Unitary-transformed Fermions Theory of Iron-based Superconductors

Unitary-transformed theory of iron-based superconductors is presented. This theory is based on the d-p model emphasizing that the electronic state of superconductors can be described by the composed fermions constructed with newly defined operators. The Hamiltonian is so modified by the unitary transformation using these fermion operators as to apply the mean field approximation. The theory is extended to the case of superconducting pnictides in considering some characteristics such as multi orbitals, antiferromagnetic metal and so on. It is indicated that the effective interaction between these composite fermions can determine the several electronic properties in superconducting pnictides. It is found that the superconductive paring wavefunction is s-like state different from the d state of cuprates, but can be changeable depending on the various conditions of the electronic or crystal structures. The relations with structural modification and carrier doping are also discussed.