Perturbation calculation of the extended Hubbard model at strong coupling

S. Pairault, David Sénéchal, A.–M. S. Tremblay

E.S. Caixeiro, A. Troper

S. N. Coppersmith, P. B. Littlewood

A charge-transfer instability that occurs in an extended Hubbard model with strong nearest-neighbor repulsions is examined nonperturbatively using variational wave functions. We show that a single mode approximation applied to a Hartree-Fock wave function accurately reproduces previous weak-coupling results on excitonic spectra. We then apply the method to a Gutzwiller variational wave function and extract information about the dynamic charge fluctuations in the intermediate and strong-coupling regimes.

V. M. Zharkov

P. G. J. van Dongen

The half-filled extended Hubbard model (containing nearest-neighbor interaction) is considered at strong coupling in fourth-order perturbation theory. The ground-state energies in the spin- and charge-density-wave phases and the resulting phase diagram are calculated in various dimensions: here d=1, d=\ensuremath{\infty}, high dimensions (d\ensuremath{\gg}1), and d=2,3 are discussed separately. In d=1 it is shown that, for U\ensuremath{\gtrsim}6, fourth-order perturbation theory leads to excellent agreement with existing Monte Carlo data. Second-order perturbation theory alone is valid only at unrealistically large values of U and V. In d=\ensuremath{\infty}, one needs fourth-order perturbation theory to obtain any nontrivial contribution to the phase diagram at all. As a consequence the fourth-order corrections to the ground-state energy and to the phase diagram are large also in d=2,3. The formalism used to obtain the perturbative expansion is discussed in an appendix.

Li Liu, Hong Yao, Steven Allan Kivelson