Impurity Effect on the Anisotropy of the Upper Critical Field of Nb—Ta Alloys
Substantial advance has been made on the subject of the anisotropy of H c2 since its discovery in Nb by Tilley et al. 1 Hohenberg and Werthamer2 (hereafter HW) first pointed out that for materials with cubic symmetry like Nb the Fermi velocity anisotropy may be related to the H c2 anisotropy through nonlocal terms. As non-locality becomes progressively unimportant with increase in temperature and impurity the theory predicts that the degree of anisotropy vanishes as one approaches T c and sufficiently dirty limits Experiments by Reed et al.3 qualitatively support these predictions. More recent measurements by Farrell et al.4 (Nb) and Williamson5 (Nb, V) offer detailed information on the temperature dependence of the anisotropy. On the other hand, systematic experimental study on the effect of impurity is lacking. The primary object of this work is to clarify this aspect with Ta added as an impurity to Nb. Tantalum was chosen because it alloys very well with Nb and also because it is known to have Fermi surface properties very similar to those of Nb.6 Thus it is expected that the perturbation to the host Nb properties may be kept at a minimum.
KeywordsFermi Surface Computer Calculation Impurity Effect Alloy Single Crystal Substantial Advance
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