Spatially-resolved nonlinearity measurements of YBa2Cu3O7−δ bicrystal grain boundaries

Abstract

We have developed a near-field microwave microscope to locally excite a superconducting film and measure second- and third-order harmonic responses at microwave frequencies. We study the local nonlinear response of a YBa2Cu3O7−δ 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 500μm. 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≅104∼105 A/cm2 is also extracted from the data, indicating that the grain boundary weak link is the dominant nonlinear source in this case.