Comparison of the scaling analysis of mixed-state magnetization data with direct measurements of the upper critical field for YBa(2)Cu(3)O(7-x).

By comparison of recent direct measurements of the temperature dependence of the upper critical field H(c2) of an YBa(2)Cu(3)O(7-x) high-T(c) superconductor with the scaling analysis of magnetization data, collected in fields [Formula: see text], we demonstrate that the temperature dependence of the Ginzburg-Landau parameter κ is negligible. Another conclusion is that the normalized temperature dependence of H(c2) is independent of the orientation of the magnetic field with respect to the crystallographic axes of the sample. We also discuss the fact that isotropy of the temperature dependence of H(c2) straightforwardly follows from the Ginzburg-Landau theory if κ does not depend on the temperature.

Unusual magnetic-field-induced phase transition in the mixed state of superconducting NbSe2.

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.