First-order phase transition driven by competing charge-order fluctuations in 1T'-TaTe${}_2$

Abstract

First-order phase transitions, characterized by a discontinuous change in the order parameter, are intriguing phenomena in condensed matter physics. However, the underlying, material-specific, microscopic mechanisms often remain unclear. Here, we unveil a high-temperature incommensurate charge-order precursor with the wave vector ${\mathbf{q}}^{\ast }=(0,\frac{1}{4}+\hat{I}´,\frac{1}{2})$ in the 1T' phase of TaTe${}_2$, which competes with fluctuating high-temperature Ta trimer bonding states at ${\mathbf{q}}_{\mathrm{CO}}=(0,\frac{1}{3},0)$. The precursor state follows the temperature dependence of the hidden incommensurability of the $\text{quasi}$-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at ${\mathbf{q}}_{\mathrm{CO}}$, characterized by a charge disproportionation of the inequivalent Ta sites, appears to be driven by local chemical bonding. Dynamical lattice calculations identify an imaginary optical mode at ${\mathbf{q}}^{\ast }$, involving an in-plane vibration of the Ta atoms forming a chain-like structure that renormalizes below $T_{\mathrm{CO}}$. Our experimental and theoretical observations suggest that the controversial first-order phase transition, as captured by phenomenological Ginzburg-Landau theory, results from the competition between two order parameters: one involving Fermi surface nesting and the other involving local chemical bonding.