Theoretical studies of the effects of orbital ordering on spin fluctuations and superconductivity in FeSe

FeSe is currently one of the most hotly debated iron-based systems due in part to its very high T$_{c}$ when monolayers are placed on STO substrates, and in part due to the fact that the material exhibits a structural distortion near T$_{S}$, $\sim$ 90K without any concomitant magnetic order. In addition, undoped bulk FeSe samples, which become superconducting below T $\sim$ 8K, display evidence of orbital ordering setting in near T$_{S}$. We discuss the normal and superconducting properties of FeSe using a ten orbital tight-binding model, and include the effect of ferro-orbital ordering. The model reproduces the essential features of FeSe band structure seen in ARPES [1] and quantum oscillation experiments [3]. Using this model, the spin lattice relaxation rate is calculated and the results are compared with recent NMR experiments [2]. We next discuss the consequences of a spin fluctuation mediated superconducting pairing in FeSe and the resulting gap structure. Finally, the local density of states derived from our calculations is compared to STM experiments [4]. \\[4pt] [1] T. Shimojima et al., Phys. Rev. B \textbf{90}, 121111(R) (2014).\\[0pt] [2] S.-H. Baek, et al., ArXiv:1408.1875.\\[0pt] [3] T. Terashima et al., Phys. Rev. B \textbf{90}, 144517 (2014).\\[0pt] [4] C. L. Song et al$.,$ Science \textbf{332}, 1410 (2010).