Using cluster perturbation theory, we explain the origin of the strongly dispersive feature found at high binding energy in the spectral function of the Hubbard model. By comparing the Hubbard and t−J−3s model spectra, we show that this dispersion does not originate from either coupling to spin fluctuations (∝ J) or the free hopping (∝ t). Instead, it should be attributed to a long-range, correlated hopping ∝ t 2 /U , which allows an effectively free motion of the hole within the same antiferromagnetic sublattice. This origin explains both the formation of the high energy anomaly in the single-particle spectrum and the sensitivity of the high binding energy dispersion to the next-nearest-neighbor hopping t ′ .PACS numbers: 71.10. Fd, 74.72.Cj,
I. INTORDUCTIONHigh-temperature superconductivity in the cuprate oxides has attracted significant attention over the past thirty years. However, the precise origin of this phenomenon is not well understood due to the complex physics even in a minimal model used to describe the correlated nature of the electrons [1][2][3] : the 2D Hubbard model. It is often assumed that a first step in understanding the physics of this model is to study its spectral properties 4-17 in the simple undoped limit. The spectral function of the undoped 2D Hubbard model when the free electron bandwidth W is comparable to the Hubbard interaction U consists of two prominent features in the band structure, cf. Fig. 1. At low binding energies (LBE), a sharply defined quasiparticle-like excitation disperses downward from (π/2, π/2) reaching an energy of the order of spin exchange J = 4t 2 /U near the Γ-point (0,0). This quasiparticle, often labeled the spin polaron (SP) 18 , represents a hole heavily dressed by spin excitations from the antiferromagnetic (AF) ground state with its bandwidth no longer governed by the free electron hopping t but by the spin exchange J 9,18-23 . At higher binding energies (HBE), another feature becomes prominent approaching the Γ-point. The delineation of the LBE spin polaron from the HBE feature has been widely associated with the "high energy anomaly" or "waterfall" seen in a number of cuprate photoemission results 11,[21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] . While the spin polaron is well understood, the characterization remains poor for the feature at higher energies. One may naively expect that a broad, non-dispersive Hubbard band should appear at the Γ-point. However, as shown in Fig. 1, it is clear that there is a sharp dispersion. Previous studies have attributed this feature to scenarios such as spin-charge separation 16,17,34,35,[41][42][43][44][45] or a weak-coupling spin-density wave 9,10,46 . However, these interpretations remain controversial.The aim of this paper is to understand the nature of the HBE feature and what separates it from the SP. Using cluster perturbation theory we examine both the Hubbard, t−J and t−J−3s models to provide an in-depth examination of what controls the quasiparticle dispersion and int...