Precursor phase with full phonon softening above the charge-density-wave phase transition in $2H$-TaSe${}_2$

Abstract

Research on charge-density-wave (CDW) ordered transition-metal dichalcogenides continues to unravel new states of quantum matter correlated to the intertwined lattice and electronic degrees of freedom. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of the canonical CDW compound $2H$-TaSe${}_2$ complemented by angle-resolved photoemission spectroscopy. Our results rule out the central-peak scenario for the CDW transition in $2H$-TaSe${}_2$ and provide evidence for a novel precursor phase above the CDW transition temperature $T_{CDW}$. The phase at temperatures between $T^{\ast }(=128.7,\mathrm{K})$ and $T_{CDW}(=121.3\mathrm{K})$ is characterized by a fully softened phonon mode and medium-range ordered ($\hat{I}{¾}_{corr}=100\mathring{\mathrm{A}}-200\mathring{\mathrm{A}})$ static CDW domains. Only $T_{CDW}$ is detectable in our photoemission experiments. Thus, $2H$-TaSe${}_2$ exhibits structural before electronic static order and emphasizes the important lattice contribution to CDW transitions.