Ab initio molecular dynamics, supported by inelastic neutron scattering and nuclear resonant inelastic x-ray scattering, showed an anomalous thermal softening of the $M_5^{-}$ phonon mode in $B2$-ordered FeTi that could not be explained by phonon-phonon interactions or electron-phonon interactions calculated at low temperatures. A computational investigation showed that the Fermi surface undergoes a novel thermally driven electronic topological transition, in which new features of the Fermi surface arise at elevated temperatures. The thermally induced electronic topological transition causes an increased electronic screening for the atom displacements in the $M_5^{-}$ phonon mode and an adiabatic electron-phonon interaction with an unusual temperature dependence.

• Received 22 March 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.076402