Structural instability and charge modulations in the kagome superconductor AV3Sb5

Abstract

Recently, both charge density wave (CDW) and superconductivity have been observed in the kagome compounds $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$. However, the nature of the CDW that results in many novel charge modulations is still under hot debate. Based on the first-principles calculations, we discover two kinds of CDW states in $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$, namely, the trimerized-hexamerized $2\ifmmode\times\else\texttimes\fi{}2$ phase and the dimerized $4\ifmmode\times\else\texttimes\fi{}1$ phase. Our phonon spectrum and electronic Lindhard function calculations reveal that the most intensive structural instability in $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ originates from a combined in-plane vibration mode of V atoms through the electron-phonon coupling, rather than the Fermi surface nesting effect. More importantly, a metastable $4\ifmmode\times\else\texttimes\fi{}1$ phase with the V-V dimer pattern and twofold symmetric bow-tie-shaped charge modulation is revealed in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$. Further analyses demonstrate that both the in-plane phonon instability and $c$ direction interaction play important roles during the $4\ifmmode\times\else\texttimes\fi{}1$ phase formation, which provides key understanding for the mechanism of the $4\ifmmode\times\else\texttimes\fi{}1$ CDW and relative novel phenomena in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$.