Kagome Materials I: SG 191, ScV$_6$Sn$_6$. Flat Phonon Soft Modes and Unconventional CDW Formation: Microscopic and Effective Theory
Kagome Materials with flat bands exhibit wildly different physical properties depending on symmetry group, and electron number. For the case of ScV$_6$Sn$_6$ in space group 191, we investigate the existence of a charge density wave (CDW) at vector $\bar{K}=(\frac{1}{3},\frac{1}{3},\frac{1}{3})$ and its relationship with the phonon behavior. The experimental findings reveal a $\sim$95K CDW without nesting/peaks in the electron susceptibility at $\bar{K}$. Notably, ScV$_6$Sn$_6$ exhibits a collapsed phonon mode at $H=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ and an imaginary flat phonon band in the vicinity of $H$. The soft phonon is attributed to triangular Sn ($Sn^T$) mirror-even vibrations along the $z$-direction. We develop a simple force constant model to describe the entire soft phonon dispersion. By employing a new (Gaussian) approximation of the hopping parameter, we demonstrate the renormalization of the phonon frequency and the consequent collapse of the $H$ phonon. Additionally, we propose an effective model with two order parameters (OPs) to explain the appearance of the CDW at $\bar{K}$, which competes with the collapsed phonon at $H$. Through comparisons with experimental data, we show that the $H$ OP undergoes a second-order phase transition while exhibiting substantial fluctuations, ultimately inducing the first-order transition of the $\bar{K}$ OP. Furthermore, we extend our analysis to the similar compound YV$_6$Sn$_6$, which lacks a CDW phase, attributing this difference to the participation of the heavier Y atom in the out-of-plane phonon behavior. Our comprehensive study not only elucidates the CDW in ScV$_6$Sn$_6$ but also presents a significant advancement in modeling complex electronic systems, fostering collaborations between ab-initio simulations and analytical approaches.