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Physical review. B, Condensed matter

Bethe-Salpeter eigenvalues and amplitudes for the half-filled two-dimensional Hubbard model

Niyazi Bulut, Douglas J. Scalapino, Steven R. White

Published Jun 1, 1993• 50 citations
Publisher

Abstract

Monte Carlo simulations are used to determine the eigenvalues and eigenfunctions of the particle-hole and particle-particle Bethe-Salpeter equations for 8\ifmmode\times\else\texttimes\fi{}8 half-filled Hubbard lattice with U/t=4 and U/t=8. In the particle-hole channel, the dominant eigenvalue corresponds to the Q=(\ensuremath{\pi},\ensuremath{\pi}) antiferromagnetic correlations. In the particle-particle channel the amplitude of the leading low-temperature eigenvalue is an even-frequency ${\mathit{d}}_{\mathit{x}}^{2}$-${\mathit{y}}^{2}$ singlet. Odd-frequency p-wave-singlet and s-wave-triplet amplitudes are also found.

DOI
10.1103/physrevb.47.14599

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Monte Carlo simulations are used to determine the eigenvalues and eigenfunctions of the particle-hole and particle-particle Bethe-Salpeter equations for 8\ifmmode\times\else\texttimes\fi{}8 half-filled Hubbard lattice with U/t=4 and U/t=8. In the particle-hole channel, the dominant eigenvalue corresponds to the Q=(\ensuremath{\pi},\ensuremath{\pi}) antiferromagnetic correlations. In the particle-particle channel the amplitude of the leading low-temperature eigenvalue is an even-frequency dx2{\mathit{d}}_{\mathit{x}}^{2}dx2​-y2{\mathit{y}}^{2}y2 singlet. Odd-frequency p-wave-singlet and s-wave-triplet amplitudes are also found.
The Bethe-Salpeter equation is combined with the temperature-cutoff functional renormalization group approach to analyze the order parameter structure for the leading instabilities of the 2D $t\text{\ensuremath{-}}{t}^{\ensuremath{'}}$ Hubbard model. We find significant deviations from the conventional sss-, ppp-, or ddd-wave forms, which is due to the frustration of antiferromagnetism at small and intermediate ${t}^{\ensuremath{'}}$. With adding a direct antiferromagnetic spin-exchange coupling the eigenfunctions in the particle-hole channel have extended sss-wave form, while in the particle-particle singlet pairing channel a higher angular momentum component arises besides the standard ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ wave component, which flattens the angular dependence of the gap. For ${t}^{\ensuremath{'}}$ closer to t∕2t∕2t∕2 we find a delicate competition of ferromagnetism and triplet pairing with a nontrivial pair-wavefunction.
The irreducible particle-particle interaction ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{I}}$ of the two-dimensional Hubbard model is calculated by quantum Monte Carlo simulations. The Bethe-Salpeter equation for the pair-wave function is then solved using the Monte Carlo data for ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{I}}$ and the single-particle Green's function. The dominant pairing correlations in the intermediate-coupling regime and at small dopings are discussed.
The extended Hubbard model describes fermions on a lattice coupled through on-site, UUU, and first-neighbor, VVV, interactions. In the context of high-Tc{T}_{c}Tc​ cuprates, antiferromagnetic fluctuations may lead to an attractive channel, hence to superconductivity. Despite interest in the two-dimensional version of the model, the current knowledge about the phase diagram is still far from complete. Here, we report on the results of extensive determinant quantum Monte Carlo simulations for this model at half filling, in which we have used the average sign of the product of fermionic determinants as an additional observable to locate critical points. We arrive at a ground state phase diagram in the $U\text{\ensuremath{-}}V$ plane in which the boundaries involving antiferromagnetic, charge-ordered, sss- and ddd-wave superconductivity, and phase-separated phases are quantitatively set with good accuracy. We have also proposed a partial phase diagram, Tc(U,V){T}_{c}(U,V)Tc​(U,V), featuring critical temperatures for the charge-density wave and sss-wave superconducting phases.
Magnetic properties of the two-dimensional Hubbard-model are inferred from results of Monte Carlo simulations. Lattice sizes up to 8\ifmmode\times\else\texttimes\fi{}8 and temperatures down to T=t20T=\frac{t}{20}T=20t​ (t=hoppingt=\mathrm{hopping}t=hopping) were studied. The half-filled system is found to exhibit antiferromagnetic long-range order for all values of the Coulomb repulsion UUU. The low-temperature magnetic properties are found to be well described by spin-wave theory with renormalized local moment and spin-wave velocity. Numerical evidence presented suggests that when doped the system loses the long-range order immediately away from half filling.
The Hubbard model at half-filling is a collective, antiferromagnetic insulator. We study added electrons or holes. The insulating energy gap and the dispersion of the added carriers are calculated variationally in two dimensions with use of a Monte Carlo evaluation of the electronic correlation functions in the insulating phase. Both Eg{\mathit{E}}_{\mathit{g}}Eg​ and m∗{\mathit{m}}^{\mathrm{*}}m∗ are found to be temperature dependent.