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Orbital upper critical field and its anisotropy of clean one- and two-band superconductors

The Helfand-Werthamer (HW) scheme\cite{HW} of evaluating the orbital upper critical field is generalized to anisotropic superconductors in general, and to two-band clean materials, in particular. Our formal procedure differs from those in the literature; it reproduces not only the isotropic HW limit, but also the results of calculations for the two-band superconducting MgB$_2$\cite{MMK,DS} along with the existing data on $H_{c2}(T)$ and its anisotropy $γ(T)=H_{c2,ab}(T)/H_{c2,c}(T)$ ($a,c$ are the principal directions of a uniaxial crystal). Using rotational ellipsoids as model Fermi surfaces we apply the formalism developed to study $γ(T)$ for a few different anisotropies of the Fermi surface and of the order parameters. We find that even for a single band d-wave order parameter $γ(T)$ decreases on warming, however, relatively weakly. For order parameters of the form $ Δ(k_z) = Δ_0(1+η\cos k_za)$,\cite{Xu} according to our simulations $γ(T)$ may either increase or decrease on warming even for a single band depending on the sign of $η$. Hence, the common belief that the multi-band Fermi surface is responsible for the temperature variation of $γ$ is proven incorrect.