Anomalous photovoltaic response in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Several experiments were carried out to study the origin of the anomalous photovoltaic effect in high-quality ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ thin films. In particular, the issue of whether the signal is induced by thermal gradients was addressed. Using samples with well-characterized orientation and texture, and using low-power cw lasers, it was determined that the signal was proportional to the thermal gradient along the c axis. However the magnitude of the signal was more than 10 times larger than that produced by thermoelectric power. Additionally, the temperature dependence of this photovoltaic signal was found to be very sample dependent even for high-quality films where all the resistance-temperature curves were apparently the same. The photovoltaic signal may be related to the recently discovered time-dependent photoinduced changes in superconductivity.