IQIM Postdoctoral and Graduate Student Seminar

Abstract: Superconductors can be classified into two distinctive groups, with either spin-singlet or spin-triple pairing.  Almost all superconductors, including the high T_c cuprates as well as the recently discovered pnictides and chalcogenides, have singlet pairing, and triplet-paired materials have been extremely difficult to find.

Strontium ruthenate (Sr₂RuO₄) is one of the few prime candidates for a triplet-pairing superconductor, but despite a large theoretical and experimental effort an unambiguous determination of the detailed structure of the order parameter (spatial and spin part) in this material has proved elusive.  Multiple experimental and theoretical studies have provides compelling support for triplet pairing and an odd-parity, p-wave order parameter symmetry.  At the same time, seemingly contradictory experimental results have left important open questions concerning the detailed structure and coupling of the orbital and spin parts of the order parameter.  One example of this predicament is conflicting evidence as to whether the p-wave order parameter is chiral.

Here we present the results of small-angle neutron scattering studies of the vortex lattice in Sr₂RuO₄ with the field applied close to the crystalline basal plane.  Taking advantage of the transverse magnetization in this highly anisotropic superconductor, it was possible to measure the vortex lattice anisotropy as a function of the field angle.  From the measurements we were able to determine the intrinsic superconducting anisotropy between the c-axis and the Ru-O basal planes (~60), which greatly exceeds the upper critical field anisotropy (~20).  This imposes significant constraints on possible models of triplet pairing in Sr₂RuO₄ and raises questions concerning the direction of the zero spin projection axis.