Charge Density Wave Order and Fluctuations above T_CDW and below Superconducting T_c in the Kagome Metal CsV₃Sb₅

The phase diagram of the kagome metal family AV(3)Sb(5) (A = K, Rb, Cs) features both super-conductivity and charge density wave (CDW) instabilities, which have generated tremendous recent attention. Nonetheless, significant questions remain. In particular, the temperature evolution and demise of the CDW state has not been extensively studied, and little is known about the coexistence of the CDW with superconductivity at low temperatures. We report an x-ray scattering study of CsV3Sb5 over a broad range of temperatures from 300 to similar to 2 K, below the onset of its superconductivity at T-c similar to 2.9 K. Order parameter measurements of the 2 x 2 x 2 CDW structure show an unusual and extended linear temperature dependence onsetting at T* similar to 160 K, much higher than the susceptibility anomaly associated with CDW order at T-CDW = 94 K. This implies strong CDW fluctuations exist to similar to 1.7 x T-CDW. The CDW order parameter is observed to be constant from T = 16 to 2 K, implying that the CDW and superconducting order coexist below T-c, and, at ambient pressure, any possible competition between the two order parameters is manifested at temperatures well below T-c, if at all. Anomalies in the temperature dependence in the lattice parameters coincide with T(CDW )for c(T) and with T* for a(T)