Nonbolometric laser-induced voltage signals in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films at room temperature

We report on detailed studies of laser-induced voltage signals observed in unbiased ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ 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 Ω at 30 mJ/${\mathrm{cm}}^2$, and because they represent potentials which are forbidden by the known crystal symmetry of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ 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 λ=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 to 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, ${\mathrm{\tau }}_{\mathit{s}}$, of the transient on the film thickness is not unique. For films with thicknesses below 4000 Å, ${\mathrm{\tau }}_{\mathit{s}}$ 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.