Vortex Redistribution below the First-Order Transition Temperature in the \beta-Pyrochlore Superconductor KOs_2O_6

TL;DR

This study investigates vortex behavior in the -pyrochlore superconductor KOs, revealing vortex redistribution and changes in magnetic properties associated with a first-order transition at 8 K linked to K ion rattling.

Contribution

It provides the first local magnetic induction measurements showing vortex redistribution and magnetic property changes across the first-order transition in KOs.

Findings

Lower critical field decreases below T_p.

Remanent magnetization drops below T_p.

Large, position-dependent magnetic jumps at T_p.

Abstract

A miniature Hall sensor array was used to detect magnetic induction locally in the vortex states of the $\beta$-pyrochlore superconductor KOs$_2$O$_6$. Below the first-order transition at $T_{\rm p}\sim 8$ K, which is associated with a change in the rattling motion of K ions, the lower critical field and the remanent magnetization both show a distinct decrease, suggesting that the electron-phonon coupling is weakened below the transition. At high magnetic fields, the local induction shows an unexpectedly large jump at $T_{\rm p}$ whose sign changes with position inside the sample. Our results demonstrate a novel redistribution of vortices whose energy is reduced abruptly below the first-order transition at $T_{\rm p}$.