Colossal c-axis response and lack of rotational symmetry breaking within the kagome plane of the CsV₃Sb₅ superconductor

The kagome materials AV₃Sb₅ (A = K, Rb, Cs) host an intriguing interplay between unconventional superconductivity and charge-density-waves. Here, we investigate CsV₃Sb₅ by combining high-resolution thermal-expansion, heat-capacity and electrical resistance under strain measurements. We show that the superconducting and charge-ordered states strongly compete, and that this competition is dramatically influenced by tuning the crystallographic c-axis. Furthermore, we report the absence of additional bulk phase transitions within the charge-ordered state, notably associated with rotational symmetry-breaking within the kagome planes. This suggests that any breaking of the C₆ invariance occurs via different stacking of C₆-symmetric kagome patterns. Finally, we find that the charge-density-wave phase exhibits an enhanced A_1g-symmetric elastoresistance coefficient, whose large increase at low temperature is driven by electronic degrees of freedom.