Hubbard model versus t-J model: The one-particle spectrum

The origin of the apparent discrepancies between the one-particle spectra of the Hubbard and t-J models is revealed: Wavefunction corrections, in addition to the three-site terms, should supplement the bare t-J. In this way a quantitative agreement between the two models is obtained, even for the intermediate-$U$ values appropriate for the high-Tc cuprate superconductors. Numerical results for clusters of up to 20 sites are presented. The momentum dependence of the observed intensities in the photoemission spectra of Sr2CuO2Cl2 are well described by this complete strong-coupling approach.Comment: 4 two-column RevTeX pages, including 4 Postscript figures. Uses epsf. Accepted for publication in Physical Review B, Rapid Communicatio

Reduction of three-band model for copper oxides to single-band generalized t~-~J model

A three-band model for copper oxides in the region of parameters where the second hole on the copper has energy close to the first hole on the oxygen is considered. The exact solution for one hole on a ferromagnetic background of the ordered copper spins is obtained. A general procedure for transformation of the primary Hamiltonian to the Hamiltonian of singlet and triplet excitations is proposed. Reduction of the singlet-triplet Hamiltonian to the single-band Hamiltonian of the generalized t~-~J model is performed. A comparison of the solution for the generalized t~-~J model on a ferromagnetic background with the exact solution shows a very good agreement.Comment: 20 pages (LATEX

Phase diagram of the two-dimensional t--J model at low doping

The phase diagram of the planar t--J model at small hole doping is investigated by finite size scaling of exact diagonalisation data of NXN clusters (up to 26). Hole-droplet binding energies, compressibility and static spin and charge correlations are calculated. Short range antiferromagnetic correlations can produce attractive forces between holes leading to a very rich phase diagram including a liquid of d-wave hole pairs (for $J/t\gtrsim 0.2$), a liquid of hole droplets (quartets) for larger J/t ratios ($J/t\gtrsim 0.5$) and, at even larger coupling J/t, an instability towards phase separation.Comment: 3 pages, latex, 5 postscript figures, uuencode

An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model

We have calculated S(q) and the single particle distribution function for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site lattice with periodic boundary conditions; we justify the use of this lattice in compariosn to those of having the full square symmetry of the bulk. This new cluster has a high density of vec k points along the diagonal of reciprocal space, viz. along k = (k,k). The results clearly demonstrate that when the single hole problem has a ground state with a system momentum of vec k = (pi/2,pi/2), the resulting ground state for N holes involves a shift of the peak of the system's structure factor away from the antiferromagnetic state. This shift effectively increases continuously with N. When the single hole problem has a ground state with a momentum that is not equal to k = (pi/2,pi/2), then the above--mentioned incommensurability for N holes is not found. The results for the incommensurate ground states can be understood in terms of rigid--band filling: the effective occupation of the single hole k = (pi/2,pi/2) states is demonstrated by the evaluation of the single particle momentum distribution function . Unlike many previous studies, we show that for the many hole ground state the occupied momentum states are indeed k = (+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include

Phase separation in the large-spin t-J model

We investigate the phase diagram of the two-dimensional t-J model using a recently developed technique that allows one to solve the mean-field model Hamiltonian with a variational calculation. The accuracy of our estimate is controlled by means of a small parameter 1/q, analogous to the inverse spin magnitude 1/s employed in studying quantum spin systems. The mathematical aspects of the method and its connection with other large-spin approaches are discussed in detail. In the large-q limit the problem of strongly correlated electron systems turns into the minimization of a total-energy functional. We have performed this optimization numerically on a finite but large L x L lattice. For a single hole the static small-polaron solution is stable except for small values Of J, where polarons of increasing sizes have lower energy. At finite doping we recover phase separation above a critical J and for any electron density, showing that the Emery et al. picture represents the semiclassical behavior of the t-J model. Quantum fluctuations are expected to be very important, especially in the small-J-small-doping region, where Phase separation may also be suppressed