Study of Local Nonlinear Properties Using a Near-Field Microwave Microscope

TL;DR

This paper introduces a near-field microwave microscope capable of locally measuring nonlinear electromagnetic responses in superconducting films and substrates, revealing spatial variations and boundary effects.

Contribution

The study presents a novel near-field microwave microscopy technique for spatially resolving local nonlinear responses in superconductors and substrates, including harmonic measurements and boundary identification.

Findings

Large nonlinear response observed near superconductor edges.

Spatially identified grain boundaries via harmonic responses.

Quantified nonlinearity through a scaling current density JNL.

Abstract

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 Tl_2Ba_2CaCu_2O_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 YBa_2Cu_3O_7-d thin film grown on a bi-crystal SrTiO_3 (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 JNL 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