Theory of Raman response in a superconductor with extended s-wave symmetry: Application to Fe-pnictides

We argue that Raman study of Fe-pnictides is a way to unambiguously distinguish between various superconducting gaps proposed for these materials. We show that $A_{1g}$ Raman intensity has a true resonance peak below $2\Delta$ for extended s-wave superconducting gap, $\Delta(\mathbf{k}) = \Delta (\cos k_x + \cos k_y)/2$ in the folded Brillouin zone. No such peak emerges for a pure s-wave gap, a d-wave gap, and another extended s-wave gap with $\Delta(\mathbf{k}) = \Delta \cos{\frac{k_x}{2}} \cos{\frac{k_y}{2}}$ proposed by several groups.