Crystalline electric field as a probe for long-range antiferromagnetic order and superconducting state of CeFeAsO1-xFx

We use inelastic neutron scattering to study the crystalline electric field (CEF) excitations of Ce3+ in CeFeAsO1-xFx (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 the Fe ions order antiferromagnetically. For superconducting CeFeAsO0.84F0.16 (Tc=41K), where the static antiferromagnetic order is suppressed, the Ce CEF levels have three magnetic doublets at ω=0, 18.7, 58.4 meV at all temperatures. Careful measurements of the intrinsic linewidth Γ and the peak position of the 18.7 meV mode reveal a clear anomaly at Tc, consistent with a strong enhancement of local magnetic susceptibility χ′′( ω) below Tc. 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.