Unconventional Pairing Originating from the Disconnected Fermi Surfaces of Superconducting ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$

For a newly discovered iron-based high $T_c$ superconductor ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$, we have constructed a minimal model, where inclusion of all five Fe $d$ bands is found to be necessary. The random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin-fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended $s$-wave pairing, while $d$-wave pairing can also be another candidate.

Figure 1

(a) The band structure of the five-band model in the unfolded BZ, where the interlayer hoppings are included. To compare with the ten-band model (thick red lines in the inset; the symbols are the present local-density approximation results), note the original (dashed lines) and the unfolded (solid lines) BZ shown in (b). (b) Fermi surface for $n=6.1$ (with the interlayer hoppings ignored), with the arrows indicating the nesting vectors. Inset depicts the original (dashed lines) and reduced (solid lines) unit cell in real space.

Figure 2

RPA result for the spin susceptibility ${\chi }_s$ (a), the gap functions ${\varphi }_3$ (c) and ${\varphi }_4$ (d), $\sqrt{(\widehat{\varphi }{\widehat{\varphi }}^{†}{)}_{44}}$ (e) for $U=1.2$, $U^{\prime }=0.9$, $J=J^{\prime }=0.15$, $n=6.1$, and $T=0.02$ (in eV). In (c) and (d), the black (green or light gray) solid lines represent the Fermi surfaces (gap nodes). In (b), the fully gapped extended $s$-wave (upper panel) and $d_{x^2-y^2}$-wave gaps are schematically shown.