Optoacoustical spectrum of the metal-insulator transition compound NdNiO3: Sub-picosecond pump-probe study

A pump-probe optical spectroscopy technique was used to study the time-resolved reflectivity ΔR(t)/R in the range 0-160ps of the metal-insulator transition compound NdNiO3. It enables, for the first time, the direct real-time observation of the non-equilibrium electron dynamics in the insulating and in the metallic state of that compound and the acoustical response. Strong change of the transient reflectivity is observed as a function of temperature. First of all, the magnitude of the fast component of transient reflectivity ({ΔR(t)/R}max, 0-3ps) increases drastically when T<TMI. Moreover, the relaxation time of the fast-component increases when the insulating state is reached. Vanishing of the acoustic echo magnitude with decreasing temperature is also reported. All these singular properties could be connected to the gap opening mechanism below MIT