Fermi-surface instability at the 'hidden-order' transition of URu(2)Si(2)

Solids with strong electron correlations generally develop exotic phases of electron matter at low temperatures. Among such systems, the heavy-fermion semimetal URu(2)Si(2) exhibits an enigmatic transition at To=17.5K to a 'hidden-order' state for which the order parameter remains unknown after 23years of intense research. Various experiments point to the reconstruction and partial gapping of the Fermi surface when the hidden order establishes. However, up to now, the question of how this transition affects the electronic states at the Fermi surface has not been directly addressed by a spectroscopic probe. Here we show, using angle-resolved photoemission spectroscopy, that a band of heavy quasiparticles drops below the Fermi level on the transition to the hidden-order state. Our data provide the first direct evidence of a large reorganization of the electronic structure across the Fermi surface of URu(2)Si(2) occurring during this transition, and unveil a new kind of Fermi-surface instability in correlated electron systems.