Superconductivity of anomalous pseudospin

TL;DR

This paper reveals that anomalous pseudospin symmetry in certain superconductors causes unusual magnetic responses, independent of local inversion symmetry breaking, broadening the understanding of superconducting behaviors in non-symmorphic materials.

Contribution

It demonstrates that anomalous pseudospin, not dependent on local inversion symmetry breaking, explains unusual magnetic responses in a wide range of non-symmorphic superconductors.

Findings

Anomalous pseudospin exists even with global inversion symmetry.

This pseudospin leads to fully gapped 'nodal' superconductors.

It provides insights into the breakdown of Blount's theorem.

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 non-symmorphic materials with momentum space spin-textures that exhibit an anomalous pseudospin with different symmetry properties than usual spin-1/2. We find that this anomalous pseudospin does not depend on the existence of local $I$ breaking crystal sites and it optimizes the unusual magnetic response traditionally associated with locally noncentrosymmetric superconductors, dramatically extending the range of relevant materials. We further show 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.