Spatially-resolved nonlinearity measurements of YBa2Cu3O7Àd bicrystal grain boundaries

We have developed a near-field microwave microscope to locally excite a superconducting film and measure secondand third-order harmonic responses at microwave frequencies. We study the local nonlinear response of a YBa2Cu3O72d thin film grown on a bicrystal SrTiO3 substrate. The location of the bicrystal grain boundary is clearly identified by the microscope through higher harmonic response, and the spatial resolution is on the order of the magnetic loop diameter, about 500mm. The harmonic power and spatial resolution are modeled with a one-dimensional extended Josephson junction simulation. From the model, the second-order harmonic response is dominated by Josephson vortex generation and flow. A geometry-free nonlinear scaling current density JNL >10;10 A/cm is also extracted from the data, indicating that the grain boundary weak link is the dominant nonlinear source in this case. © 2003 American Institute of Physics. @DOI: 10.1063/1.1561152#