Search for an infrared electro-optic effect in thin high temperature superconducting films

Infrared spectroscopy is used to investigate the effect of transport currents on the ab-plane superconducting state properties of thin c-axis oriented YBa2Cu3O6+x films on sapphire and magnesium oxide substrates.; The films were first characterized in both the normal and superconducting states, through optical reflectivity and transmittance measurements over a wide frequency range (40--350 cm-1) in the far-infrared. We observed fairly high reflectivity and low transmittance in the superconducting state at frequencies below 150 cm-1. This is in contrast to the lower reflectivities and higher transmittances which are observed in the normal state over the same frequency range.; It is well known from nonlinear transport studies on thin superconducting films below their transition temperature, Tc, that large currents exceeding the critical current Ic destroy superconductivity, triggering a transition to the ohmic lossy regime typical of the normal state. We studied the effect of transport currents on the superconducting state transmittance in the far infrared region below 350 cm-1. The first set of spectra was taken at 75 K---about 10 K below Tc for both films---and a set of spectra was also taken at low temperature (40 K) for the film on sapphire substrate.; The films on magnesium oxide showed no electro-optic response up to very high currents, while at high currents, the film on sapphire showed an increase in transmittance at low frequency accompanied by a suppression of the transmittance at higher frequencies. At lower temperature a similar effect is observed in the film on sapphire, even at currents much lower than the critical value. We believe the changes observed are spurious, and may be attributed to ohmic heating. This has been further verified by simulations of the dielectric response using a simple two-fluid model of the superconducting state with a current dependent superfluid oscillator strength.