Superconductivity of anomalous pseudospin

Abstract

In materials with both time-reversal (T ) and inversion symmetry (I), superconductivity is formed by pairing fermion pseudospin partners at momenta k and −k. Typically, pseudospin shares the same symmetry properties as usual spin-1/2. Here we consider non-symmorphic materials with momentum space spin-textures that exhibit an anomalous pseudospin with different symmetry properties than usual spin-1/2. We provide a comprehensive list of space groups for which anomalous pseudospin occurs on planes in momentum space and carry out a complete categorization and analysis of superconductivity for Fermi surfaces centered on all possible T , I invariant momenta (TRIM) in these planes. We show that superconductivity from this anomalous pseudospin leads to a variety of unusual consequences for superconductivity including: extremely large Pauli limiting fields and residual Knight shifts for pseudospin singlet superconductors; field induced pair density wave states; field induced pseudospin singlet to pseudospin triplet transitions; fully gapped 'nodal' superconductors; and additional insight into the breakdown of Blount's theorem for pseudospin triplet superconductors. We apply our results to UPt3, BiS2-based superconductors, Fe-based superconductors, and paramagnetic UCoGe. I. INTRODUCTIONMomentum space spin-textures of electronic bands are known to underlie spintronic and superconducting properties of quantum materials [1-3]. In the spintronics context, Rashba-like spin textures allow control of electronic spin through applied electric fields [1,3]. In superconductors, these same spin textures lead to unusual and counter-intuitive magnetic response, such as the robustness of spin-singlet superconductivity to applied magnetic fields, pair density wave states, and singlet-triplet mixing [2]. While such spin-textures are common when inversion symmetry is broken, it has been realized that these can occur when inversion symmetry is present. This has lead to the notion of hidden spin-textures [4] and locally non-centrosymmetric superconductivity [5], where inversion related sectors each allow a Rashba-like spin-texture due to the local inversion symmetry breaking. These spin-textures are of opposite sign on the two sectors, so that global inversion symmetry is restored. These hidden spin-textures allows the novel physics associated with spin-orbit coupling (SOC) to emerge even when inversion symmetry is not broken. It further allows for new physics to emerge. One notable example is a field induced transition from an even-parity (pseudospin singlet) to odd-parity (pseudospin triplet) observed in CeRh 2 As 2 [6][7][8][9].Key to observing novel physics associated with these spin-textures in inversion symmetric materials, is that the inversion related sectors are weakly coupled [5,[9][10][11]. Theoretical proposals for how to achieve this fall under two approaches: the first is to tailor weak coupling between the inversion related sectors, for example by separating two inversion symmetry related layers so that the interlayer coupling...