Isotope effect and the role of phonons in the iron-based superconductors

We studied the isotope effect of phonon in the Fe-based superconductors using a phenomenological two band model for the sign-changing s-wave (+/- s-wave) state. Within this mean-field model, we showed that the large isotope effect is not inconsistent with the +/- s-wave pairing state and its high transition temperature. In principle, a large phonon isotope coefficient alpha implies a large phonon coupling constant. However, the asymmetric density of states between two bands substantially enhances the value of alpha, so that a moderate value of the phonon coupling constant (lambda(ph)approximate to 0.4) can produce a very large value of alpha(approximate to 0.4) as well as a high transition temperature together with an antiferromagnet-induced interaction.