Terahertz radiation imaging of supercurrent distribution in vortex-penetrated YBa2Cu3O7−δ thin film strips

Abstract

We have developed a supercurrent distribution imaging system for high Tc superconductive thin films and demonstrated the visualization of the supercurrent distribution in the vortex-penetrated YBa2Cu3O7−δ thin film strips. The terahertz (THz) radiation and detection system with a scanning femtosecond laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radiation into the free space by optical supercurrent modulation, and the radiation amplitude is proportional to the local supercurrent density at the optically excited area. Prior to the observation of the supercurrent distribution, we studied optical excitation effects on the vortices trapped in the strips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser power dependence of the THz radiation in the remanent state revealed that the excitation with powers larger than the relatively weak finite value (about 10 mW in the present case) strongly affects the vortices trapped in the films. We attributed this behavior to the optically excited depinning effect. We derived a calibration function from the THz radiation images into the supercurrent density distributions by observing the bias-current dependence of the THz radiation, and applied it for the diagnosis of the distributions in the vortex-penetrated strips. The THz radiation images were successfully transferred into the supercurrent density distributions with quantitative agreement. The minimum magnetic flux resolution at the optically excited area was roughly estimated to be 3 φ0 where φ0 is a single flux quantum. The measurement of the laser beam profile indicated that the spatial resolution of the THz radiation images is limited by the laser beam diameter: 25 μm in our case. The observed distributions revealed that the vortices easily penetrate into the strip under an external magnetic field BEX of 0.9 mT, and the persistent supercurrent exists only near the strip edges in the remanent state after removal of the field. The calculations of the convolution between the observed laser pattern and the trial functions suggested that the supercurrent distribution width in the remanent state after removal of the field of 0.9 mT is estimated to be less than 1 μm. The temperature dependence of the supercurrent distributions revealed that, below 60 K, the thermal activation produces no significant effects on the penetrated vortices at BEX=0.9 mT, whereas, the vortices in the remanent state after removal of the field of 15 mT were strongly affected by the thermal activation. The decreasing rate of the supercurrent density at the edge with increasing temperature was larger than that inside the strip. This suggested that the vortices trapped near the edges exhibit rather different behavior from the ones that penetrated into the inner part of the strip.