Multiphase superconductivity in CeRh₂As₂

The new superconductor CeRh₂As₂ that was recently discovered at our institute by the group of S. Khim represents a large part of our current activity. We revealed that this compound has an extraordinary phase diagram showing a huge critical field and a phase transition inside the superconducting state for a magnetic field applied along the crystallographic c-axis. For fields in the plane, the phase diagram is more conventional but with an enhanced critical field. The submitted manuscript was published in Science. Such multiphase superconductivity is extremely rare in nature. In collaboration with Daniel Agterberg (U. Wisconsin) and Philip Brydon (U. Otago), leading theorists in Rashba superconductivity, we could identify the possibility that this transition is one from an even parity to an odd parity superconducting state, induced by spin-orbit coupling due to local symmetry. At the Ce position, the inversion symmetry is locally broken, although the crystal is centrosymmetric. As such, it appears to be the first compound where an odd parity superconducting state can be realised in a spin singlet pairing channel by a change of sign of the superconducting order parameter between two adjacent Ce layers. Additionally to the extraordinary superconducting state, the compound also shows signatures of quantum criticality and multipolar order, two other fascinating phenomena that we explore further.

Our group aims at using CeRh₂As₂ as a new platform to study the interplay of strong correlations with locally broken inversion symmetry and the subsequent Rashba interaction and their effect on superconductivity and other orders.