Neutron scattering studies of itinerancy and ``hidden order'' in URu$_{2}$Si$_{2}$

The heavy fermion superconductor URu$_{2}$Si$_{2}$ has been of interest for the last two decades but the nature of the ``hidden order'' in the phase below T$_{0}$ = 17.5 K has not been identified. The large specific heat jump at this temperature cannot be accounted for by the weak ordered moment which is only a fraction of a Bohr magneton on the uranium site (0.03 $\mu _{B})$. In recent neutron scattering experiments we set new limits for the maximum size of an ordered moment which could be observed in the ``hidden order'' phase, and we investigated the possibility of orbital currents. Furthermore, our experiments have unveiled a new feature in the inelastic spectrum -- the precursor phase above 17.5 K exhibits incommensurate fluctuations that are highly correlated in Q and extend to high energies. This suggests that (1) the spins are itinerant, rather than localized and (2) these excitations must play a role in the heat capacity anomaly at the phase transition, as they become gapped at incommensurate wave-vectors below T$_{0}$ and cover a large region of phase space. Implications of these new measurements will be discussed.