Origin of competing charge density waves in kagome metal ScV6Sn6

Abstract

Understanding competing charge density wave (CDW) orders in the bilayer kagome metal ScV₆Sn₆ remains challenging. Experimentally, upon cooling, short-range order with wave vector ${\mathbf{q}}_2=\left(\frac{1}{3},\frac{1}{3},\frac{1}{2}\right)$ forms, which is subsequently suppressed by the condensation of long-range ${\mathbf{q}}_3=\left(\frac{1}{3},\frac{1}{3},\frac{1}{3}\right)$ CDW order at lower temperature. Theoretically, however, the q₂ CDW is predicted as the ground state, leaving the CDW mechanism elusive. Here, using anharmonic phonon-phonon calculations combined with density functional theory, we predict a temperature-driven structural phase transitions from the high-temperature pristine phase to the q₂ CDW, followed by the low-temperature q₃ CDW, explaining experimental observations. We demonstrate that semi-core electron states stabilize the q₃ CDW over the q₂ CDW. Furthermore, we find that the out-of-plane lattice parameter controls the competing CDWs, motivating us to propose compressive bi-axial strain as an experimental protocol to stabilize the q₂ CDW. Finally, we suggest Ge or Pb doping at the Sn site as another potential avenue to control CDW instabilities. Our work provides a full theory of CDWs in ScV₆Sn₆, rationalizing experimental observations and resolving earlier discrepancies between theory and experiment.

Competing charge density wave states were recently reported in the bilayer Kagome metal ScV₆Sn₆, but the mechanism remained unclear. Here the authors present a microscopic theory, highlighting the crucial role of anharmonic phonon-phonon interactions, and reconcile previous experimental and theoretical reports.