Fig. 1: Charge density wave in CsV₃Sb₅.

a T-P phase diagram and crystal structure of CsV₃Sb₅ (space group P6 = mmm, no. 191). The phase diagram is separated by two characteristic pressures, P_c1 and P_c2. The curvature of resistivity at T_CDW changes sign at P_c1. At the same pressure, superconductivity reaches its first peak. The green markers indicate pressures where the high-pressure X-ray diffraction measurement was performed (Fig. 3). b A schematic of van Hove singularities at the M point of the hexagonal Brillouin zone. Theoretically, the van Hove filling induce Fermi surface instabilities with three nesting wavevector Q_1,2,3. Integrated CDW intensity vs temperature measured at Q_CDW = (3, 0.5 0.5) (c) and (3, 0.5, 0) (d) using meV-resolution elastic X-ray scattering. A sharp jump of the order parameter at the CDW transition indicates a weak first order phase transition and is consistent with nuclear magnetic resonance measurement⁴¹. The intensity jump is much stronger in the out-of-plane CDW peak at (3, 0.5, 0.5) compared to the in-plane one at (3.5, 0.5, 0). The inset of c shows the CDW peak intensity measured at 92.5 K and 92.75 K. The specific heat data shown in the inset of d reveal a sharp transition at 92.5 K, consistent with the diffraction data. The error bars in c, d represent 1-standard deviation assuming Poisson counting statistics.