Upper critical fields of the superconducting layered compounds Nb1?x Ta x Se2

Abstract

The upper critical fieldsH c2 (T) of the superconducting layered compounds Nb1−x Ta x Se2 (0≤x≤0.20) have been measured with the magnetic field oriented either perpendicular or parallel to the layer plane. Just belowT c , positive (upward) curvature is seen inH c2 (T) for both field directions and for all compositions. The amount of positive curvature is not a function of crystal quality. At low temperatures,H c2 (T) displays enhanced linearity as compared to the prediction of the isotropic theory, and shows no evidence of Pauli paramagnetic limiting for either field orientation. The critical field slopesdH c2 /dT nearT c change only slightly with composition.H c2 (T) has been calculated for two Fermi surface models for NbSe2 using the band structure calculated by Wexler and Woolley. TheseH c2 (T) calculations indicate that Fermi surface anisotropy can explain the positive curvature and enhanced linearity seen in the experimental data. The calculations also show that theH c2 (T) data are better explained with a Fermi surface that consists of open, undulating cylinders with an additional smaller closed piece, than with a Fermi surface model that consists only of the open, undulating cylinders of the original Wexler and Woolley model.