Four-layer charge density waves and chirality in CsV$_3$Sb$_5$

Abstract: The kagome superconductor CsV$_3$Sb$_5$ is the only one in the AV$_3$Sb$_5$ family (A=K,Rb,Cs) that shows a $2\times2\times4$ charge-density-wave (CDW) ground state competing with the more common $2\times2\times2$. In addition, it is also the only one that shows second-harmonic transport and thus broken inversion symmetry, suggesting these two features are connected. In this work, we test whether the $2\times2\times4$ CDW can break inversion symmetry by analyzing its stacking energetics with a real-space Ginzburg-Landau free energy. In the limit where each layer is forced to adopt a fixed, threefold symmetric tri-hexagonal configuration, we find an analytical phase diagram mostly occupied by inversion preserving AB and ABCD stacked solutions. Relaxing this rigid layer constraint in a physically meaningful way consistent with \emph{ab initio}--calculated parameters, we find several new phases including an AABC solution and a distorted ABCD solution, which break inversion and all mirrors and are thus chiral. However, these phases occupy only small fractions of phase space, suggesting other mechanisms for inversion symmetry breaking may be at play in CsV$_3$Sb$_5$.