Bulk-sensitive photoemission study of A Cu₃ Ru₄ O12 (A=Ca, Na, and La) with heavy-fermion behavior

We have studied the d -electron heavy-fermion states in A Cu₃ Ru₄ O12 (A=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=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 Cu₃d and Ru₄d orbitals. The A -site substitution controls the filling of the Cu₃d and Ru ₄d hybridized bands. Interestingly, while the resonancelike peak position roughly follows the rigid-band shift for A=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 Cu2p core-level spectra.