Ab initio investigation of photoinduced non-thermal phase transition in β -cristobalite

Using the linear-response method, we investigate the phonon properties of β -cristobalite crystal under electronic excitation effect. We find that the transverse-acoustic phonon frequency becomes imaginary as the electron temperature is increased, which means that the lattice of β -cristobalite becomes unstable under intense laser irradiation. In addition, for the optic phonon mode, the LO(H)-TO(H) splitting disappears when the electronic temperature reaches a certain value, corresponding to the whole transverse-acoustic phonon branches becoming negative. It means that the electronic excitation destroys the macroscopic electric field of β -cristobalite. Based on the calculated phonon band structures, some thermodynamic properties are calculated as a function of temperature at different electronic temperatures. These investigations provide evidence that non-thermal melting takes place during a femtosecond pulse laser interaction with β -cristobalite.