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 Tp∼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 Tp
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 Tp.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let