Superconductivity of anomalous pseudospin in nonsymmorphic materials

Abstract

Spin-orbit coupling driven by broken inversion symmetry (I) is known to lead to unusual magnetic response of superconductors, including extremely large critical fields for spin-singlet superconductors. This unusual response is also known to appear in materials that have I, provided there is local I-breaking: fermions participating in superconductivity reside on crystal sites that lack I. Here we show that this unusual response exists even when the crystal sites preserve I. Indeed, we argue that the symmetry of Kramers degenerate fermionic pseudospin is more relevant than the local crystal site symmetry. We examine and classify nonsymmorphic materials with momentum space spin-textures that exhibit an anomalous pseudospin with different symmetry properties from usual spin-1/2. Since this anomalous pseudospin does not depend on the existence of local I-breaking crystal sites, and since it optimizes the unusual magnetic response traditionally associated with locally noncentrosymmetric superconductors, it dramatically extends the range of relevant materials. We further show that this anomalous pseudospin leads to fully gapped “nodal” superconductors and provides additional insight into the breakdown of Blount's theorem for pseudospin triplet superconductors. We apply our results to UPt_{3}, BiS_{2}-based superconductors, Fe-based superconductors, and paramagnetic UCoGe.