The normal state of URu$_2$Si$_2$: spectroscopic evidence for an anomalous Fermi liquid

We report on new measurements of the optical conductivity of the heavy fermion compound URu2Si2 using a new technique that reduces the noise level of the spectra by an order of magnitude. In the normal state, above the hidden order transition at 17.5 K, we find that the optical resistivity, defined as \rho(\omega)=Re(1} /\sigma(\omega)) can be accurately described by a generalized Fermi liquid like formula where \rho(\omega,T)=A'(\omega^2+b(\pi T)^2)+\rho_0. The optical measurement is able to establish both A' and b and can be compared with dc transport that yields A'b. We find that instead of the value of b=4 expected for a Fermi liquid, our experiments show that b \approx 1. This value of b is a signature of elastic scattering. Since the scattering is intrinsic we discuss this normal state in terms of an unconventional incoherent Fermi liquid, Fermi molasses.