AC Response of Flux-Line Liquid in High $T_c$ Superconductors

TL;DR

This paper models the ac response of a viscous flux-line liquid in high-temperature superconductors, revealing how viscoelastic effects and high viscosity influence magnetic field penetration and induce characteristic peaks in ac permeability.

Contribution

It introduces a hydrodynamics-based theory that captures the crossover between liquid-like and solid-like vortex responses, including the effects of high viscosity and viscoelasticity.

Findings

Identification of two frequency-dependent length scales for ac field penetration.

Prediction of a new low-frequency peak in ac permeability due to solid-liquid transition.

Demonstration of viscous screening effects in high-viscosity flux-line liquids.

Abstract

We use a hydrodynamics theory to discuss the response of a viscous flux-line liquid to an ac perturbation applied at the surface of the sample. The theory incorporates viscoelastic effects and describes the crossover between liquid-like and solid-like response of the vortex array as the frequency of the perturbation increases. A large viscosity from flux-line interactions and entanglement leads to viscous screening of surface fields. As a result, two frequency-dependent length scales are needed to describe the penetration of an ac field. For large viscosities the imaginary part of the ac permeability can exihibit, in addition to the well-know peak associated with flux diffusion across the sample, a new low-frequency peak corresponding to the transition from solid-like to liquid-like behavior.