Upper critical field of Nb and Nb+1 at % Ta determined by calorimetric and magnetic methods

The upper critical field H/sub c/2 of single-crystal Nb and arc-melted Nb+1 at % Ta near the critical temperature T/sub c/ has been measured using both calorimetric and magnetic methods. In contrast with the H/sub c2/ curve determined by the magnetic measurement, which shows a clear upward curvature, the calorimetric measurement gives a nearly straight line near T/sub c/. The data disagree with available models. A possible explanation is given