High Energy Spin and Charge Excitations in La sub 2 CuO sub 4

Previous Raman studies of La sub 2 CuO sub 4, the insulating parent compound of the high T sub c superconductor, have found well-defined spin pair excitations of B sub (1g) symmetry at ~3000 cm sup (-1), corresponding to J~1100 cm sup (-1). Extending the spectra to Raman shifts of 9000 cm sup (-1) has provided a detailed line shape for the B sub (1g) spin pair excitation and has revealed two unexpected high energy excitations of A sub (1g) symmetry. These new modes at ~3500 and ~5000 cm sup (-1) are very broad, but exhibit good selection rules. All of these high energy modes exhibit strong resonant enhancement in the visible. Two single crystal samples have been studied, the first was grown in a PbO flux and thus contains of order 1% Pb as a substitutional impurity. The second crystal, having a T sub N of 260 K, was grown from a CuO flux. It was cleaved to expose a clean (001) surface free of flux.