Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned $\rm YBa_2Cu_3O_7$

TL;DR

This study measures the complex resistivity of the vortex lattice in untwinned YBa2Cu3O7, revealing a sharp collapse of the shear modulus at the melting transition, indicating a loss of lattice rigidity.

Contribution

It provides detailed RF measurements of vortex lattice inductance and shear modulus near melting, showing a significant collapse of shear modulus while pinning remains finite.

Findings

Shear modulus $c_{66}$ collapses by over 4 decades at $T_m$

Vortex-lattice inductance exhibits a cusp at melting temperature

Pinning parameter $$ remains finite at $T_m$

Abstract

The complex resistivity $\hat{\rho}(\omega)$ of the vortex lattice in an untwinned crystal of 93-K $\rm YBa_2Cu_3O_7$ has been measured at frequencies $\omega/2\pi$ from 100 kHz to 20 MHz in a 2-Tesla field $\bf H\parallel c$, using a 4-probe RF transmission technique that enables continuous measurements versus $\omega$ and temperature $T$. As $T$ is increased, the inductance ${\cal L}_s(\omega) ={\rm Im} \hat{\rho}(\omega)/ \omega$ increases steeply to a cusp at the melting temperature $T_m$, and then undergoes a steep collapse consistent with vanishing of the shear modulus $c_{66}$. We discuss in detail the separation of the vortex-lattice inductance from the `volume' inductance, and other skin-depth effects. To analyze the spectra, we consider a weakly disordered lattice with a low pin density. Close fits are obtained to $\rho_1(\omega)$ over 2 decades in $\omega$. Values of the pinning parameter $\kappa$ and shear modulus $c_{66}$ obtained show that $c_{66}$ collapses by over 4 decades at $T_m$, whereas $\kappa$ remains finite.