Instanton sector of correlated electron systems as the origin of   populated pseudo-gap and flat "band" behavior: analytic solution
  Finite temperature instantons between meta-stable vacua of correlated electronic system are solved analytically for quasi one-dimensional Hubbard model. The instantons produce dynamic symmetry breaking and connect metallic state with the dual vacua: superconducting (SC) and spin-density wave (SDW) states. The instantons spread along the Matsubara's imaginary time and possess the structure similar to the coordinate-space solitonic lattices previously discovered in quasi one-dimensional Peierls model. On the microscopic level the inter-vacua excursion is described by mutual transformations between the "resonating quartets" of the couples of electron-hole and Cooper pairs. Spectral properties of the electrons in the "instantonic crystal" reveal pseudo-gap (PG) behavior, with finite fermionic density of states in the center of the PG and ``flat-band'' outside of it. Analytically derived inverse temperature scaling of the pseudo-gap and the densities of the SC and SDW condensates is discussed in the context of ARPES and STM data in high-Tc cuprates. 
S.I. Mukhin