight suggests that the hybridization of the Pr 4{ital f} states with the in-plane O states is reduced, as compared to PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. This is plausible, as the interplanar spacing is larger in this material than in PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. We infer that thismorexa0» hybridization is important for the quenching of the superconductivity in PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and explains why the superconductivity is not quenched in Pb{sub 2}Sr{sub 2}Pr{sub 1{minus}{ital x}}Ca{sub {ital x}}Cu{sub 3}O{sub 8}.«xa0less

Observation of Hexagonally Ordered Flux Quanta in YBa 2 Cu 3 O 7

The high resolution Bitter Pattern technique has been used to reveal the structure of the array of flux lines which is present when single crystal samples of the high Tc superconductor YBa2Cu3O7 are placed in a magnetic field. At 4.2K with the magnetic field parallel to the c axis, the patterns formed are strongly reminiscent of the analogous structures in ordinary type II superconductors. The structures typically consist of flux spots with short range hexagonal correlations. We interpret the patterns to consist of singly quantized vortices. Very uniform patterns are observed in thin, flat samples cooled in a constant field. Sampling the spot density yields a vortex density consistent with a flux quantum of hc/2e.n Experiments at 77K, the only other temperature studied, showed no spatially varying magnetic structure. Recent mechanical measurements which are sensitive to the bulk modulus and dissipation within the vortex lattice show evidence for a mobility transition near 75K, which would be consistent with this observation. We believe this stems from materials properties rather than some more fundamental property of the new superconductors.