Low-energy optics of the heavy-fermion compound UNi2Al3

Heavy-fermion materials are intermetallic compounds with unusual metallic behavior at low temperatures. From the optical point of view, the Drude response (the transport relaxation time is enhanced in the same way as the effective mass) and the so-called hybridization gap (a signature of the peculiar band structure due to electronic interactions) are of particular interest. The low energy scales of heavy fermions call for optics at very low frequencies and in a broad range, but the only material studied in detail so far is UPd2Al3, where we found an extremely narrow Drude response (around 5 GHz) and an optical excitation at 100 GHz in the antiferromagnetic state. To generalize those previous results, we focus here on the related heavy-fermion compound UNi2Al3. We have grown high-quality thin films and studied them with a combination of microwave and optical techniques in a very broad frequency range. At temperatures below 30 K, we find a strongly frequency-dependent optical conductivity: the Drude roll-off resides below 20 GHz, but above 100 GHz another broad conductivity maximum occurs. In addition to the frequency and temperature dependence of the conductivity, we also present its anisotropy, and we discuss them in the context of the different energy scales of this material.