Observation of an optical non-Fermi-liquid behavior in the heavy fermion   state of YbRh$_{2}$Si$_{2}$
  We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy quasiparticle state, a linear dependence of the low-energy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an antiferromagnetic quantum critical point. 
S. Kimura, J. Sichelschmidt, J. Ferstl, C. Krellner, C. Geibel, F.   Steglich