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

We have developed a supercurrent distribution imaging system for high T-c s uperconductive thin films and demonstrated the visualization of the supercu rrent distribution in the vortex-penetrated YBa2Cu3O7-delta thin film strip s. The terahertz (THz) radiation and detection system with a scanning femto second laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radi ation into the free space by optical supercurrent modulation, and the radia tion amplitude is proportional to the local supercurrent density at the opt ically excited area. Prior to the observation of the supercurrent distribut ion, we studied optical excitation effects on the vortices trapped in the s trips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser pow er 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 fil ms. We attributed this behavior to the optically excited depinning effect. We derived a calibration function from the THz radiation images into the su percurrent density distributions by observing the bias-current dependence o f 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 ar ea was roughly estimated to be 3 phi(0) where phi(0) is a single flux quant um. The measurement of the laser beam profile indicated that the spatial re solution of the THz radiation images is limited by the laser beam diameter: 25 mu m in our case. The observed distributions revealed that the vortices easily penetrate into the strip under an external magnetic field B-EX 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 conv olution between the observed laser pattern and the trial functions suggeste d that the supercurrent distribution width in the remanent state after remo val of the field of 0.9 mT is estimated to be less than 1 mu m. The tempera ture dependence of the supercurrent distributions revealed that, below 60 K , the thermal activation produces no significant effects on the penetrated vortices at B-EX = 0.9 mT, whereas, the vortices in the remanent state afte r removal of the field of 15 mT were strongly affected by the thermal activ ation. The decreasing rate of the supercurrent density at the edge with inc reasing temperature was larger than that inside the strip. This suggested t hat the vortices trapped near the edges exhibit rather different behavior f rom the ones that penetrated into the inner part of the strip. (C) 2000 Ame rican Institute of Physics. [S0021-8979(00)00410-2].