NONBOLOMETRIC LASER-INDUCED VOLTAGE SIGNALS IN YBA2CU3O7-DELTA THIN-FILMS AT ROOM-TEMPERATURE

We report on detailed studies of laser-induced voltage signals observed in unbiased YBa2Cu3O7-delta thin films at room temperature. The induced signals are measured as a function of incident energy, bias current, light polarization, and wavelength. The observed signals are remarkable because of their magnitude, up to 2 V across 4-OMEGA at 30 mJ/cm2, and because they represent potentials which are forbidden by the known crystal symmetry of YBa2CU3O7-delta if two obvious explanations, pyroelectricity and off-diagonal thermoelectricity, are considered. The signal proves to be independent of the polarization of light as well as its wavelength for lambda = 1064, 532, and 355 nm. Our measurements provide strong evidence that the observed signals do not have a bolometric origin. The signal amplitude is proportional to the magnitude of the incident energy and not its energy density. The time constant of the bolometric response, determined by applying a bias current, is different from that of the transient. The dependence of the time constant, tau-s of tbe transient on the film thickness is not unique. For films with thicknesses below 4000 angstrom, tau-s, agrees with simulated thermal diffusion times. Thicker films appear to show time constants independent of thickness, in contrast with predictions for thermal diffusion. Although the reflectivity is a symmetric function of the angle of incidence about the normal, the voltage signals are not.