In-plane upper critical field anisotropy in Sr2RuO4 and CeIrIn5 - art. no.052502

Experiments on tetragonal Sr2RUO4 and CeIrIn5 indicate the presence of supe rconductivity with a multicomponent superconducting order parameter. Such a n order parameter should exhibit an in-plane anisotropy in the upper critic al field near the superconducting transition temperature that does not occu r for single-component superconductors. Here, this anisotropy is determined from microscopic calculations for arbitrary gap functions. It is shown tha t this anisotropy is generally not small and, in some cases, independent of impurity scattering, Furthermore, this anisotropy is calculated for many d etailed microscopic models of Sr2RuO4. For these models this anisotropy is found to be large, which is in sharp contrast to the small anisotropy obser ved experimentally. However, an accidental cancellation of the anisotropy f or gape, on different Fermi surface sheets can lead to a result that is con sistent with experiment.