Spatially Resolved Nonlinearity Measurements of YBa$_2$Cu$_3$O$_{7-d}$ Bi-crystal Grain Boundaries

TL;DR

This study introduces a near-field microwave microscope to spatially resolve and analyze the nonlinear electromagnetic response of YBa₂Cu₃O₇-d superconducting thin films at grain boundaries, revealing vortex dynamics and weak link behavior.

Contribution

It demonstrates a novel microwave microscopy technique for local nonlinear measurements of superconducting grain boundaries with high spatial resolution and theoretical modeling.

Findings

Higher harmonic response identifies grain boundary location.

Second harmonic response dominated by Josephson vortex flow.

Extracted nonlinear current density indicates weak link behavior.

Abstract

We have developed a near-field microwave microscope to locally excite a superconducting film and measure second and third harmonic responses at microwave frequencies. We study the local nonlinear response of a YBa$_2$Cu$_3$O$_{7-d}$ thin film grown on a bi-crystal SrTiO$_3$ substrate. The location of the bi-crystal grain boundary is clearly identified by the microscope through higher harmonic response, and the spatial resolution is on the order of the magnetic loop probe size, about 500$\mu m$. The harmonic power and spatial resolution are successfully modeled with a one-dimensional extended Josephson junction simulation. From the model, the 2nd order harmonic response is dominated by Josephson vortex generation and flow. A geometry-free nonlinear scaling current density $J_{NL}\simeq 10^4\sim 10^5 A/cm^2$ is also exstracted from the data, indicating that the grain boundary weak link is the dominant nonlinear source in this case.