MIDINFRARED ELECTROREFLECTANCE IN LA2CUO4+Y

Polarized electroreflectance spectra are reported for single-crystal L a2CuO4+y. Depending on the electrode structure, the primary effect of modulating the electrode potential is addition of charge carriers or v ariation of the electric field in the space-charge region. By adding c harge carriers we observe changes in the optical spectra similar to th ose caused by chemical doping. For modulation of the electric field th e perturbed absorption spectrum is very different, corresponding to el ectric-field-induced crystal-field transitions. Bands near 1.4 and 1.6 eV respond to the electric field in ways consistent with d(x2-y2)-->d (xy) and d(x2-y2)-->d(zx),d(yx) transitions, respectively. Our observa tions are consistent with Perkins et al. [Phys. Rev. Lett. 71, 1621 (1 993)] who ascribe transitions near 0.4 and 1.5 eV to the d(x2-y2)-->d3 z2-r2 and d(x2-y2)-->d(xy) excitons.