Flat quasiparticle dispersion and a hidden small energy scale in the 2D   t-J model
  A model of weakly interacting hole quasiparticles is proposed to describe the normal state of the high temperature superconductors. The effect of strong correlations is contained in the dispersion relation of the holes, which is obtained accurately using a numerical technique and the t-J model on $12\times 12$ clusters. Saddle-points generated by many-body effects induce quasiparticle $flat$ bands similar to those observed in recent angle-resolved photoemission experiments. The near degeneracy between momentum states along the ${\rm X-Y}$ direction is a hidden small energy scale in the problem, which produces a strongly temperature dependent Hall coefficient, as well as a large Fermi surface similar to those observed in the cuprates. 
Elbio Dagotto, Alexander Nazarenko and Massimo Boninsegni