Thermodynamic origin of the peak effect in the superconductor Nb3Sn

TL;DR

This study investigates the thermodynamic origins of the peak effect in Nb3Sn, revealing a complex vortex phase diagram with an intermediate glass phase and a first-order melting transition influenced by magnetic fields and thermal fluctuations.

Contribution

It provides the first thermodynamic evidence linking the peak effect in Nb3Sn to vortex phase metastability and identifies an intermediate glass phase affecting vortex dynamics.

Findings

Peak effect linked to vortex phase metastability

Intermediate glass phase with enhanced flux pinning identified

First-order vortex melting transition observed with hysteresis

Abstract

We report a pronounced peak effect in the magnetization and the magnetocaloric coefficient in a single crystal of the superconductor Nb3Sn. As the origin of the magnetization peak effect in classical type-II superconductors is still strongly debated, we performed an investigation of its underlying thermodynamics. Calorimetric experiments performed during field sweeps at constant temperatures reveal that the sharp increase in the current density occurs concurrently with additional degrees of freedom in the specific heat due to thermal fluctuations and a liquid vortex phase. No latent heat due to a direct first-order melting of a Bragg glass phase into the liquid phase is found which we take as evidence for an intermediate glass phase with enhanced flux pinning. The Bragg glass phase can however be restored by a small AC field. In this case a first-order vortex melting transition with a clear hysteresis is found. In the absence of an AC field the intermediate glass phase is located within the field range of this hysteresis. This indicates that the peak effect is associated with the metastability of an underlying first-order vortex melting transition.