Spin and orbital fluctuations and pairing mechanism in the Fe-based superconductors

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Superconductivity in the Fe-based compounds, first reported by Japanese researchers in 2008, have attracted tremendous interests of researchers because of its very high transition temperature up to 56K. Since there exists antiferromagnetically ordered phase adjacent to the superconducting phase, it was argued in the beginning that the superconductivity is due to the spin fluctuations. On the other hand, it was also suggested from the multi-band character of the Fe 3d electrons that orbital degree of freedom may also play an essential role. In this work, we have tried to clarify the role of the orbital degree of freedom in the Fe-based superconductors. We showed that the anisotropic spin correlation observed in the paramagnetic phase can be explained by the random phase approximation, whereas, slight structural distortion was observed by X-ray and neutron diffraction in the paramagnetic phase, which may be related to the orbital degree of freedom. We also worked on the 123 systems.