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Unconventional Superconductivity

Orbital Fluctuation Mechanism in Fe-based Superconductors

Phase diagram of Fe-based superconductors.
In FeSe, no magnetic order emerges in the nematic
state below TS. The compound-dependence of the
phase diagrams is naturally explained by considering
the Aslamasov-Larkin vertex corrections.

Fe-based superconductors, which was discovered in 2008, exhibit high-Tc next to the cuprate superconductors. After thediscovery, lots of researched have been performed for these hogh-Tc superconductors all over the world. To solve important unsolved issues, we have focused on the many-body effect beyond the mean-field-type approximations, called the vertex corrections.

In Fe-based superconductors, the conduction electrons acquire d-orbital degrees of freedom, and interesting “orbital physics” emergenear the superconducting phase. We discovered that the electronic nematic transition in Fe-based superconductors, which is the spontaneous rotational symmetry breaking of correlated electrons, originates from the orbital order driven by vertex corrections [1,2]. The strong orbital fluctuations mediate the s-wave SC gap without sign reversal, called the s++ wave state [3,4]. The s++ wave state is supported by the tiny impurity effect on Tc [5] and the resonance-like hump structure in neutron inelastic scattering measurements [6].

Recently, novel electronicproperties in FeSe attract increasing attention. We revealed that the “nonmagnetic nematic state” in FeSe is naturally explained as the orbital order due to the vertex correction [7,8]. The increment of Tc under the pressure (Tc~40K) and heavily electron-doping (~65K) is understood as the orbital fluctuation pairing mechanism.

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