Study of local nonlinear properties using a near-field microwave microscope

We have developed a near-field microwave microscope to locally apply microwave frequency currents and fields to superconductors, and dielectric substrates, and measure the locally generated 2nd and 3rd harmonic responses. We measure the local nonlinear response of a Tl₂Ba₂CaCu₂O_y film grown on an MgO substrate, and observe a large response due to the enhanced current density near the edge. We also study the local nonlinear response of a YBa₂Cu₃O_7-δ thin film grown on a bi-crystal SrTiO₃ (STO) substrate, and spatially identify the grain boundary through higher harmonic measurements. The spatial resolution is determined by the magnetic loop probe size. A scaling current density J_NL is extracted to quantify the magnitude of the nonlinearity of the superconductor. Preliminary results on the nonlinear properties of some commonly used substrates, e.g., MgO and STO, have also been obtained.