A near-field microwave microscope was developed in order to excite locally a superconducting film and measure second- and third-order harmonic responses at microwave frequencies. The local non-linear response of a YBa2Cu3O7–δ thin film grown on a bicrystal SrTiO3 substrate was studied. The location of the bicrystal grain boundary was clearly identified by the microscope through higher harmonic response, and the spatial resolution was of the order of the magnetic loop diameter (about 500µm). The harmonic power and spatial resolution were modeled with a one-dimensional extended Josephson junction simulation. From the model, the second-order harmonic response was dominated by Josephson vortex generation and flow. A geometry-free non-linear scaling current density JNL of some 104 to 105A/cm2 was also extracted from the data, indicating that the grain boundary weak link was the dominant nonlinear source in this case.

Spatially-Resolved Non-Linearity Measurements of YBa2Cu3O7– δ Bicrystal Grain Boundaries. S.C.Lee, S.M.Anlage: Applied Physics Letters, 2003, 82[12], 1893-5