The extended Hubbard model with attractive interactions

Abstract

We discuss the phase diagram of the extended Hubbard model with both attractive and repulsive local and nonlocal interactions. The extended dynamical mean-field theory (EDMFT) and the dual boson method (DB) are compared. The latter contains additional nonlocal correlation effects that are not incorporated in EDMFT. We find that EDMFT and DB give almost identical results in the attractive V regime, where phase separation occurs. This is quite a difference with the previously studied repulsive V regime, where EDMFT and DB give very different phase boundaries for the checkerboard order phase, especially at small U . arXiv:1709.06379v2 [cond-mat.str-el]

Evidence for Symmetry Breaking in the Ground State of the Si(111)‐7 × 7: The Origin of Its Metal–Insulator Transition

“…For the DFA, these substrate atom are only visible in the corner holes of this idealized model. DFT calculations of the DAS structure [ 16–25 ] show that these first bilayer atoms of the substrate are also significantly shifted and perturbed from their bulk positions, many atoms by 0.3–0.7 Å, [ 24c ] as they bond to the top layer. This makes these so‐called substrate atoms an integral part of the 7 × 7 ad‐layer.…”

“…[ 59,60 ] Although the details differ, the structural similarities support the idea that strong electron correlations occur for the DFA as proposed theoretically for the 111 surfaces and the DAS structure. [ 13–16 ]…”

“…[ 6,7 ] Meanwhile, the most stable 2D terminal layer of Si that occurs on the Si(111) surface forms a 7 × 7 reconstructed layer which shows an energy gap at low temperature that disappears with increasing temperature. [ 8–12 ] This has been interpreted as a Mott Hubbard metal–insulator transition arising from a highly correlated electron system, [ 13–16 ] but for the time being it is considered here as a metal–insulator transition of undetermined origin. This experimentally observed energy gap at low temperatures is in conflict with T = 0 density functional theory (DFT) calculations that show a metallic surface.…”

“…DFT which use the local density approximation (LDA) cannot account for strong electron correlation and other approaches have been used to include them in such systems. [ 13–16 ] Progress in understanding this energy gap has been limited by more specific information about the nature of the Si(111)‐7 × 7 surface and its lowest energy state—its ground state. Hereafter, the Si(111)‐7 × 7 reconstructed surface is referred to simply as the 7 × 7.…”

Extending Density Matrix Embedding: A Static Two-Particle Theory

“…[20]. A rigorous approach to couple these effects necessitates the introduction of a frequency-dependent 'screened' Coulomb interaction in the cluster model, described as 'Extended'-DMFT [21][22][23][24][25][26][27][28][29][30]. This approach is more involved than the comparatively simple one-body embedding of DMFT, but has demonstrated that it can qualitatively describe strongly correlated materials in a fully ab initio approach, including (for example) systems of interest to high-T c superconductivity [31].…”