Optical investigation of coherent and thermal phonons in high-T c superconductors

Low-energy phonons in NdBa₂Cu₃O_7?x and Bi₂Sr₂CaCu₂O_8+x superconducting single crystals have been studied by Raman scattering and reflectivity measurements with femtosecond-scale resolution. Raman scattering provides information on equilibrium thermal phonons, whereas information about a coherent state of the phonon system is obtained by measuring in the time domain when the phonon system is pumped by an ultrashort pulse and subsequently probed by a properly gated second pulse. It is shown that both methods yield similar results for the phonon mode energies, while the energy relaxation and dephasing rates exhibit a tendency to disagreement.