Inelastic neutron scattering studies of Crystal Field Levels in   PrOs$_4$As$_{12}$
  We use neutron scattering to study the Pr$^{3+}$ crystalline electric field (CEF) excitations in the filled skutterudite PrOs$_4$As$_{12}$. By comparing the observed levels and their strengths under neutron excitation with the theoretical spectrum and neutron excitation intensities, we identify the Pr$^{3+}$ CEF levels, and show that the ground state is a magnetic $\Gamma_4^{(2)}$ triplet, and the excited states $\Gamma_1$, $\Gamma_4^{(1)}$ and $\Gamma_{23}$ are at 0.4, 13 and 23 meV, respectively. A comparison of the observed CEF levels in PrOs$_4$As$_{12}$ with the heavy fermion superconductor PrOs$_4$Sb$_{12}$ reveals the microscopic origin of the differences in the ground states of these two filled skutterudites. 
Songxue Chi, Pengcheng Dai, T. Barnes, H. J. Kang, J. W. Lynn, R.   Bewley, F. Ye, M. B. Maple, Z. Henkie, and A. Pietraszko