Optical Conductivity and Electronic Structures in Ce-Filled Skutterudites

Abstract

Optical conductivity [σ(ω)] spectra have been measured for eight Ce-filled skutterudite compounds CeT4X12 (T =Fe, Ru, Os; X=P, As, Sb) to systematically study their microscopic electronic structures around Fermi level. The σ(ω) spectra show large differences among X=P, As and Sb, while they show smaller differences among T =Fe, Ru and Os for a given X element. This indicates that the choice of X has larger effects on the electronic structures of CeT4X12 than that of T . The spectral shape and temperature dependence of σ(ω) suggest that the electronic structures of CeT4P12 are similar to that of a band semiconductor and those of CeT4Sb12 to that of a heavy fermion metal or a Kondo semiconductor. In the case of CeT4As12, the σ(ω) spectra suggest intermediate characteristics between CeT4P12 and CeT4Sb12, namely they are similar to those shown by a semiconductor with a very narrow energy gap.KEYWORDS: Ce-filled skutterudites, optical conductivity, heavy fermion, Kondo semiconductorCompounds having the filled skutterudite structure have attaining much interest recently.1 The chemical formula of a filled skutterudite compound is RT 4 X 12 , where R is either a rare earth metal, an alkali metal or an alkaline earth metal, T is a transition metal such as Fe, Ru, Os, and X is a pnictogen such as P, As and Sb. In a rare earth-filled skutterudite, the rare earth atom is confined in a cage consisting of 12 X atoms. This large cordination number may result in a strong hybridization between the wave functions of f electrons at the rare earth atom and the p electrons at the X ligand. Such a situation is in contrast to those in usual rare earth intermetallics, where the hybridization of the f electron is generally weak because the f electron wave function is located to the inner portion of the atom. It is believed that many interesting physical phenomena found for the filled skutterudite compounds are related with the strong hybridization of the f electrons with conduction electrons in this particular crystal structure. 1In the case of Ce-filled skutterudites CeT 4 X 12 , the reported physical properties are widely varied depending on the constituent atoms T and X. They can be summarized as follows. For X=P, the three compounds with T =Fe, Ru, and Os are all semiconductors with energy gaps of 130, 90, and 40 meV, respectively, as estimated from the electrical resistivity and Hall coefficient experiments.2-4 For X=As, they have exhibited semiconductor-like characteristics with energy gaps of 5-10 meV suggested by electrical resistivity, but also metallic characteristics suggested by specific heat experiment. 5,6 For X=Sb, the observed properties for the three compounds with T =Fe, Ru, and Os are basically metallic, with heavy fermion-like mass enhance- * Present Address: Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581 † Author to whom all correspondence should be addressed. E-mail: okamura@kobe-u.ac.jp ment seen in the electronic specific heat data. However, CeRu 4 Sb 12 also shows strong non-Fermi liqui...

Optical Conductivity and Electronic Structure of CeRu4Sb12 under High Pressure

“…[In the case of CeX 4 P 12 (X=Fe, Ru, Os), 38) an energy gap with an onset at 0.2-0.3 eV was actually observed ] To explain the above result and the metallic characteristics observed in other physical properties, we suggest that the actual band structure should be similar to that described in Fig. 7(b).…”

“…Note that in CeM 4 P 12 and CeM 4 As 12 , whose lattice constants are smaller than that of CeM 4 Sb 12 , an energy gap is in fact observed. [6][7][8][9][10]38)…”

Kondo scaling of the pseudogap in CeOs4Sb12and CeFe4P12

“…The extracted DOS(E F ) of CeFe 4 P 12 thus reveals a V-type depletion of spectral weight within ∼110 meV of E F , and E F would be centred at the middle of the gap. While an early optical spectroscopy study of the material reported a gap size of about 150 meV [45], a recent study has shown a gap of 0.22-0.24 eV [23]. Thus, UPS is measuring roughly half the gap seen in optical spectroscopy.…”

“…However, the Hall coefficient for CeFe 4 P 12 is positive between 25 and 300 K and increases with decreasing temperature, thus suggesting that the dominant carriers are holes and its concentration decreases with decreasing temperature [17]. An optical gap of about 0.22-0.24 eV has been reported recently for single crystal CeFe 4 P 12 [23]. Also, the magnetic susceptibility below room temperature shows a continuous decrease in the effective magnetic moment for CeFe 4 P 12 [22].…”