Laser-induced coherent acoustical phonons mechanisms in the metal-insulator transition compoundNdNiO3: Thermal and nonthermal processes

Abstract

Coherent acoustic phonon generation processes on the picosecond time scale were investigated in the metal-insulator transition compound ${\text{NdNiO}}_{3}$. Time-resolved optical pump-probe methods were employed. While at high temperature thermoelastic process drives the photogeneration of acoustic phonons, an additional contribution can be identified when the transition to the insulating state at $T<{T}_{\text{MI}}$ takes place. The latter additional photogenerated stress is possibly of electronic stress nature (deformation-potential mechanism). We show that it competes with thermoelastic stress, leading to reduction in the magnitude of a photogenerated acoustic phonon pulse. In that insulating state, we suggest that the electron-phonon deformation-potential coupling in ${\text{NdNiO}}_{3}$ leads to a laser-induced lattice cell volume contraction. Furthermore, in the temperature range where a first-order phase transition takes place and where both metallic and insulating metastable phases coexist, the photogenerated ultrashort picosecond acoustic pulse magnitude cannot be described by a classical thermoelastic process. We suggest that such a phenomenon is connected to unusual thermoelasticity in a metastable state. In particular, we discuss the definition of the thermal-expansion coefficient of solids in a metastable state subjected to ultrafast optical excitation.