Tetragonal-to-orthorhombic distortion in CeRh₂As₂

CeRh₂As₂ is best known for its two superconducting phases, which are observed when a magnetic field is applied along the c axis of its tetragonal unit cell. The emergence of superconductivity, with Tc ≈ 0.31K, is preceded by an ordered state (phase I), with an ordering temperature T₀ ≈ 0.46K. The precise nature of the order parameter in phase I is still unknown, although evidence of dipolar ordering was found. To investigate a possible symmetry breaking at T₀, we conducted longitudinal thermal-expansion measurements along the crystallographic [100], [010], [110], and [11̄0] directions on a single-crystalline sample cut into an octagonal shape. Our results demonstrate the breaking of the fourfold in-plane symmetry below T₀, evidencing a small tetragonal-to-orthorhombic distortion (orthorhombicity δ~ 10⁻⁷). The breaking of the tetragonal symmetry at T₀ might imply that the superconducting order parameter that develops within phase I cannot be multi-component. More generally, the distortion places restrictions on the possible order parameters for phase I.