Resonant x-ray diffraction measurements in charge ordered kagome superconductors KV₃Sb₅and RbV₃Sb₅

We report on (resonant) x-ray diffraction experiments on the normal state properties of kagome-lattice superconductors KV₃Sb₅and RbV₃Sb₅. We have confirmed previous reports indicating that the charge density wave (CDW) phase is characterized by a doubling of the unit cell in all three crystallographic directions. By monitoring the temperature dependence of Bragg peaks associated with the CDW phase, we ascertained that it develops gradually over several degrees, as opposed to CsV₃Sb₅, where the CDW peak intensity saturates promptly just below the CDW transition temperature. Analysis of symmetry modes indicates that this behavior arises due to lattice distortions linked to the formation of CDWs. These distortions occur abruptly in CsV₃Sb₅, while they progress more gradually in RbV₃Sb₅and KV₃Sb₅. In contrast, the amplitude of the mode leading to the crystallographic symmetry breaking fromP6/mmmtoFmmmappears to develop more gradually in CsV₃Sb₅as well. Diffraction measurements close to the V K edge and the Sb L₁edge show no sensitivity to inversion- or time-symmetry breaking, which are claimed to be associated with the onset of the CDW phase. The azimuthal angle dependence of the resonant diffraction intensity observed at the Sb L₁edge is associated with the difference in the population of unoccupied states and the anisotropy of the electron density of certain Sb ions.