Nematicity driven by hybridization in iron-based superconductors

In this Rapid Communication, we study an effective model for the normal state of iron-based superconductors. It has separate but interacting itinerant and localized degrees of freedom, originating from the $d_{xz}$ and $d_{yz}$ and from the $d_{xy}$ iron orbitals, respectively. At low temperatures, below a mean-field phase transition, these different states condense together in an excitonic order parameter. We show that, at even lower temperatures, after another phase transition, this ordered state can spontaneously break the $C_4$ lattice symmetry and become nematic. We propose this mechanism as an explanation of the tendency towards nematicity observed in several iron-based compounds.

Figure 1

Plot of the solutions for $\varphi$ (purple line), $b$ (green line), and $\lambda$ (red dashed line) of the self-consistency equations,[45] that show the mean-field phase transition at $T_c$. The delocalization of the $d$ electrons is described by the appearance of finite negative $\varphi$ and positive $b$. The inset shows a zoom of the $T_c$ region.

Figure 2

On the left is the Feynman diagram that contributes to ${\gamma }_2$ because of the ellipticity of the $c_3$ and $c_4$ bands. On the right is the diagram of a process generated by the interband interactions.

Figure 3

Plot of $b$ (green line), ${\varphi }_3$ (red line), and ${\varphi }_4$ (blue line) that minimizes the GL functional.[46] Below $T\approx 0.94T_c$, the effect of positive ${\gamma }_2$ overcomes the linear coupling with $b$, and an additional solution appears with ${\varphi }_3\ne {\varphi }_4$. In the inset, we show that the free energy of this solution (red line) is below the free energy of the symmetric solution (purple line). The new order parameter breaks the $C_4$ symmetry.