Phase diagram and optical conductivity ofLa1.8−xEu0.2SrxCuO4

Abstract

The absence of nesting between electron and hole-pockets in LiFeAs with $T_c = 18$K attracts great attention, as an important hint to understand the pairing mechanism of Fe-based superconductors. Here, we study the five-orbital model of LiFeAs based on the recently-developed orbital-spin fluctuation theories. It is found that the experimentally observed gap structure of LiFeAs, which is a "fingerprint" of the pairing mechanism, is quantitatively reproduced in terms of the orbital-fluctuation-mediated $s_{++}$-wave state. Especially, the largest gap observed on the small two hole-pockets composed of ($d_{xz}, d_{yz}$) orbitals can be explained, and this is a hallmark of the orbital-fluctuation-mediated superconductivity. The $s_{++}$-wave gap structure becomes more anisotropic in the presence of weak spin fluctuations. As the spin fluctuations increase, we obtain the "hole-$s_\pm$-wave state", in which only the gap of the large hole-pocket made of $d_{xy}$-orbital is sign-reversed, due to the cooperation of orbital and spin fluctuations. %out of the five pockets. This gap structure with "sign-reversal between hole-pockets" is similar to that recently reported in (Ba,K)Fe$_2$As$_2$.