Evidence of a magnetic gaplike excitation in URu2Si2

We report the observation of a sharp gaplike magnetic excitation at low temperatures by inelastic neutron scattering in the heavy-fermion $5f$ system U${\mathrm{Ru}}_2$ ${\mathrm{Si}}_2$. At $T=10\mathrm{}$ K this sharp excitation occurs at an energy transfer of 5.5 meV and has a Lorentzian relaxational-type tail at higher-energy transfer. The gap rapidly disappears as the sample is heated and at $T=50\mathrm{}$ K the response function consists of only one quasielastic line. The $Q$ dependence of the low-temperature peak suggests that antiferromagnetic correlations are important.