The crystalline electric field as a probe for long range   antiferromagnetic order and superconductivity in CeFeAsO$_{1-x}$F$_x$
  We use inelastic neutron scattering to study the crystalline electric field (CEF) excitations of Ce$^{3+}$ in CeFeAsO$_{1-x}$F$_{x}$($x=0,0.16$). For nonsuperconducting CeFeAsO, the Ce CEF levels have three magnetic doublets in the paramagnetic state, but these doublets split into six singlets when Fe ions order antiferromagnetically. For superconducting CeFeAsO$_{0.84}$F$_{0.16}$ ($T_c=41$ K), where the static AF order is suppressed, the Ce CEF levels have three magnetic doublets at $\hbar\omega=0,18.7,58.4$ meV at all temperatures. Careful measurements of the intrinsic linewidth $\Gamma$ and the peak position of the 18.7 meV mode reveal clear anomaly at $T_c$, consistent with a strong enhancement of local magnetic susceptibility $\chi^{\prime\prime}(\hbar\omega)$ below $T_c$. These results suggest that CEF excitations in the rare-earth oxypnictides can be used as a probe of spin dynamics in the nearby FeAs planes. 
Songxue Chi, D. T. Adroja, T. Guidi, R. Bewley, Shiliang Li, Jun Zhao,   J. W. Lynn, C. M. Brown, Y. Qiu, G. F. Chen, J. L. ...