Revealing Anharmonic Interactions as the Key Driver for Phase Transition in Charge Density Waves

A charge density wave (CDW) is a periodic modulation of electron density and lattice structure that arises at low temperatures. CDWs can be classified as commensurate, where the CDW period is an integer multiple of the lattice period, or incommensurate, where this relationship does not hold. These phase transitions are closely associated with remarkable phenomena such as colossal magnetoresistance, superconductivity, and electronic nematicity, motivating significant interest in understanding their underlying mechanisms. In this computational study, we explored the phase transitions in H-TaSe22, a prototypical CDW material, which exhibits transitions from commensurate to incommensurate and eventually to a normal phase as temperature increases. Using a temperature-dependent phase diagram generated from highly accurate interatomic potentials (<0.5 meV/atom) derived from first-principles calculations, we analyzed the role of anharmonic interactions in driving these transitions. Eigenmode decomposition of the total energy reveals that these anharmonic interactions are responsible for the commensuration locking and the appearance of satellite Fourier peaks, both of which are key features of the incommensurate CDW phase observed experimentally.