Temperature-dependent anisotropy of London pen- etration depth in single-gap weak-coupling model

Temperature-dependent anisotropy, γ_λ(T ) = λ_c/λ_ab, of the London penetration depth, λ, is often considered to be a hallmark of multi-gap superconductivity. Similarly, larger than 1.76 values of Δ(0)/T_c ratio are often considered to be due to strong coupling (Δ(0) is the gap magnitude at T=0). Here we use weak-coupling self-consistent theory applied to various types of the order parameter and show that all types of the temperature dependencies of γ_λ(T ) are possible in a single- gap scenario. On the other hand, if observed, temperature-dependent γ_λ(T ) imposes certain restrictions on the structure of the order parameter. For example, pure d−wave results in temperature-independent γ_λ even on anisotropic Fermi surface, whereas Maki’s s + g order parameter leads to a non-monotonic variation of γ_λ(T ). The description of the experimental evaluation of γ_λ(T ) from small-amplitude AC susceptibility will complete the discussion.