Gutzwiller Density Functional Studies of FeAs-Based Superconductors: Structure Optimization and Evidence for a Three-Dimensional Fermi Surface

The electronic structures of FeAs compounds are sensitive to FeAs bonding, which is described unsuccessfully by the local density approximation (LDA). Treating the multiorbital fluctuations from ab inito LDA + Gutzwiller method, we can now predict the correct FeAs bond length and bonding strength, which will explain the observed soft phonon.'' The bands are narrowed by a factor of 2 from their LDA widths. The d(3z2-r2) orbital is pushed up to cross the Fermi level, forming a three-dimensional Fermi surface, which reduces the anisotropy. The interorbital Hund's coupling J rather than U plays a crucial role in obtaining these results.