The discovery of a rotational symmetry breaking, or nematic, state in the superconducting doped topological insulator MxBI2Se3 (M = Cu, Sr, Nb) indicates a multicomponent superconducting order parameter which is topological in nature. Many questions remain in this family of materials. In particular, the nematic axis of twofold symmetry is always pinned along one of three rotationally equivalent directions in the threefold symmetric basal plane in the rhombohedral crystal, suggestive of nematic domains in a single crystal. Crystallographic strain has been proposed as the origin of the choice of pinning axis. Furthermore, depending on the choice of pinning axis, nodes in the superconducting gap are a theoretical possibility. In this talk, I will discuss the origins and consequences of nematic pinning of the superconducting state, focusing on the SrxBi2Se3 compound, for which cm-scale single crystals approaching 100% superconducting volume fraction are available. I will summarize our magnetotransport, magnetization, penetration depth, and calorimetry measurements, which demonstrate the nematic behavior, and will discuss the impact of controlled disorder via proton irradiation on the nematic superconducting state. In a nodeless anisotropic superconductor, increasing disorder scattering should systematically decrease anisotropy; in a nodal superconductor, increasing disorder should rapidly suppress the superconducting transition. Neither effect is observed, which may be a consequence of the topological nature of the state. Additionally, I will discuss our recent work on unique multimodal sub-kelvin synchrotron diffraction measurements to look for a structural origin of the choice of nematic pinning axis as well as results from other groups using more conventional techniques.
Work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. This research used resources of the APS, a US DOE Scientific User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. Work at Brookhaven was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy.
Keywords: Topological superconductor, Nematic superconductor