Tanmoy Das, A. V. Balatsky
Recent observation of pressure induced second superconducting phase in $A_y$Fe$_{2-x}$Se$_2$ [$A$=K, (Tl,Rb)] calls for the models of superconductivity that are rich enough to allow for multiple superconducting phases. We propose a model where pressure induced changes in Fermi surface topology lead to appearance of the hole pocket at $\Gamma$ point at high pressure. We suggest the phase assignment where low pressure superconducting state with no hole pocket at $\Gamma$ point is a nodeless d-wave state. It evolves into a $s^{\pm}$ state at higher pressure when the Fermi surface topology changes and hole pocket appears. We analyze the pairing interactions using five band tight binding fitted band structure and find the pairing strength dependents on the Fermi surface topology. We also analyze the energies and momentum dependence of neutron spin resonances in each of the phases as verifiable predictions of our proposal.
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http://arxiv.org/abs/1208.2468
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