Driving Mechanism and Dynamic Fluctuations of Charge Density Waves in the Kagome Metal ScV${}_6$Sn${}_6$

Abstract

In a new family of V-based kagome metals RV${}_6$Sn${}_6$ (R = Sc, Y, La), only ScV${}_6$Sn${}_6$ was observed to exhibit the unusual charge density wave (CDW) with a $\sqrt{3}\times \sqrt{3}$ in-plane ordering and a tripling of the unit cell along the $c$-axis. However, the driving mechanism of such a $\sqrt{3}\times \sqrt{3}\times$3 CDW order is elusive. Here, using first-principles density-functional theory calculations, we demonstrate that the $\sqrt{3}\times \sqrt{3}\times$2 or $\sqrt{3}\times \sqrt{3}\times$3 CDW order is driven by a Jahn-Teller-like effect where the interlayer dimerization of Sn atoms in kagome bilayers is accompanied by a charge redistribution between such Sn atoms and its associated gap opening. It is revealed that the free energy of the $\sqrt{3}\times \sqrt{3}\times$3 phase becomes lower than that of the $\sqrt{3}\times \sqrt{3}\times$2 phase above $T_{\mathrm{CDW}}$ mostly due to the emergence of a large configurational entropy. Such a high-entropy phase shows dynamic fluctuations between its degenerate configurations but, as the temperature lowers below $T_{\mathrm{CDW}}$, it can be kinetically trapped to release a sizable entropy. Our findings not only identify the driving mechanism and order-disorder transition of the CDW in ScV${}_6$Sn${}_6$ but also provide an ideal platform for investigating strong CDW fluctuations in V-based kagome metals.