Incoherent vs. coherent behavior in the normal state of copper oxide superconductors

The self-consistent quantum fluctuations around the mean-field Hartree-Fock state of the Hubbard model provide a very good description of the ground state and low temperature properties of a 2-D itinerant antiferromagnet. Very good agreement with numerical calculations and experimental data is obtained by including the one- and two-loop spin wave corrections to various physical quantities. In particular, the destruction of the long-range order above the Neel temperature can be understood as a spontaneous generation of a length-scale epsilon(T), which should be identified as the spin correlation length. For finite doping, the question of the Hartree-Fock starting point becomes a more complex one since an extra hole tends to self-trap in antiferromagnetic background. Such quantum defects in an underlying antiferromagnetic state can be spin-bags or vortex-like structures and tend to suppress the long-range order. If motion of the holes occurs on a time-scale shorter than the one associated with the motion of these quantum defects of a spin background, one obtains several important empirical features of the normal state of CuO superconductors like linear T-dependence of resistivity, the cusp in the tunneling density of states, etc. As opposed to a familiar Fermi-liquid behavior, the phenomenology of the above system is dominated by a large incoherent piece of a single hole propagator, resulting in many unusual normal state properties

Dimer Impurity Scattering, "Reconstructed" Nesting and Density-Wave Diagnostics in Iron Pnictides

While the impurity-induced nanoscale electronic disorder has been extensively reported in the underdoped iron pnictides, its microscopic origins remain elusive. Recent scanning tunneling microscopy (STM) measurements reveal a dimer-type resonant structure induced by cobalt doping. These dimers are randomly distributed but uniformly aligned with the antiferromagnetic a axis. A theory of the impurity-induced quasiparticle interference patterns is presented that shows the local density of states developing an oscillatory pattern characterized by both geometry and orbital content of the {\em reconstructed} Fermi pockets, occasioned by the pocket density-wave (PoDW) order along the b axis. This pattern breaks the $C_4$ symmetry and its size and orientation compare well with the dimer resonances found in the STM experiments, hinting at the presence of a "hidden" PoDW order. More broadly, our theory spotlights such nanoscale structures as a useful diagnostic tool for various forms of order in iron pnictides.Comment: 5 pages, 6 figures, published versio