Limits on Phase Separation for Two-Dimensional Strongly Correlated Electrons

From calculations of the high temperature series for the free energy of the two-dimensional t-J model we construct series for ratios of the free energy per hole. The ratios can be extrapolated very accurately to low temperatures and used to investigate phase separation. Our results confirm that phase separation occurs only for J/t greater than 1.2. Also, the phase transition into the phase separated state has Tc of approximately 0.25J for large J/t.Comment: 4 pages, 6 figure

Violation of Luttinger's Theorem in the Two-Dimensional t-J Model

We have calculated the high temperature series for the momentum distribution function n_k of the 2D t-J model to 12th order in inverse temperature. By extrapolating the series to T=0.2J we searched for a Fermi surface of the 2D t-J model. We find that three criteria used for estimating the location of a Fermi surface violate Luttinger's Theorem, implying the 2D t-J model does not have an adiabatic connection to a non-interacting model.Comment: 4 pages, 5 figures. Version with grayscale figures available upon reques

Phase separation at all interaction strengths in the t-J model

We investigate the phase diagram of the two-dimensional t-J model using a recently developed Green's Function Monte Carlo method for lattice fermions. We use the technique to calculate exact ground-state energies of the model on large lattices. In contrast to many previous studies, we find the model phase separates for all values of J/t. In particular, it is unstable at the hole dopings and interaction strengths at which the model was thought to describe the cuprate superconductors.Comment: Revtex, 4 pages, 3 figures. Some minor changes were made to the text and figures, and some references were adde

Stripes and the t-J model

We investigate the two-dimensional t-J model at a hole doping of x = 1/8 and J/t = 0.35 with exact diagonalization. The low-energy states are uniform (not striped). We find numerous excited states with charge density wave structures, which may be interpreted as striped phases. Some of these are consistent with neutron scattering data on the cuprates and nickelates.Comment: 4 pages; 4 eps figures included in text; Revte

Green's Function Monte Carlo for Lattice Fermions: Application to the t-J Model

We develop a general numerical method to study the zero temperature properties of strongly correlated electron models on large lattices. The technique, which resembles Green's Function Monte Carlo, projects the ground state component from a trial wave function with no approximations. We use this method to determine the phase diagram of the two-dimensional t-J model, using the Maxwell construction to investigate electronic phase separation. The shell effects of fermions on finite-sized periodic lattices are minimized by keeping the number of electrons fixed at a closed-shell configuration and varying the size of the lattice. Results obtained for various electron numbers corresponding to different closed-shells indicate that the finite-size effects in our calculation are small. For any value of interaction strength, we find that there is always a value of the electron density above which the system can lower its energy by forming a two-component phase separated state. Our results are compared with other calculations on the t-J model. We find that the most accurate results are consistent with phase separation at all interaction strengths.Comment: 22 pages, 22 figure

Phase separation and stripe formation in the 2D t-J model: a comparison of numerical results

We make a critical analysis of numerical results for and against phase separation and stripe formation in the t-J model. We argue that the frustrated phase separation mechanism for stripe formation requires phase separation at too high a doping for it to be consistent with existing numerical studies of the t-J model. We compare variational energies for various methods, and conclude that the most accurate calculations for large systems appear to be from the density matrix renormalization group. These calculations imply that the ground state of the doped t-J model is striped, not phase separated.Comment: This version includes a revised, more careful comparison of numerical results between DMRG and Green's function Monte Carlo. In particular, for the original posted version we were accidentally sent obsolete data by Hellberg and Manousakis; their new results, which are what were used in their Physical Review Letter, are more accurate because a better trial wavefunction was use