Testing the limits for critical currents in YBa$_{2}$Cu$_{3}$O$_{7}$ films

Vortex pinning in YBa$_{2}$Cu$_{3}$O$_{7}$ films can be very strong. At low temperatures and in the absence of applied magnetic field (\textbf{H}), critical current densities J$_{c}$ of about 20{\%} of the depairing limit have been obtained. This is as high as the best achieved in commercial Nb-based superconducting wires after decades of optimization. Remarkably, similar J$_{c}$s are attained in YBa$_{2}$Cu$_{3}$O$_{7}$ films grown by various methods that produce vastly different nanostructures, suggesting that perhaps we are close to an effective J$_{c}$ limit regardless of the details of the pinning mechanisms. In contrast, the different types of pinning centers (either naturally occurring or artificially introduced by material nanoengineering) produce distinctively different J$_{c}$ behavior as a function of \textbf{H} strength and orientation. I will present a comparison of pinning mechanisms in YBa$_{2}$Cu$_{3}$O$_{7}$ films and will analyze the possibilities of further improvements.