Synthetic Metals, 29 (1989) F563-F568 F563 PHOTOINDUCED ABSORPTION IN THE LA-CU-O AND Y-BA-CU-O SYSTEMS J.M. GINDER, M.G. ROE," R.P. MCCALL, T. KUSUMOTO,t and A.J. EPSTEIN Department of Physics,The Ohio StateUniversity, Columbus, Ohio 43210-1106(U.S.A.) E. E H R E N F R E U N D Department of Physics and Solid State Institute, Technion, Halfa (Israel) Y. SONG and J.R. GAINES Department of Physics, University of Hawaii, Honolulu, Hawaii 96822 (U.S.A.) S.I. SHAH Central Research and Development Station, E.I. DuPont de Nemours & Company, Wilm- ington, Delaware 19898 (U.S.A.) ABSTRACT Near-steady-state photoinduced absorption experiments on samples belonging to the high-Te families of materials give insight into the types of charged excitations in them, as well as the nature of their ground states and mechanisms of superconductivity. In powdered sam- pies of La2CuO4-t at low temperature, excitation by 2.54 eV laser photons induces several features: bleaching above 2 eV, photolumlnescence whose intensity increases rapidly above 2 eV, and two new absorptions at 0.5 and 1.4 eV. The behavior of these photoinduced fea- tures suggests that a bimolecular type process governs the decay of the induced excitations. The presence of the photoinduced bleaching and photolumlneseence is consistent with a 2 eV energy gap, while the photoinduced absorptions suggest that defect states in the gap are created. Similar photoinduced experiments on thin Klms of YBa2CusOffi also reveal several features. We examine our experimental results in light of recently developed theories that predict soliton, polaron, or bipolaron defects to be the low-lylng excited states of these materials. INTRODUCTION Much effort has been placed over the past two years in studying the mechanisms of high temperature superconductivity in the La-Sr(Ba)-Cu-O and Y-Ba-Cu-O systems. Various * Present address: Naval Research Laboratory, Washington D.C. 20375 (U.S,A.) t Permanent address: Idemitsu Kosan Co. Ltd., Kimltsu, Chiba (Japan) 0379-6779/89/$3.50 © Elsevier Sequoia/Printed in The Netherlands