Nature of Heavy Quasiparticles in Magnetically Ordered Heavy FermionsUPd2Al3and

Abstract

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 *...

Low energy electrodynamics of the Kondo-lattice antiferromagnet CeCu2Ge2

“…As in other recent papers [12][13][14][15] that measure low frequency conductivity in heavy fermion compounds, we find evidence of mass enhancement through a dynamic measurement. At temperatures well above the onset of magnetic order and the coherence temperature, the Drude conductivity is broad and featureless, which is consistent with the strong scattering in the normal state.…”

“…The model shows that two Drude peaks are sufficient to fit the data: one that is large and broader than the experimental spectral range, the other narrower that accounts for the heavy fermion renormalizations at low T. Overall the model is a reasonable parametrization of the measured spectra. An assumption of this fitting is that there are no peaked spectral features in σ 1 between the lower end of our spectral range and DC [13,14]. A posteriori motivation for this assumption can be found in the fact that effective masses we extract from the analysis below are in close agreement with those found via thermodynamic measurements.…”

Bandwidth-controlled Mott transition inκ−(BEDT−TTF)2Cu[N(CN)Dumm1, Faltermeier2, Drichko3 et al. 2009Phys. Rev. B9215970View full textAdd to dashboardCite

“…To extract information about the frequencydependent scattering rate Γ 1 (T, ω) and effective mass, we analyze the low-temperature spectra using a generalized Drude model; 33,55 this approach is commonly applied to correlated-electron systems like heavy fermions and hightemperature superconductors: 24,41,56,57 …”

The dual nature of 5f electrons and the origin of heavy fermions in U compounds

“…Detailed Fermi surface studies for UGa 3 [10] and high-resolution photoemission measurements for URu 2 Si 2 [11] show that the observed Fermi surfaces cannot be explained by assuming all 5 f electrons to be itinerant or localized. Measurements of the optical conductivity in UPd 2 Al 3 and UPt 3 [12] indicate that the enhanced effective masses m * of the quasiparticles should result from the interaction of delocalized states with localized magnetic moments.…”