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 [1]. 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 [2]. 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 [3]. 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.

[1] S. Khim, J. F. Landaeta, J. Banda, N. Bannor, M. Brando, P. M. R. Brydon, D. Hafner, R. Küchler, R. Cardoso-Gil, U. Stockert, A. P. Mackenzie, D. F. Agterberg, C. Geibel, and E. Hassinger, Science 373, 1012 (2021).
[2] T. Yoshida, M. Sigrist, and Y. Yanase, Phys. Rev. B 86, 134514 (2012).
[3] D. C. Cavanagh, T. Shishiduo, M. Weinert, P. M. R. Brydon, and D. F. Agterberg, arXiv:2106.02698 (2021).

Keywords: odd-parity superconductivity, spin-orbit coupling, heavy fermion