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Physical Review B

Functional calculation of partition function and electron Green’s function for a D -dimensional Hubbard model

Yu-Liang Liu

Published Aug 24, 2005• 1 citation
Publisher

Abstract

Using the eigenfunctional method, we derive general functional expressions of the partition function and electron Green's function of the $D$-dimensional Hubbard model for zero and finite temperatures, in which the sign problem of fermions is avoided. These expressions can be calculated by solving the equation of a phase field and taking the functional average over a Lagrange multiplier field, while for some special cases they can be solved exactly. Moreover, we compare our results with the exact solution of the dynamical mean-field theory in the infinite-dimension limit.

DOI
10.1103/physrevb.72.075134

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Using the eigenfunctional method, we derive general functional expressions of the partition function and electron Green's function of the DDD-dimensional Hubbard model for zero and finite temperatures, in which the sign problem of fermions is avoided. These expressions can be calculated by solving the equation of a phase field and taking the functional average over a Lagrange multiplier field, while for some special cases they can be solved exactly. Moreover, we compare our results with the exact solution of the dynamical mean-field theory in the infinite-dimension limit.
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An approximate partition functional is derived for the infinite-dimensional Hubbard model. This functional naturally includes the exact solution of the Falicov-Kimball model as a special case, and is exact in the uncorrelated and atomic limits. It explicitly keeps spin symmetry. For the case of the Lorentzian density of states, we find that the Luttinger theorem is satisfied at zero temperature. The susceptibility crosses over smoothly from that expected for an uncorrelated state with antiferromagnetic fluctuations at high temperature to a correlated state at low temperature via a Kondo-type anomaly at a characteristic temperature T∗{\mathit{T}}^{\mathrm{*}}T∗. We attribute this anomaly to the appearance of the Hubbard pseudogap. The specific heat also shows a peak near T∗{\mathit{T}}^{\mathrm{*}}T∗. The resistivity goes to zero at zero temperature, in contrast to other approximations, rises sharply around T∗{\mathit{T}}^{\mathrm{*}}T∗, and has a rough linear temperature dependence above T∗{\mathit{T}}^{\mathrm{*}}T∗.
The method of a generating functional of Green’s functions was further developed within the framework of the Hubbard model and single-impurity Anderson model. In contrast to the earlier proposed works, the equations in the variational derivatives for the partition functions are presented here in the closed form, i.e. the role of variables is played by the physical matrix parameters of the systems rather than by the external local fluctuating fields. The solutions to these equations are the generating functionals of different Green’s functions. It is shown that the simplest iterative solutions in terms of the parameters U / W and W / U in the case of the Hubbard model or U /Δ and Δ/ U for the Anderson model, where U is the Coulomb repulsion on a site, W is the width of a free electron zone, and Δ is the width of an impurity level, lead to the well-known results of the weak and strong coupling limits.
We calculate the one-particle density of states for the Mott-Hubbard insulating phase of the Hubbard model on a Bethe lattice in the limit of infinite coordination number. We employ the Kato-Takahashi perturbation theory around the strong-coupling limit to derive the Green function. We show that the Green function for the lower Hubbard band can be expressed in terms of polynomials in the bare hole-hopping operator. We check our technique against the exact solution of the Falicov-Kimball model and give explicit results up to and including second order in the inverse Hubbard interaction. Our results provide a stringent test for analytical and numerical investigations of the Mott-Hubbard insulator and the Mott-Hubbard transition within the dynamical mean-field theory. We find that the Hubbard-III approximation is not satisfactory beyond lowest order, but the local-moment approach provides a very good description of the Mott-Hubbard insulator at strong coupling.