The electrical conduction of metals is governed by how freely mobile electrons can move throughout the material. This movement is hampered by scattering with other electrons, as well as with impurities or thermal excitations (phonons). Experimentally, the scattering processes of single electrons are not observed, but rather the overall response of all mobile charge carriers within a sample. The ensemble dynamics can be described by the relaxation rates, which express how fast the system approaches equilibrium after an external perturbation. Here we measure the frequency-dependent microwave conductivity of the heavy-fermion metal UPd2Al3 (ref. 4), finding that it is accurately described by the prediction for a single relaxation rate (the so-called Drude response). This is notable, as UPd2Al3 has strong interactions among the electrons that might be expected to lead to more complex behaviour. Furthermore, the relaxation rate of just a few gigahertz is extremely low--this is several orders of magnitude below those of conventional metals (which are typically around 10 THz), and at least one order of magnitude lower than previous estimates for comparable metals. These observations are directly related to the high effective mass of the charge carriers in this material and reveal the dynamics of interacting electrons.
The optical conductivity of the heavy fermions UPd2Al3 and UPt3 has been measured in the frequency range from 10 GHz to 1.2 THz (0.04 meV to 5 meV) at temperatures 1 K < T < 300 K. In both compounds a well pronounced pseudogap of less than a meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective electronic mass and scattering rate we derive the energies essential for the heavy quasiparticle. We find that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the pre-requisite to the formation of the heavy quasiparticle and eventually of superconductivity.PACS numbers: 71.27.+a, 74.70.Tx, 72.15.Qm In common metals itinerant electrons of the conduction band are responsible for the electrical transport. The conductivity is frequency independent until the scattering with phonons and imperfections leads to a drop in the real part of the optical conductivity σ 1 (ω) around the scattering frequency Γ. This behaviour is well described by the Drude model:σ(ω) = σ 1 + iσ 2 = (N e 2 /m)/(Γ − iω) with the charge carrier mass m and scatting rate Γ assumed to be frequency independent. Here N is the carrier concentration, e is the electronic charge, and ω is the frequency [1]. While at room temperature also heavy-fermions metals follow this scenario, at liquid-helium temperatures large deviations are observed which are caused by many-body effects.Heavy fermions show an unusual interplay of electronic correlations due to an interaction of itinerant electrons and local magnetic moments. At low temperatures these intermetallic compounds exhibit significant increase of the magnetic susceptibility and the electronic contribution to the specific heat compared to most metals. This is commonly explained by an enhanced effective mass m * of some hundred times the free electron mass due to the highly correlated electronic behaviour and is described in the context of interacting Fermi liquids [2]. The strong interaction of the quasi-free conduction-band electrons with nearly localized f -electrons leads to a many-body resonance, i.e., an enhanced density of states at the Fermi energy [3]. The pile-up of this narrow resonance sets in below the so-called coherence temperature T * , which is experimentally determined by a drop in the dc-resistivity and a cusp in the susceptibility, and typically is between 10 and 100 K.Also the optical properties show clear signatures of the coherent many-body ground state: As a consequence of the large effective mass m * the spectral weight1/2 is called the renormalized plasma frequency. The relevant electronic excitations shift to very low energies, best characterized by an effective scattering rate Γ * [4]. With decreasing temperature the electronic correlations become more pronounced, leading to a gradual increase of the effective mass m *...
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