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arXiv:cond-mat/9407097 · Physical review. B, Condensed matter

Hole pockets in thet-Jmodel

R. Eder, Y. Ohta

Published Mar 1, 1995• 22 citations
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Abstract

We present an exact diagonalization study of the electron momentum distribution n(k) for small clusters of the model t-J model for different hole concentrations and t/J. To obtain reliable information about the Fermi surface, we focus on the regime tJ, where the backflow effect is weak, and we suppress the formation of a bound state by introducing a density repulsion between holes. We find clear signatures of a Fermi surface which takes the form of small hole pockets. Comparison of the scaling of n(k) and that of the quasiparticle weight with t/J suggests that these pockets persist also for t>J.

DOI
10.1103/physrevb.51.6041

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arXiv:cond-mat/9308010•Physical review. B, Condensed matter•Feb 1, 1994•27 citations
We present an exact diagonalization study of the electron momentum distribution n(k) for small clusters of the model t-J model for different hole concentrations and t/J. To obtain reliable information about the Fermi surface, we focus on the regime tJ, where the backflow effect is weak, and we suppress the formation of a bound state by introducing a density repulsion between holes. We find clear signatures of a Fermi surface which takes the form of small hole pockets. Comparison of the scaling of n(k) and that of the quasiparticle weight with t/J suggests that these pockets persist also for t>J.
The evolution of the Fermi surface upon hole doping is studied in the t-J model of correlated electrons by exact diagonalization of chains and planes. In one dimension at low hole doping, the momentum distribution function n(k) indicates the presence of pocketlike features at the (noninteracting) Fermi momentum, while increasing the density of holes a large Fermi surface is observed. Although the results in two dimensions are consistent with this picture, conclusive evidence for the existence of hole pockets (at intermediate doping levels) cannot be provided in the present study of 4\ifmmode\times\else\texttimes\fi{}4 and \ensuremath{\surd}18 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}18 square lattices. In order to improve the resolution in momentum space, twisted boundary conditions are used for the two-dimensional clusters.
The 2D t-J model is studied with exact-diagonalization techniques applied to clusters of sixteen to twenty sites with various hole doping levels. Already for two holes (\ensuremath{\ge}10% doping), a large electronic Fermi surface, compatible with Luttinger's theorem, is observed. The full single-particle Green's function is examined, and is shown to exhibit quasiparticlelike behavior, with dispersion consistent with the band structure of the noninteracting limit, and bandwidth scaling approximately as J for J smaller than t.
We investigate the effect of doping in the hole spectral function by exact diagonalization of a 16-site cluster with two holes. It is shown that for more than one hole the spectral function differs qualitatively from the one in the single-hole limit. In particular, we find no evidence for the formation of a Fermi surface having hole ``pockets'' localized around (\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2) in momentum space. On the contrary, a large low-energy peak appears in the spectral function of the hole propagator at k=(\ensuremath{\pi},0) or (0,\ensuremath{\pi}) which is then the best candidate for the existence of those pockets. A (\ensuremath{\pi}/2,\ensuremath{\pi}/2) quasihole state is obtained as a small energy excitation of the two-hole system. We discuss some consequences for photoemission and optical-conductivity experiments in high-Tc{\mathit{T}}_{\mathit{c}}Tc​ materials at intermediate doping.
We present an exact diagonalization study of the doping dependence of the single-particle Green's function in 16-, 18-, and 20-site clusters of the t-J model. We find evidence for the validity of the rigid-band picture starting from the half-filled case: upon doping, the topmost states of the quasiparticle band observed in the photoemission spectrum at half-filling cross the chemical potential and reappear as the lowermost states of the inverse photoemission spectrum. Features in the inverse photoemission spectra, which are inconsistent with the rigid-band picture, are shown to originate from the nontrivial point-group symmetry of the ground state with two holes, which enforces different selection rules than at half-filling. Deviations from rigid-band behavior which lead to the formation of a large Fermi surface in the momentum distribution are found to occur at energies far from the chemical potential. A Luttinger Fermi surface and a nearest-neighbor hopping band do not exist.
We study the momentum distribution function n(k) for the two-dimensional t-J model by high-temperature expansions. Starting with a k-independent n(k) at T=\ensuremath{\infty}, we observe the buildup of a sharp Fermi-surface-like feature as the temperature is lowered. For very small density of electrons, our results are clearly consistent with Luttinger's theorem. At all fillings the contours of constant n(k) closely resemble those of the tight-binding model. No evidence for pocketlike Fermi surfaces is seen. Far from the Fermi surface n(k) has a particle-hole asymmetry similar to that found in the one-dimensional, U=\ensuremath{\infty} Hubbard model.
We have calculated S(q) and the single particle distribution function 〈nq{\mathit{n}}_{\mathbf{q}}nq​〉 for N holes in the t-J model on a nonsquare \ensuremath{\surd}8 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}32 16-site lattice with periodic boundary conditions. We justify the use of this lattice by appealing to results obtained from the conventional 4\ifmmode\times\else\texttimes\fi{}4 16-site cluster, and an undoped 32-site system, each having the full square symmetry of the bulk. This new cluster has a high density of k points along the diagonal of reciprocal space, viz. along k=k(1,1). The results clearly demonstrate that when the single-hole problem has a ground state with a system momentum of k=(\ensuremath{\pi}/2,\ensuremath{\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 q=(\ensuremath{\pi},\ensuremath{\pi}). 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=(\ensuremath{\pi}/2,\ensuremath{\pi}/2), something that may easily be accomplished through the use of the t-t'-J model with t'/t small and positive, then the above-mentioned incommensurability for N holes is not found---the maximum of S(q) remains at q=(\ensuremath{\pi},\ensuremath{\pi}) for all N.The existence of the incommensurate ground states are presented in conjunction with the electron and hole momentum distribution functions. Our results may be interpreted as evidence for rigid-band filling of the dipolar spin distortion states of Shraiman and Siggia---by studying the hole-hole correlation function we demonstrate that the holes on our cluster tend to stay as far apart as is possible, and thus rigid-band filling is not unexpected. Thus, these results demonstrate that in some instances important results for moderately doped CuO2{\mathrm{CuO}}_{2}CuO2​ planes can be predicted from a knowledge of the properties of weakly doped planes. However, the electron Fermi surface that we obtain also satisfies Luttinger's theorem, and has dimples at the wave vectors of the single hole ground state, k=(\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2). Finally, we comment on the relation of all of these results to the incommensurability found in S(q).