Bulk-sensitive photoemission study of $A{\text{Cu}}_3{\text{Ru}}_4{\text{O}}_{12}$ ($A=\text{Ca}$, Na, and La) with heavy-fermion behavior

We have studied the $d$-electron heavy-fermion states in $A{\text{Cu}}_3{\text{Ru}}_4{\text{O}}_{12}$ ($A=\text{Ca}$, Na, and La) using x-ray photoemission spectroscopy and high-resolution photoemission spectroscopy at bulk-sensitive photon energy (7 eV). The bulk-sensitive photoemission measurements for $A=\text{Ca}$, Na, and La show that a resonancelike structure within a pseudogap commonly evolves in the low-temperature range where the Kadowaki-Woods relation holds. The origin of the resonancelike peak and the heavy-fermion behavior is basically attributed to the hybridization between the $\text{Cu}3d$ and $\text{Ru}4d$ orbitals. The $A$-site substitution controls the filling of the $\text{Cu}3d$ and $\text{Ru}4d$ hybridized bands. Interestingly, while the resonancelike peak position roughly follows the rigid-band shift for $A=\text{Na}$ and La, the resonancelike peak deviating from the rigid-band behavior is located just at the Fermi level as expected in a Kondo system. The difference in the resonancelike peak near the Fermi level is correlated with that in the $\text{Cu}2p$ core-level spectra.

Figure 1

(Color online) (a) Valence-band XPS spectra of ${\text{NaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, ${\text{LaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, and ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$ taken at 300 K. (b) $\text{Ru}3d$ photoemission spectra of ${\text{NaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, ${\text{LaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, and ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$ taken at 300 K. (c) $\text{Cu}2p$ photoemission spectra of ${\text{NaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, ${\text{LaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, and ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$ taken at 300 K.

Figure 2

(Color online) Temperature dependence of the angle-integrated photoemission spectra near the Fermi level for (a) ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, (b) ${\text{NaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, and (c) ${\text{LaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$.

Figure 3

(Color online) Temperature dependence of the photoemission spectra near the Fermi level divided by fitting functions of gold spectra for (a) ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, (b) ${\text{NaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, and (c) ${\text{LaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$. The spectra taken at 300 K are shown by the black dashed curves and are compared to the spectra taken at 30, 50, 100, and 200 K. (d) Photoemission spectra at 50 K of ${\text{CaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, ${\text{NaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$, and ${\text{LaCu}}_3{\text{Ru}}_4{\text{O}}_{12}$. The peak positions are indicated by the arrows for the Na and La systems.