Nematic and antiferromagnetic order in iron-pnictide superconductors

Iron-based high-temperature superconductors exhibit a transition to an electronic nematic phase in which the crystal rotation symmetry is spontaneously broken. We consider a model for correlated electrons consisting of the Fermi surfaces of an electron and a hole pocket separated by a nesting vector Q and interactions between electrons giving rise to itinerant antiferromagnetism (AF). Assuming that the vector Q is commensurate with the lattice (Umklapp with Q = G/2), pairs of electrons can be transferred between the pockets. This process may lead to a superconducting (S+) dome at low temperatures. The Pomeranchuk effect transforms circular orbits into elliptical ones and yields an electronic nematic phase (n). The AF and the n order can gradually be suppressed by mismatching the nesting of the Fermi surfaces. Both, the boundary between the nematic and paramagnetic phases and the boundary between the nematic and antiferromagnetic phases merge into the superconducting dome giving rise to several phases: a pure S+-phase and two mixed phases (S+ with n (nematic), and S+ with n and AF). The system has a quantum critical point (QCP) at the T = 0 endpoint of the boundary between the paramagnetic and S(+ )phases.