Colossal c-axis response and lack of rotational symmetry breaking within the kagome plane of the CsV$_3$Sb$_5$ superconductor

Abstract

The kagome materials AV4$_3$Sb$_5$ (A = K, Rb, Cs) host an intriguing interplay between unconventional superconductivity and charge-density-waves. Here, we investigate CsV$_3$Sb$_5$ by combining high-resolution thermal-expansion, heat-capacity and electrical resistance under strain measurements. We directly unveil that the superconducting and charge-ordered states strongly compete, and that this competition is dramatically influenced by tuning the crystallographic c-axis. In addition, 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 C$_6$ invariance occurs via different stacking of C$_6$-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.