Low field irreversibility in twinned and untwinned YBaCuO crystals

An abrupt collapse of the irreversibility line is observed in both twinned and untwinned YBaCuO crystals within a critical regime. The exponent of the power law describing the irreversibility line H_irr changes from at high fields to larger values (between 1.5 and 2) below the collapse. We propose that the observed low field anomaly, which extends down to temperatures well below the irreversibility line, is associated with the thermal softening boundary which increases as T² and crosses H_irr shifting it to lower fields. Below this crossover, which occurs when the mean-square thermal displacement √ u² of the vortices exceeds coherence length ξ, the strength of the pinning is strongly reduced.     

Pressure-induced valence changes in EuS and SmTe

Abstract

X-ray absorption spectroscopy was applied to study the pressure-induced valence changes in EuS and SmTe, which are divalent semiconductors of NaCl-type structure at ambient pressure. In both systems the Eu-L_III and Sm-L_III thresholds exhibit the onset of intermediate valencies of the rare earth ions at 15 GPa and 4 GPa, respectively. In EuS, one observes only a small increase of the valency within the investigated pressure range (36 GPa), while in SmTe a full transition to trivalency is observed above 20 GPa. The transition to the CsCl-Type high-pressure phase has no significant influence on the valence in both systems.     

The effect of pressure on lattice parameter,magnetic susceptibility and reflectivity of SmS

The lattice parameter of SmS single crystals has been measured under hydrostatic pressure up to 70 kbar. The fit for the metallic phase by means of the Birch equation gives an unusually large pressure derivative of the bulk modulus B' = 11 and hence a large pressure dependence of B. No change in the nonmagnetic intermediate valence state of Sm is observed up to 24 kbar as concluded from our magnetic susceptibility measurements. Reflectivity measurements under hydrostatic pressure p > 6.5 kbar show a small plasma edge shift (～0.08 eV) towards lower energy upon cooling from 77 to 4.2 K. Hence the low temperature resistivity anomaly is attributed to an enhanced conduction electron scattering.     

Vortex phases and dynamics in high-temperature and conventional amorphous superconductors

The vortex phases and dynamics of high-temperature and conventional amorphous superconductors are investigated over a broad frequency range. A second-order vortex-solid melting transition in both YBa₂Cu₃O₇ single crystals with random point defects and amorphous Mo₃Si films is manifested by a set of universal critical exponents (ν≈2/3 and z≈3) from both ac and dc transport as well as ac magnetic susceptibility measurements. In contrast, the melting transition in YBa₂Cu₃O₇ single crystals with correlated columnar defects is found to be consistent with a Bose-glass phase transition characterized by a different set of critical exponents (ν≈1/2 and z′≈4).