Evaluation of Critical Current Density in Superconductors by an Inductive Method Measuring Third-harmonic Voltages

—Critical Current Density in Superconducting Films and Bulk Materials—

Abstract

We have investigated an inductive method measuring third-harmonic voltages to evaluate the local critical current density J_c in superconducting films and bulk materials. When a sinusoidal drive current I₀cosωt flows in a small coil close to the flat surface of superconductors, harmonic voltages V_ncos(nωt+θ_n) are induced in the coil, because of the nonlinear response of superconductors. We have considered the AC response of superconductors and the induced third-harmonic voltage V₃ as the function of I₀, and we present the precise technique to measure the critical current density J_c in superconducting films as well as J_c in bulk superconductors.