Optical observation of non-Fermi-liquid behavior in the heavy fermion state ofYbRh2Si2

Abstract

We report far-infrared optical properties of YbRh2Si2 for photon energies down to 2 meV and temperatures 0.4 -300 K. In the coherent heavy quasiparticle state, a linear dependence of the lowenergy 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 YbRh2Si2 which is due to its close vicinity to an antiferromagnetic quantum critical point.PACS numbers: 71.10. Hf, 71.27.+a, The investigation of 4f -containing metals by farinfrared optical spectroscopy provides valuable insight into the nature of strong electronic correlations. This in particular holds true for heavy fermion (HF) compounds where at low temperatures a weak 4f -conduction electron (cf -)hybridization generates mass-renormalized quasiparticles with a coherent ground state which is in many HF systems of the Landau Fermi liquid (LFL) type.[1] The quasiparticles influence thermodynamic quantities which are described in terms of a large effective mass m * exceeding the free electron mass m 0 by three orders of magnitude. Furthermore, in typical HF materials, below a single-ion Kondo temperature (T K ), the coherent state is characterized by a dynamical screening of the 4f magnetic moments through the conduction electrons. Several highly correlated metals exhibit so-called nonFermi liquid (NFL), i.e., strong deviations from a renormalized LFL behavior when T → 0 K. [1] The system YbRh 2 Si 2 studied in this paper is one of a few clean stoichiometric HF metals with pronounced NFL behavior at ambient pressure which is related to both antiferromagnetic (AF) as well as ferromagnetic quantum critical spin fluctuations in close proximity to an AF quantum critical point (QCP). [2, 3, 4] Those NFL effects manifest as a divergence of the 4f -derived increment to the specific heat ∆C/T ∝ − ln T and in the electrical resistivity ρ(T ) showing a power law exponent close to 1 in a temperature range substantially larger than one decade and extending up to T ≃ 10 K.[5] Transport and thermodynamic properties are consistent with a single-ion Kondo temperature T K = 25 K (associated with the crystallineelectric-field-derived doublet ground state [6]).The electrodynamical response of HF systems is characterized by an optical conductivity σ(ω) which follows at room temperature the classical Drude model [σ(ω) = N e 2 τ /m * (1 + ω 2 τ 2 ); N : charge carrier density] with frequency independent m * and scattering rate 1/τ . [7] At low temperatures, upon entering the coherent state, large deviations are observed which are caused by many-body effects. Then a narrow, renormalized peak at zero photon energy ω = 0 eV is formed and a socalled hybridization gap appears which is related to the transition between the bonding and antibonding states resulting from the cf -hybridization. [8, 9, 10] The coherent part of the underlying strong electron-electron correlations are treated in an extended Drude model by renormalized and frequency dependent m * (ω)...

Anisotropic Electronic Structure of the Kondo Semiconductor CeFe2Al10Studied by Optical Conductivity

“…This result is consistent with our previous optical studies of heavy fermion compounds. [25][26][27] Therefore, the anisotropic T opt implies that T K is also anisotropic. The similar anisotropic T K has been discussed in orthorhombic CeNiSn and CeRhAs, in which no magnetic phase transition takes place.…”

“…The temperature dependence of the residual Drude weight is common to heavyquasiparticles, whose scattering rate declines with decreasing temperature. [25][26][27] Next, the anisotropic formation processes of the c-f hybridization gap and quasiparticle state are discussed. Figure 3(a) indicates the temperature dependence of the low-energy portion of the σ(ω) spectrum along all principal axes.…”

Far-infrared optical conductivity of CeCu2Si2

“…A heavy fermion state arises upon the formation of coherence among heavy quasiparticles below a lattice coherence temperature. The latter has been shown to be characteristic for the temperature dependence of the far-infrared optical response in heavy fermion compounds like YbRh 2 Si 2 and YbIr 2 Si 2 [1,2,3]. The signature of the Kondo interaction itself was discussed in terms of a gap-like suppression in the far-infrared optical conductivity due to a conductionband (c)-f electron hybridization.…”

Infrared and Terahertz Spectroscopy of Strongly Correlated Electron Systems under Extreme Conditions

“…3) In addition, a characteristic mid-IR peak is often observed in ð!Þ, which results from a c-f hybridized state far from E F . [103][104][105][106][107][108][109][110][111][112][113] So far, high pressure IR-SR studies of the f electron state have been reported for SmTe, 114) SmS, 115) YbS, 116) CeRu 4 Sb 12 , 117) and CeIn 3 . 118) Both SmS and YbS are divalent semiconductors at ambient pressure, but undergo pressure-induced transition to a metallic IV state.…”