Spin and Orbital Nematic Susceptibility of Iron Based Superconductors*

The spin and orbital nematic susceptibility of the iron-based superconductors are calculated in the undoped limit using a three-orbital (xz, yz, xy) spin-fermion model that includes spin, orbital, and lattice degrees of freedom [1]. The results are in very good agreement with experiments by J-H. Chu et al., Science 337, 710 (2012). Recently, Raman scattering experiments[2] indicate a Curie-Weiss behavior of the orbital nematic susceptibility in BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. This behavior is observed in our numerical simulations after considering the coupling between the spin and orbital nematic order parameters. A Landau-Ginzburg formalism is used to analytically derive equations for both the spin and orbital susceptibility that fit well the numerical data. [1] S. Liang et al., arXiv:1405.6395 (2014), to appear in PRB. [2] Y.-X. Yang, et al., ``Raman scattering as a probe of charge nematic fluctuations in iron based superconductors,'' {\em JPS Conf. Proc. 3 , 015001}, 2014.