Spatial distribution analysis of critical temperature in epitaxial Y-Ba-Cu-O film using variable temperature scanning laser microscopy

We have investigated the spatial distribution of superconducting transition in an epitaxial YBa₂Cu₃O₇ film using variable temperature scanning laser microscope (VTSLM). VTSLM creates an image of the ac voltage response, δV(x, y), due to an ac modulated laser beam, which is proportional to dRdT(x, y). In the resistive transition region, there is a strong correlation between the VTSLM images and the resistance of the sample. When the sample is making a poor thermal contact to the heat bath, the large δV(x, y) region shifts toward the ends of the bridge while the sample resistance decreases. This result is due to the variation of surface temperature along the sample created by the heating at the contact resistance and/or the poor thermal contact between the sample and the heat bath. However, even after improving thermal contact, we still observe the distribution of superconducting transition. Since the local superconducting transition occurs within 1 K, we conclude that any samples with superconducting transition width larger than 1 K have local nonuniformity.