Macroscopic anisotropy in superconductors with anisotropic gaps

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 γ(T) defined as the ratio of London penetration depths, λc/λab, is evaluated for all T's. Within the two-gap model of MgB₂, γ(T) is an increasing function of T.