Electronic correlations and crystal-field effects in RCu3Ru4O12 (R = La, Pr, Nd)

Among the large class of A-site ordered perovskites of stoichiometry AC(3) B4O12, the rare-earth (R) ruthenates RCu3Ru4O12 (R = La, Pr, Nd) are interesting compounds due both to Ru-4d-derived electronic correlations and to unconventional crystal-electric-field effects of the R ions. Here we report on detailed investigations of these compounds utilizing x-ray diffraction, neutron scattering, magnetic susceptibility, and electrical resistivity measurements as well as heat capacity and nuclear resonance experiments. A broad range of external parameters is scanned and depending on the specific technique, temperatures range from 100 mK to 730 K in external magnetic fields up to 14 T. In this work LaCu3Ru4O12 serves as reference compound with a nonmagnetic A site, characterized in detail recently [S. Riegg et al., Phys. Rev. B 93, 115149 (2016)]. All compounds investigated reveal heavy-fermion behavior with a T-2 dependence of the low-temperature electrical resistivity and significantly enhanced Sommerfeld coefficients. Toward low temperatures, the compounds with R = Pr and Nd are dominated by the magnetic moments of the R ions, which occupy crystallographic positions with point-group symmetry T-h. The crystal-electric-field effects are clearly visible especially in heat capacity and inelastic neutron scattering data from which the crystal-electric-field parameters are derived. The ground state of the Pr3+ ion is identified as a triplet (Gamma((1))(4)), whereas for Nd3+ it is a quartet (Gamma(67)). Evidence for lowering of the T-h symmetry is observed at the Pr site at temperatures below 10 K, suggesting the formation of orbital order. Moreover, the spin-lattice relaxation derived from Cu-63 nuclear quadrupole resonance indicates characteristic temperatures close to 7 K and 350 mK, probably related to orbital and magnetic order, respectively.