Non-equilibrium electronic Grüneisen parameter

The electronic Grüneisen parameter, γ _e , is a fundamental parameter characterizing the relationship between thermal expansion and electronic specific heat. Conventionally, γ _e is measured at low temperatures for minimizing the lattice contribution to thermal expansion. In this paper, we extract γ _e by separating the electron and lattice contributions in time domain using ultrashort pulse excitation. We show that γ _e cannot be considered as a constant during the electron-lattice thermal nonequilibrium state. Instead, a revised expression for γ _e is proposed for the non-equilibrium electron–phonon system.