Anomalous thermal expansion and strong damping of the thermal conductivity of NdMnO3_3 and TbMnO3_3 due to 4f crystal-field excitations

We present measurements of the thermal conductivity $\kappa$ and the thermal expansion $\alpha$ of NdMnO$_3$ and TbMnO$_3$. In both compounds a splitting of the $4f$ multiplet of the $R^{3+}$ ion causes Schottky contributions to $\alpha$. In TbMnO$_3$ this contribution arises from a crystal-field splitting, while in NdMnO$_3$ it is due to the Nd-Mn exchange coupling. Another consequence of this coupling is a strongly enhanced canting of the Mn moments. The thermal conductivity is greatly suppressed in both compounds. The main scattering process at low temperatures is resonant scattering of phonons between different energy levels of the $4f$ multiplets, whereas the complex 3d magnetism of the Mn ions is of minor importance.Comment: 9 pages including 6 figure

A novel heavy-fermion state in CaCu_3Ru_4O12

We have measured susceptibility, specific heat, resistivity, and thermopower of CaCu$_3$Ti$_{4-x}$Ru$_x$O$_{12}$ and CaCu$_{3-y}$Mn$_y$Ru$_4$O$_{12}$, and have found that CaCu$_3$Ru$_4$O$_{12}$ can be regarded as a heavy-fermion oxide in d-electron systems. The Kondo temperature is near 200 K, and the susceptibility (1.4$\times10^{-3}$ emu/Cu mol) and the electron specific heat coefficient (28 mJ/Cu molK$^2$) are moderately enhanced. The resistivity is proportional to $T^2$ at low temperatures, and satisfies the Kadowaki-Woods relation. The heavy-fermion state comes from the interaction between the localized moment of Cu 3d and the conduction electron of Ru 4d. An insulator-metal transition occurs between $x=1.5$ and 4 in CaCu$_3$Ti$_{4-x}$Ru$_x$O$_{12}$, which can be regarded as a transition from magnetic insulator to heavy-fermion metal.Comment: 4 pages, 5 figures, submitted to J. Phys. Soc. Jp