Manifestations of nematic degrees of freedom in the magnetic, elastic,   and superconducting properties of the iron pnictides
  We investigate how emergent nematic order and nematic fluctuations affect several macroscopic properties of both the normal and superconducting states of the iron pnictides. Due to its magnetic origin, long-range nematic order enhances magnetic fluctuations, leaving distinctive signatures in the spin-lattice relaxation rate, the spin-spin correlation function, and the uniform magnetic susceptibility. This enhancement of magnetic excitations is also manifested in the electronic spectral function, where a pseudogap can open at the hot spots of the Fermi surface. In the nematic phase, electrons are scattered by magnetic fluctuations that are anisotropic in momentum space, giving rise to a non-zero resistivity anisotropy whose sign changes between electron-doped and hole-doped compounds. We also show that due to the magneto-elastic coupling, nematic fluctuations soften the shear modulus in the normal state, but harden it in the superconducting state. The latter effect is an indirect consequence of the competition between magnetism and superconductivity, and also causes a suppression of the orthorhombic distortion below T_{c}. We also demonstrate that ferro-orbital fluctuations enhance the nematic susceptibility, cooperatively promoting an electronic tetragonal symmetry-breaking. Finally, we argue that T_{c} in the iron pnictides might be enhanced due to nematic fluctuations of magnetic origin. 
Rafael M. Fernandes and J\"org Schmalian
cond-mat.supr-con cond-mat.str-el