, Volume 5, Issue 2, pp 161-165
Date: 13 Nov 2009

Low-energy Ce spin excitations in CeFeAsO and CeFeAsO0.84 F0.16

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We use inelastic neutron scattering to study the low-energy spin excitations of polycrystalline samples of nonsuperconducting CeFeAsO and superconducting CeFeAsO0.84F0.16. Two sharp dispersionless modes are found at 0.85 and 1.16 meV in CeFeAsO below the Ce antiferromagnetic (AF) ordering temperature of T N Ce ˜ 4 K. On warming to above T N Ce ˜ 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T N Fe ˜ 140 K. For superconducting CeFeAsO0.84F0.16, where Fe static AF order is suppressed, we find a weakly dispersive mode center at 0.4 meV that may arise from short-range Ce-Ce exchange interactions. Using a Heisenberg model, we simulate powder-averaged Ce spin wave excitations. Our results show that we need both Ce spin wave and crystal electric field excitations to account for the whole spectra of low-energy spin excitations.