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Abstract
Inversion symmetry is a key symmetry in unconventional superconductors, and even its local breaking can have profound implications. For inversion-symmetric systems, there is a competition on a microscopic level between the spin-orbit coupling associated with the local lack of inversion and hybridizing terms that ``restore'' inversion. Investigating a layered system with alternating mirror-symmetry breaking, we study this competition considering the spin response of different superconducting order parameters for the case of strong spin-orbit coupling. We find that signatures of the local noncentrosymmetry, such as an increased spin susceptibility in spin-singlet superconductors for $T\ensuremath{\rightarrow}0$, persist even into the quasi-three-dimensional regime. This leads to a direction-dependent spin response that allows us to distinguish different superconducting order parameters. Furthermore, we identify several regimes with possible topological superconducting phases within a symmetry-indicator analysis. Our results may have direct relevance for the recently reported Ce-based superconductor ${\mathrm{CeRh}}_{2}{\mathrm{As}}_{2}$ and beyond.
Highlights
In superconductors with inversion symmetry, even- and odd-parity gap functions are distinguished by symmetry, and they correspond via the Pauli principle to spin-singlet and spin-triplet pairing states, respectively
We focus on the most symmetric gap functions leading to three different gap structures: a spinsinglet that transforms like A1g, a spin-triplet order parameter with its d-vector in-plane that transforms like A2u, and a spin-triplet order parameter with the corresponding d-vector along the z axis that transforms like A1u in D4h [5]
In some globally centrosymmetric systems, the lack of inversion symmetry in subunits can influence the physical properties significantly. Such remnants of noncentrosymmetricity are particular interesting in the context of superconductivity, where inversion symmetry yields a strict distinction between spin-singlet and spin-triplet Cooper pairing
Summary
In superconductors with inversion symmetry, even- and odd-parity gap functions are distinguished by symmetry, and they correspond via the Pauli principle to spin-singlet and spin-triplet pairing states, respectively. Even in the case of dominant spin-singlet or spin-triplet order parameters, the magnetic response, such as the spin susceptibility or critical fields, is not a feasible distinguishing probe anymore, as for spin-singlet superconductors a finite spin susceptibility for T → 0 and unusually high critical fields can be expected. The recently discovered heavy-fermion superconductor CeRh2As2 with its tetragonal crystal structure belongs to the class of locally noncentrosymmetric superconductors It consists of layers with alternating inversionsymmetry breaking. Our model allows us to investigate how the system evolves from the 2D limit to the truly 3D case For this purpose, we first study the spin susceptibility in the normal state, which leads us to identify four distinct regions, going from quasi-two-dimensional (q2D) all the way to truly 3D. Note that since the system retains inversion, we can use the recently developed concept of symmetry indicators [15,16,17,18,19] to show that the system can realize both first- and second-order topological superconducting phases
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