Vibrational Entropy from Thermally-Driven Electronic Topological Transitions*

A novel thermally-driven electronic topological transition was discovered in the B2-ordered intermetallic compound FeTi. Ab-initio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic x-ray scattering, showed an anomalously large thermal softening of the M₅^– phonon mode that could not be explained by phonon-phonon interactions or electron-phonon interactions calculated at low temperatures. The softening is caused by an adiabatic electron-phonon interaction with an unusual temperature dependence. This interaction arises from the appearance of new features of the Fermi surface at elevated temperatures. This electronic topological transition (Lifshitz transition) was analyzed by electronic band unfolding, Fermi surface visualization, and enumerating the electron-phonon spanning vectors. These methodologies have also shown that there is a temperature-dependent evolution of the spanning vector distributions that is linked to anomalous thermal shifts in phonon energies in the A15 compound V₃Ge.