Nonlocal Excitations and 1/8 Singularity in Cuprates

Momentum-dependent excitation spectra of the two-dimensional Hubbard model on the square lattice have been investigated at zero temperature on the basis of the full self-consistent projection operator method in order to clarify nonlocal effects of electron correlations on the spectra. It is found that intersite antiferromagnetic correlations cause shadow bands and enhance the Mott-Hubbard splittings near the half-filling. Furthermore nonlocal excitations are shown to move the critical doping concentration $δ^{\ast}_{h}$, at which the singular quasiparticle peak is located just on the Fermi level, from $δ^{\ast}_{h}=0.153$ (the single-site value) to $δ^{\ast}_{h}=0.123$. The latter suggests the occurance of an instability such as the stripe at $δ^{\ast}_{h}=1/8$.

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