Superconductors with non-trivial pairing symmetries expand our understanding of correlated quantum matter and show promise for applications in quantum computing. Odd-parity superconductivity is interesting in this regard due to its robustness to magnetic field and possible topological surface states. The phenomenon only occurs in a few materials, the most recognised cases of which are strongly correlated uranium-based systems with weak ferromagnetism. Another candidate is CeRh2As2, which exhibits a magnetic-field-induced transition between two superconducting phases, currently understood as states of even- and odd-parity pairing. Here, the odd-parity pairing is thought to be stabilisied not by ferromagnetism, but by the staggered Rashba spin-orbit interaction caused by the absence of inversion symmetry at the Ce sites. Since the tetragonal crystal structure is centrosymmetric, states of distinct parity are allowed [1,2]. But the superconductivity is not the only mystery of CeRh2As2. Similarly to other unconventional superconductors, the material hosts a coexisting weak ordered state named Phase I that can be suppressed by pressure [3-5]. While antiferromagnetic order in Phase I is evidenced by NMR/NQR [6] and muSR [7] measurements, the strong in-plane field enhancement of the transition temperature is hard to reconcile with a simple magnetic order but can be explained by considering quadrupolar degrees of freedom [3,8]. This unconventional magnetic state and its role for superconductivity are currently in the focus of research on this compound. In my talk, I will highlight new experimental results under different tuning parameters such as pressure and magnetic field, showing an absence of clear quantum criticality caused by the suppression of Phase I and evidence for anisotropic coupling of the quadrupolar component of the ordered state in CeRh2As2.

References

[1] S. Khim & J. Landaeta et al., Science 373, 1012–1016 (2021).

[2] J. Landaeta et al., Phys. Rev. X 12, 031001 (2022).

[3] D. Hafner et al., Phys. Rev. X 12, 011023 (2022).

[4] M. Pfeiffer et al., Phys. Rev. Lett. 133, 126506 (2024).

[5] K. Semeniuk et al., Phys. Rev. B 110, L100504 (2024).

[6] M. Kibune et al., Phys. Rev. Lett. 128, 057002 (2022).

[7] S. Khim et al., Phys. Rev. B 111, 115134 (2025).

[8] B. Schmidt and P. Thalmeier, Phys. Rev. B 110, 075154 (2024).