Anisotropy of the upper critical field in a heavy-fermion superconductor

Exotic Multigap Structure in UPt_{3} Unveiled by a First-Principles Analysis

A heavy-fermion superconductor UPt_{3} is a unique spin-triplet superconductor with multiple superconducting phases. Here, we provide the first report on a first-principles analysis of the microscopic superconducting gap structure. We find that the promising gap structure is an unprecedented E_{2u} state, which is completely different from the previous phenomenological E_{2u} models. Our obtained E_{2u} state has in-plane twofold vertical line nodes on small Fermi surfaces and point nodes with linear dispersion on a large Fermi surface. These peculiar features cannot be explained in the conventional spin 1/2 representation, but is described by the group-theoretical representation of the Cooper pairs in the total angular momentum j=5/2 space. Our findings shed new light on the long-standing problems in the superconductivity of UPt_{3}.

Is the anisotropy of the upper critical field of Sr2RuO4 consistent with a helical p-wave state?

We calculate the angular and temperature T dependencies of the upper critical field Hc2(θ, ϕ, T) for the C4v point group helical p-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli limiting and strong spin-orbit coupling that locks the spin-triplet d-vectors onto the layers. Good fits to the Sr2RuO4 Hc2,a(θ, T) data of Kittaka et al (2009 Phys. Rev. B 80 174514) are obtained. Helical states with d(k) =kˆxxˆ −kˆyy and kˆyxˆ +kˆxy (or kˆxxˆ +kˆyyˆ and kˆyxˆ -kˆxyˆ ) produce Hc2(90°, ϕ, T) that greatly exceed (or do not exhibit) the fourfold azimuthal anisotropy magnitudes observed in Sr2RuO4 by Kittaka et al and by Mao et al (2000 Phys. Rev. Lett. 84 991), respectively.