Microstructure-controlled vortex phases and two-phase superconductivity in (TaNb)0.7(HfZrTi)0.5 revealed by ac magnetostrictive coefficients

TL;DR

This study reveals how annealing-induced microstructural changes in a high-entropy alloy superconductor influence vortex phases and enable the coexistence of two superconducting phases, with implications for flux pinning control.

Contribution

It demonstrates the microstructure-vortex state relationship in (TaNb)0.7(HfZrTi)0.5 and shows how thermal processing can tailor flux pinning and phase coexistence in complex superconductors.

Findings

Intermediate annealing enhances vortex pinning and induces fishtail effects.

High-temperature annealing reveals two-phase superconductivity with distinct critical fields.

Microstructure connectivity controls the visibility of two-step superconducting responses.

Abstract

We investigate flux dynamics in the high-entropy alloy superconductor (TaNb)0.7(HfZrTi)0.5 after annealing (as-cast, 500 {\deg}C, 550 {\deg}C, and 1000 {\deg}C) using a sensitive ac composite magnetoelectric method that measures the complex ac magnetostrictive coefficient (d{\lambda}/dH)ac. The resulting vortex phase diagrams show that intermediate annealing (500-550 {\deg}C) induces nanoscale clustering, enhances pinning, and produces a pronounced fishtail effect with successive elastic- and plastic-vortex-glass regimes. Flux-jump instabilities appear at 550 {\deg}C and persist at 1000 {\deg}C, indicating strong pinning and thermomagnetic instability in the low-temperature, low-field regime. Remarkably, the 1000 {\deg}C sample exhibits a two-step superconducting response-a double plateau or drop in d{\lambda}'/dH and two dissipation peaks in d{\lambda}''/dH-demonstrating the coexistence of two superconducting phases with distinct irreversibility and critical-field value. We further show that the resolvability of the two-step (d{\lambda}/dH)ac signature is governed by the topological connectivity of the phase-separated microstructure, which controls magnetic shielding between the TaNb-rich network and the (TaNb)0.7(HfZrTi)0.5 parent phase. These results establish a direct microstructure-vortex-state correlation and provide a route to tailoring flux pinning in chemically complex superconductors via thermal processing.