PHOTOEXCITED CARRIER RELAXATION IN YBA2CU3O7-DELTA BY PICOSECOND RESONANT RAMAN-SPECTROSCOPY

The temperature dependence of the energy relaxation of photoexcited (P E) carriers is used as a probe of the electronic structure of YBa2Cu3O 7-delta in the insulating (delta approximate to 0.8) and metallic (del ta approximate to 0.1) phases. The energy relaxation rate to phonons i s obtained by measuring the nonequilibrium phonon occupation number, n (neq), with pulsed Raman Stokes/anti-Stokes spectroscopy using 1.5 and 70 ps long laser pulses. We can distinguish between relaxation via ex tended band states and localized states, since theoretically in the fo rmer, the relaxation is expected to be virtually T independent, while in the latter it is strongly T dependent. From the experiment-which sh ows strong temperature dependence of n(neq)-we deduce that at least pa rt of the PE-carrier relaxation proceeds via hopping between localized states and we propose a simple theoretical model of the relaxation pr ocess. In addition, we compare the coupling of different vibrational m odes to the carriers to find that the apical O vibrational mode is sig nificantly more involved in the energy relaxation process than the in- plane 340 cm(-1) mode. This implies that the localized states are main ly (but not entirely) coupled to out-of plane vibrations.