First-order phase transition driven by competing charge-order fluctuations in
Phys. Rev. B 114, 065120 – Published 15 July, 2026
DOI: https://doi.org/10.1103/xg7k-8tbx
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 in the 1T' phase of , which competes with fluctuating high-temperature Ta trimer bonding states at . The precursor state follows the temperature dependence of the hidden incommensurability of the quasi-1D nested Fermi surface. In contrast, the low-temperature commensurate charge order at , 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 , involving an in-plane vibration of the Ta atoms forming a chainlike structure that renormalizes below . 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.
