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Philosophical Magazine Letters

Variational solution of the one-dimensional t–J model

M. Gulácsi

Published Oct 1, 2004• 1 citation
Publisher

Abstract

A new variational scheme based on a modified Bethe–Peierls method is used to study the ground state properties of the one-dimensional t–J model. Expectation values are evaluated by cutting out a four-site cluster from a correlated Fermi sea, the ground state of which is described by a variational trial wave function. We study a generalized Gutzwiller state where nearest-neighbour hole–hole correlations are controlled variationally. From the electron concentration dependence of the ground state energy, we determine the true thermodynamic boundary where segregation into an electron-rich, and purely hole phase sets in. We also determine the spinodal line and pair susceptibilities. The variational method is applied also to an extended t–J–V model, where V is the coupling constant of the charge interaction term.

DOI
10.1080/09500830512331325073

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arXiv:cond-mat/9506142•Physical review. B, Condensed matter•Mar 1, 1996•14 citations
A new variational scheme based on a modified Bethe–Peierls method is used to study the ground state properties of the one-dimensional t–J model. Expectation values are evaluated by cutting out a four-site cluster from a correlated Fermi sea, the ground state of which is described by a variational trial wave function. We study a generalized Gutzwiller state where nearest-neighbour hole–hole correlations are controlled variationally. From the electron concentration dependence of the ground state energy, we determine the true thermodynamic boundary where segregation into an electron-rich, and purely hole phase sets in. We also determine the spinodal line and pair susceptibilities. The variational method is applied also to an extended t–J–V model, where V is the coupling constant of the charge interaction term.
We find the Gutzwiller projected Fermi sea wave function(GWF) has the correct phase structure to describe the kink nature of the doped holes in the ground state of the one dimensional t−Jt-Jt−J model. We find the failure of the GWF for general value of J/tJ/tJ/t and electron density nnn can be attributed to the residual charge correlation in the ground state. We find such residual charge correlation is well described by a XXZ-type effective Hamiltonian. Based on these observations, a Pfaffian-type variational wave function is proposed and is found to reproduce correctly the global phase diagram and corresponding correlation functions of the one dimensional t−Jt-Jt−J model, including the Luther-Emery phase in the low electron density and large J/tJ/tJ/t region.
Generalized Gutzwiller-type trial ground states for the Hubbard model are considered, which include extra variational parameters to control the nearest-neighbour charge-charge, charge-spin, and spin-spin correlations. The correlation between doubly occupied and empty sites is found to be particularly important. The results for the large- U regime show qualitative improvement when compared to the predictions of the simple Gutzwiller Ansatz: We get the correct order of magnitude for the energy density, and the band state occupation number is found to be a monotonically decreasing function, with the leading term corresponding to that of the t/U -expansion. We find that to get the small- U limit right, the pair-correlated Ansatz has to be used in conjunction with a cluster Gutzwiller approximation, and introduce a four-site cluster approximation which gives promising results. We argue that small-cluster Gutzwiller approximations get accurate in the limit of large lattice dimensionality.
The one-dimensional t-J model is investigated by the variational Monte Carlo method. A variational wave function based on the Bethe-ansatz solution is proposed, where the spin-charge separation is realized and a long-range correlation factor of Jastrow-type is included. In most regions of the phase diagram, this wave function provides an excellent description of the ground-state properties characterized as a Tomonaga-Luttinger liquid; both the amplitude and exponent of correlation functions are correctly reproduced. For the spin-gap phase, another trial state of correlated singlet pairs with a Jastrow factor is introduced. This wave function shows generalized Luther-Emery-liquid behavior, exhibiting enhanced superconducting correlations and exponential decay of the spin correlation function. Using these two variational wave functions, the whole phase diagram is determined. In addition, relations between the correlation exponent and variational parameters in the trial functions are derived. \textcopyright{} 1996 The American Physical Society.
We find that the Gutzwiller projected Fermi sea wavefunction (GPWF) has the correct phase structure to describe the kink nature of the doped holes in the ground state of the one dimensional t-J model. We find the residual charge correlation beyond the GPWF is well described by an XXZ-type effective Hamiltonian. A Pfaffian-type variational wavefunction with only one parameter is proposed based on this observation and is found to reproduce correctly the global phase diagram and correlation functions of the one dimensional t-J model in both the Tomonaga-Luttinger regime and the Luther-Emery regime.
The well-known Gutzwiller wave function is generalized by including new variational parameters to control nearest-neighbour charge-charge, charge-spin, and spin-spin correlations. The non-magnetic state of the one-dimensional, half-filled Hubbard model is studied. Within the Gutzwiller approximation, the expression for the ground state energy can be worked out analytically. The correlation between empty and doubly occupied sites is found to play the most essential role. Minimization in the large-U limit shows that the Brinkman-Rice transition has been pushed to U → ∞, and the leading term of the ground state energy density is of order −t2/ U . In contrast to results obtained with the Gutzwiller wave function, we find that the band occupation number nk is monotonically decreasing both above and below kF. The dominant k–dependence is given by ~(t/U) cos k, in agreement with t/U–expansion results. nk has also a weak step-function component, with the discontinuity at kF vanishing as (t/U)2 in the limit U/t ≫ 1.
The one-dimensional (1D) t-J model is investigated by using a Gutzwiller-Jastrow-type variation method and the exact diagonalization of small systems. Variational expectation values are estimated by the variational Monte Carlo method with sufficient accuracy. First, we give detailed descriptions of the preceding paper [Phys. Rev. Lett. 67, 3610 (1991)], where we discussed the properties of the Fermi-liquid-type Jastrow wave function as well as the Gutzwiller wave function. Secondly, these wave functions are compared with the Tomonaga-Luttinger-liquid-type wave function proposed by Hellberg and Mele. It is found that the correlation factors in short distances control bulk quantities like energy and the magnitude of the correlation functions, while the long-range part of the correlation factors determines the critical behavior of correlation functions. Finally, using these functions, charge, spin susceptibilities, and magnetization curve are estimated, which agree with the exact results. It is shown that the Mott transition in the 1D t-J model is quite different from the Brinkman-Rice transition. \textcopyright{} 1996 The American Physical Society.