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It is shown within the weak-coupling model that the macroscopic superconducting anisotropy for materials with a gap varying on the Fermi surface cannot be characterized by a single number, unlike the case of clean materials with isotropic gaps. For clean uniaxial materials, the anisotropy parameter (T) defined as the ratio of London penetration depths, ₂/₀₁, is evaluated for all T's. Within the two-gap model of MgB₂, (T) is an increasing function of T.
V. G. Kogan (Thu,) studied this question.