Doping evoluton of antiferromagnetic order and structural distortion in LaFeAsO1−xFx

We use neutron scattering to study the structural distortion and antiferromagnetic (AFM) order in LaFeAsO1−xFx as the system is doped with fluorine (F) to induce superconductivity. In the undoped state, LaFeAsO exhibits a structural distortion, changing the symmetry from tetragonal (space group P4/nmm) to orthorhombic (space group Cmma) at 155 K, and then followed by an AFM order at 137 K. Doping the system with F gradually decreases the structural distortion temperature, but suppresses the long range AFM order before the emergence of superconductivity. Therefore, while superconductivity in these Fe oxypnictides can survive in either the tetragonal or the orthorhombic crystal structure, it competes directly with static AFM order.

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http://arxiv.org/pdf/0809.4816.pdf