Disentangling thermal and non-thermal excited states in a charge-transfer insulator by ultrafast optical spectroscopy

by Goran Zgrablic, Sincrotone Trieste

Thursday 29 Apr 2010, 11:00 → 12:00 Europe/Berlin

AER 19, Room 3.11 (AER 19)

Pump-probe setup with broadband detection from 330 to 950 nm and 50 fs time resolution has been used to study the excited state dynamics of a model strongly correlated system – insulating CuGeO3 crystal [1]. By changing the energy and polarization of the pump photons we are able to clarify the origin of the spectrally-resolved transmissivity variation. When the system is excited at 3.14 eV and polarization is along the c-axis, the charge-transfer edge broadens significantly on the sub-ps timescale. The broadening persists on a time scale of several hundreds of ps and cannot be attributed to the local increase of the effective temperature as a consequence of the pump pulse absorption. In fact, in this experimental configuration the pump pulse photo-injects the holes to the Cu-O4 plaquette where they become delocalized on the O-2p oxygen orbitals [2]. On the contrary, for the pump pulse energy of 3.14 eV and polarization along the b-axis, or 1.55 ev pump energy and polarization along the c-axis, the excitonic delocalized transitions are strongly quenched. In the former configuration the holes become localized in the O-2p orbitals, whereas in the latter phonon-assisted Cu 3d-3d intraband transitions are induced. For both pump configurations the charge-transfer edge broadening is of thermal origin as it can attributed to the local heating of the lattice. The measured modifications of the optical properties indicate that delocalization of holes strongly perturbs the potential experienced by electrons on the Cu-3d levels and leaves the system in a non-thermal metastable phase. Other activities will also be briefly presented: measurements of time resolved dielectric function of femtosecond melted silicium, charge transfer dynamics in layers of photomagnetic molecules FeCo(CN)6 and some preliminary XMCD measurements on permalloy using our table top HHG source. [1] C. Gianetti, G. Zgrablić, C. Consani, A. Crepaldi, D. Nardi, G. Ferrini, G. Dhalenne, A. Revcolevschi, F. Parmigiani, Phys. Rev. B, 80, 235129 [2] H. Eskes, L. H. Tjeng, G. Sawatzky, Phys. Rev. B 41, 288 (1990)