Theory of magnetism and triplet superconductivity in LiFeAs

Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by its (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet $p$-wave superconductivity.

Figure 1

(a) Band structure (dashed lines) of the three-band model Eq. (1) from Ref. 25 and (solid lines) the model for LiFeAs. (b) Fermi surface and dominant orbitals for the LiFeAs model; ${\alpha }_1$ and ${\alpha }_2$ are hole pockets, ${\beta }_1$ and ${\beta }_2$ are electron pockets. Parameters: $t_1=0.02$, $t_2=0.12$, $t_3=0.02$, $t_4=-0.046$, $t_5=0.2$, $t_6=0.3$, $t_7=-0.15$, $t_8=-t_7/2$, $t_9=-0.06$, $t_{10}=-0.03$, $t_{11}=0.014$, ${\Delta }_{\mathit{xy}}=1$, $\mu =0.338$. The filling is four electrons per site, energies are in eV.

Figure 2

Magnetic phase diagram of the three-band model for LiFeAs. Inset: Magnetic susceptibility ${\chi }^{\mathit{zz}}(\mathbf{q})$ for $U=0.42\hspace{0.5em}$eV, $T=35$ K, and $T=135$ K.

Figure 3

Triplet pairing. (a) Eigenvalues $\lambda$ of the leading gap symmetries as a function of $U$. (b) Gap around the Fermi pockets for the $p_x$-wave pairing, which is a member of the degenerate pair that gives the dominant triplet $p$ channel. The winding angle about the FSs $\theta$ is measured with respect to the $k_x$ axis, taken in the anticlockwise direction. (c) Gap magnitude of pairing states combining the degenerate $p_x$ and $p_y$ solutions of Eq. (12). ${\alpha }_1$, ${\alpha }_2$, ${\beta }_1$, and ${\beta }_2$ refer to the FS pockets; see Fig. 1b.