  In this paper, instabilities of the isotropic metallic phase in iron
pnictides are investigated. The relevant quartic fermionic interaction terms in
the model are identified using phase space arguments. Using the functional
integral formalism, a Hubbard-Stratonovich transformation is used to decouple
these quartic terms. This procedure introduces several bosonic fields which
describe the low-energy collective modes of the system. By studying the
behavior of these collective modes, a possible instability is found in the
forward scattering channel of the isotropic phase driven by magnetic
fluctuations. Using mean field analysis, we obtain a static and homogeneous
ground state. This ground state is metallic, but the electron Fermi pockets are
distorted unequally at different pockets in momentum space. This results in a
desirable nematic ordering which breaks the lattice C4 symmetry but preserves
translational symmetry and may explain several experimental observations.
