Ultrafast dynamical response of strongly correlated oxides: Role of coherent optical and acoustic oscillations

We describe some general features in the transient behaviour of strongly correlated transition metal oxides, following ultrafast excitation by a femtosecond laser pulse. Our analysis is based on time-resolved reflectivity measurements on (V₁-_xCr_x)₂O ₃, a prototype Mott-Hubbard material, and the manganite compound (La_0.67Sr_0.33)MnO₃, performed over a time window of several tenths of ps. We point out the contribution of coherent lattice oscillations-both optical and acoustic-to the overall signal, and show how they can depend on the crystallographic orientation of the material with respect to the laser beam wavevector and polarisation. In particular, an acoustic wave is always found to be present in our measurements, and we show that its oscillating behaviour is superposed to the average time evolution of the material, which can be instead related in different ways to its electronic properties and thermodynamic phase. All these effects are to be taken into account in any further analysis leading to the estimation of relevant physical parameters for correlated materials, and they appear to play a significant role not only for reflectivity techniques, but also in other kinds of ultrafast pump-probe experiments.