Vortex Redistribution below the First-Order Transition Temperature in the β-Pyrochlore Superconductor KOs₂O₆

A miniature Hall-sensor array was used to detect magnetic induction locally in the vortex states of the β-pyrochlore superconductor KOs₂O₆. Below the first-order transition at T_p∼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_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_p.