Polarized Neutron on URu2Si2

Abstract Polarized neutron scattering has shed a new light on URu 2 Si 2 . This compound studied for almost thirty years presents two tran- sitions: a superconducting state below 1.2K (T sc ) and a well-defined bulk transition at 17.8K (T 0 ). In spite of intensive research, no local probe has defined the order parameter associated with this transition (the famous Hidden Order (HO)). Neutron Larmor diffraction measurements combined with uniaxial stress experiments have shown that the relevant parameter that governs the magnetic properties is the a -lattice parameter. Induced magnetization distribution measured by elastic polarized neutron scattering shows a subtle change when entering the HO state that suggests a freezing of rank 5 multipoles (i.e. dotriacontapole). Polarized inelastic neutron measurements have shown the existence of two well defined excitations on the top of a broad continuum. How- ever only the AF excitation vanishes at T 0 and as well as when URu 2 Si 2 switches between HO to AF state under pressure. These excitations seem to correspond also to transitions between singlet levels.