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

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Frontiers of Physics in China 5,2 (2010) 161-165;


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) similar to 4 K. On warming to above T-N(Ce) similar to 4 K, these two modes become one broad dispersionless mode that disappears just above the Fe ordering temperature T-N(Fe) similar to 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.

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