The superconducting state in the heavy fermion compound CeRh2As2 has staggering properties: the upper critical fields far exceed the Pauli limit, and shows evidence of a field-induced transition between even- and odd-parity phases . This is believed to arise from the locally noncentrosymmetric structure of the crystal, where spin-orbit coupling at the two Ce sites of the unit cell stabilizes an odd-parity “staggered” state at high fields, characterized by a sign reversal of the superconducting order parameter between the two sublattices . This scenario requires that the spin-orbit coupling is much larger than the intersublattice hopping, which is a stringent condition. In this talk I argue that the P4/nmm nonsymmorphic crystal structure of CeRh2As2 enforces an unusual spin structure near Brillouin zone boundaries that ensures large spin-orbit interactions in these regions of momentum space . Assuming that the dominant contribution to the DOS at the Fermi energy comes from Fermi surfaces near the Brillouin zone boundary, this readily permits the high-temperature field-induced transition into the staggered state, and naturally explains the key features of the phase diagram. Including f-electron correlations in a DFT calculation, we find a Fermi surface consistent with our theory, as shown in the figure. Finally, I comment on the relevance of our results to superconducting FeSe, which also crystallizes in a P4/nmm structure.
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Keywords: odd-parity superconductivity, spin-orbit coupling, heavy fermion