Photoresponse of a YBa₂Cu₃O_x grain-boundary junction

The photoresponse of a YBa₂Cu₃O_x grain-boundary junction has been measured as a function of chopping frequency and bias current in a temperature range of 2 K≤T≤70 K. The response was found to be mostly bolometric, but a nonbolometriclike component was also identified, which appeared as a chopping-frequency independent signal while immersing the sample in the superfluid helium. The bias-current dependence of the response showed a peak at a current corresponding to the critical current of the junction, and the peak values remained constant for 30 K<T<70 K. However, below 15 K the peak of the photoresponse increased very sharply with decreasing temperature following a 1/T³ dependence, consistent with the temperature dependence of the thermal boundary resistance between the film and the substrate.