It has been known that the elemental Yb, a divalent metal at ambient pressure, becomes a mixed-valent metal under external pressure, with its valence reaching ∼ 2.6 at 30 GPa. In this work, infrared spectroscopy has been used to probe the evolution of microscopic electronic states associated with the valence crossover in Yb at external pressures up to 18 GPa. The measured infrared reflectivity spectrum [R(ω)] of Yb has shown large variations with pressure. In particular, R(ω) develops a deep minimum in the mid-infrared, which shifts to lower energy with increasing pressure. The dip is attributed to optical absorption due to a conduction (c)-f electron hybridization state, similarly to those previously observed for heavy fermion compounds. The red shift of the dip indicates that the c-f hybridization decreases with pressure, which is consistent with the increase of valence.KEYWORDS: Yb metal, valence crossover, infrared synchrotron radiation, high pressure, diamond anvil cell Physical properties of strongly correlated "heavy fermion" compounds, most typically Ce-or Yb-based compounds containing partly filled 4f shell, have attracted much attention.1) In heavy fermion compounds, the hybridization between the conduction (c) electrons and the otherwise localized f electrons leads to many interesting phenomena, such as a crossover between itinerant and localized characters of the f electrons, and the formation of heavy fermion.Elemental Yb is a divalent metal at ambient pressure, hence the f electron configuration is 4f 14 with no local magnetic moment. However, it has been shown using Xray absorption spectroscopy (XAS) that the valence of Yb increases under external pressure, reaching ∼ 2.6 at 30 GPa.2, 3) Namely, Yb is a mixed-valent metal in this pressure range. Since the number of localized 4f holes increases with this valence crossover, it can be viewed from the 4f hole point of view as a crossover from an itinerant to a localized regime. Equivalently, it can be also viewed that the c-f hybridization becomes weaker as the applied pressure increases. Hence Yb can be regarded as an f electron system where an itinerant-localized crossover can be caused, and where the c-f hybridization can be tuned, by the external pressure. In addition, Yb has another interesting property: its resistivity (ρ) increases under pressure up to ∼ 4 GPa.4) It has been suggested by band structure calculation that the increase of ρ with pressure is due to a pseudogap formation at the Fermi level (E F ). 5)In this work, we have probed the interesting electronic structures of Yb metal under pressure using infrared (IR) reflectivity [R(ω)] measurement. IR spectroscopy has been quite successful in probing the electronic structures associated with the c-f hybridized state in mixedvalent heavy fermion compounds.6-9) We have used a diamond anvil cell (DAC) to produce high pressure. It * E-mail: okamura@kobe-u.ac.jp is technically challenging to do reflectivity experiment within the limited sample space in a DAC using long wavelength IR radiation....

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...