Experimental determination of the upper critical field of epitaxially grown thin films of YBa2Cu3O7−δdown to He temperature

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Abstract
The upper critical field of YBa2(Cu1−x Fe x )3O7−δ with x=0 and 0.05 is determined over the whole temperature range from 4.2 K to T c . The data behave according to the theory of Werthamer, Helfand, and Hohenberg [N. R. Werthamer, E. Helfand, and P. C. Hohenberg, Phys. Rev. 147, 295 (1966)]. However, at low temperatures the upper critical field is larger than expected from the original expressions of Werthamer and co‐workers. Generalized equations, which take into account the anisotropy of the Fermi surface, yield values for the upper critical field, which are in excellent agreement with the experimental data.