Inductive measurements of third-harmonic voltage and critical current density in bulk superconductors

TL;DR

This paper introduces an inductive method to measure the critical current density in bulk superconductors using third-harmonic voltage detection, supported by theoretical modeling and experimental validation.

Contribution

It presents a novel inductive measurement technique for $J_c$ in bulk superconductors, combining theoretical analysis with experimental application on YBCO samples.

Findings

Third-harmonic voltage relates to $J_c$ via $V_3= G_3rac{ ext{frequency} imes I_0^2}{J_c}

The method accurately evaluates $J_c$ in bulk superconductors

Theoretical model agrees with experimental measurements

Abstract

We propose an inductive method to measure critical current density $J_c$ in bulk superconductors. In this method, an ac magnetic field is generated by a drive current $I_0$ flowing in a small coil mounted just above the flat surface of superconductors, and the third-harmonic voltage $V_3$ induced in the coil is detected. We present theoretical calculation based on the critical state model for the ac response of bulk superconductors, and we show that the third-harmonic voltage detected in the inductive measurements is expressed as $V_3= G_3\omega I_0^2/J_c$, where $\omega/2\pi$ is the frequency of the drive current, and $G_3$ is a factor determined by the configuration of the coil. We measured the $I_0$-$V_3$ curves of a melt-textured $\rm YBa_2Cu_3O_{7-\delta}$ bulk sample, and evaluated the $J_c$ by using the theoretical results.