High-field de Haas-van Alphen investigation of the filled skutterudite compound NdOs$_4$Sb$_{12}$*

The filled skutterudite compound NdOs$_4$Sb$_{12}$ displays mean-field type ferromagnetism with a Curie temperature of 0.9~K. The apparent electronic specific heat coefficient $\gamma \sim$ 0.52~Jmol$^{-1}$K$^{-2}$ is very large. In order to search for possible heavy-fermion behavior, de Haas-van Alphen (dHvA) measurements were performed on NdOs$_4$Sb$_{12}$. Experiments for H//[100] show dHvA frequencies at 695$\pm$10, 950$\pm$10, and 2560$\pm$20 T with effective masses ranging from 1.8 to 2.9 $m_e$. Moreover, the angle dependence of dHvA frequencies is very similar to that of LaOs$_4$Sb$_{12}$. Both results suggests that heavy-fermion behavior is not primary responsible for the large $\gamma$. In this context, a theoretical model by Miyake et al. indicates that the tunneling motion of the rare earth ions between off-center sites may enhance the interaction between local phonons and neighboring conduction electrons in the filled skutterudites, resulting in a large $\gamma$. This may be relevant to the extra mode found at 15~K in ultrasound measurements on NdOs$_4$Sb$_{12}$, implying abnormal behavior of the phonon spectrum in this compound.