Third harmonic generation at the charge transfer gap in Sr/sub 2/CuO/sub 2/Cl/sub 2/

Summary form only given. Progress towards understanding high transition-temperature superconductivity in the copper oxides (cuprates) is hindered by the coexistence of complex spin, charge and lattice degrees of freedom. Even in the undoped parent compounds a detailed microscopic description of excitations within the basic energy bandstructure as observed in electron and optical spectroscopies remains a formidable task. Nonlinear optical spectroscopy, however, allows precise control over excitation energies and symmetries, revealing important new information about such systems. Here, we employ third-harmonic generation (THG) spectroscopy to investigate the electronic structure around the charge-transfer gap in the strongly correlated cuprate Sr/sub 2/CuO/sub 2/Cl/sub 2/.