Anomalous Duality of $4f$ Electrons in Filled Skutterudite ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$

We study the electronic structure of the filled skutterudite ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ using photoemission spectroscopy (PES). Soft x-ray excited Ce $3d$-$4f$ resonant PES confirms the existence of Ce $4f$ states at the Fermi level ($E_F$). Temperature dependent high-resolution laser-PES spectra reveal a pseudogap formation around $E_F$, which can be explained in terms of the hybridization gap. Simultaneously, a sharp feature is formed just above $E_F$ with decreasing temperature. The heavy-fermion-like specific heat is attributed to the occupation of this feature. The results identify the origin of the anomalous coexistence of heavy-fermion and pseudogap behavior in terms of a symmetry dependent hybridization.

Figure 1

(a) Ce $3d_{5/2}$ x-ray absorption spectrum for ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$. Arrows indicate the energies at which off- and on-resonant photoemission (PES) spectra were measured. (b) Ce $3d$-$4f$ resonant PES spectra for ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ in comparison with the band-structure calculation. (c) Higher-resolution ($\Delta E=100\mathrm{meV}$) resonant PES spectra near the Fermi level. (d) Spectral density of states, obtained by divided the on- and off-resonance PES spectra by resolution-convoluted Fermi-Dirac function.

Figure 2

Temperature dependence of laser photoemission spectra for ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$, normalized at the integrated intensity between $-0.1$ and 0.3 eV ($\Delta E=6\mathrm{meV}$). The inset shows the higher-resolution ($\Delta E=2\mathrm{meV}$) data at 4.2 K, and the spectral density of states obtained by dividing the corresponding Fermi-Dirac function.

Figure 3

(a) Spectral density of states (DOS) for ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ as a function of temperature ($T$), obtained by dividing the resolution-convoluted Fermi-Dirac function for each $T$. (b) DOS for ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ as a function of $T$, normalized to that at 200 K. For clarity, each spectrum is offset by 0.5. Arrows indicate the pseudogap energy. (c) The $T$ dependence of the pseudogap energy in this laser-PES plotted with that obtained from the optical conductivity [9].

Figure 4

Schematic picture of band dispersion and density of states (DOS) realized in ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$. (a) The dispersive conduction band and two discrete Ce $4f$ states without hybridization. The ${\Gamma }_5^{-}$ and ${\Gamma }_{67}^{-}$ states are located below and above Fermi level ($E_F$), respectively. (b) Only ${\Gamma }_5^{-}$ states hybridize with the $a_u$ (${\Gamma }_5^{-}$) main conduction band, and two hybridized bands are formed below and above $E_F$. ${\Gamma }_{67}^{-}$ states remain nearly dispersionless because they hardly hybridize with the opposite parity $a_g$ (${\Gamma }_5^{+}$) band (not shown) [22]. (c) DOS in the case of (b). The arrow represents the effective pseudogap magnitude observed in this laser photoemission study. The sharp $f$-DOS just above $E_F$ due to ${\Gamma }_{67}^{-}$ states is partially occupied.