Using the dynamical cluster approximation and quantum monte carlo we calculate the singleparticle spectra of the Hubbard model with next-nearest neighbor hopping t ′ . In the underdoped region, we find that the pseudogap along the zone diagonal in the electron doped systems is due to long range antiferromagnetic correlations. The physics in the proximity of (0, π) is dramatically influenced by t ′ and determined by the short range correlations. The effect of t ′ on the low energy ARPES spectra is weak except close to the zone edge. The short range correlations are sufficient to yield a pseudogap signal in the magnetic susceptibility, produce a concomitant gap in the singleparticle spectra near (π, π/2) but not necessarily at a location in the proximity of Fermi surface.Introduction -The normal state phase of high T c superconductors at low doping, the pseudogap (PG) region, is characterized by strong antiferromagnetic (AF) correlations and a depletion of low energy states detected by both one and two-particle measurements [1]. Whereas the d-wave superconducting phase appears to be universal in the cuprates [2,3], the PG region displays different properties in the electron and hole doped materials [4,5]. In order to further develop a theory for high T c superconductivity it is essential to have a better understanding of the asymmetry and similarities between the electron and the hole doped materials.In the hole doped cuprates the antiferromagnetism is destroyed quickly upon doping (persisting to ≈ 2% doping) [6] and the angle resolved photoemission spectra (ARPES) show well defined quasiparticles close to (π/2, π/2) in the Brillouin zone (BZ) and gap states in the proximity of (0, π) [4,7,8]. In the electron doped cuprates AF is more robust (persisting to ≈ 15% doping) [9] and the ARPES at small doping (≈ 5%) shows sharp quasiparticles at the zone edge and gap states elsewhere in the BZ [5,8]. In the Hubbard model, or the closely related t-J model, the electron-hole asymmetry can be captured by including a finite next-nearest neighbor hopping t ′ [10,11]. In this Letter we employ a reliable technique, the dynamical cluster approximation (DCA) [12,13], on relatively large clusters, to investigate the PG and single-particle spectra at small doping, the asymmetry between electron and hole-doped systems, and the role of AF correlations on the PG physics.We find that in the hole doped systems, the PG emerges in the proximity of (0, π), requires only short range correlations, and its magnitude and symmetry is strongly influenced by t ′ . In the electron doped systems, the PG emerges along the diagonal direction, as a direct consequence of AF scattering, and requires long range AF correlations, but not necessarily long range order. The hopping t ′ enhances the AF correlations in the electron doped system and produces this AF gap. With reduced temperatures, the short range AF correlations suppress the low-energy spin excitations in both electron and hole

We explore the Matsubara quasiparticle fraction and the pseudogap of the two-dimensional Hubbard model with the dynamical cluster quantum Monte Carlo method. The character of the quasiparticle fraction changes from non-Fermi-liquid, to marginal Fermi liquid, to Fermi liquid as a function of doping, indicating the presence of a quantum critical point separating non-Fermi-liquid from Fermi-liquid character. Marginal Fermi-liquid character is found at low temperatures at a very narrow range of doping where the single-particle density of states is also symmetric. At higher doping the character of the quasiparticle fraction is seen to cross over from Fermi liquid to marginal Fermi liquid as the temperature increases.

We present a diagrammatic Monte Carlo study of the properties of the Hubbard-Holstein bipolaron on a two-dimensional square lattice. With a small Coulomb repulsion, U , and with increasing electron-phonon interaction, and when reaching a value about two times smaller than the one corresponding to the transition of light polaron to heavy polaron, the system suffers a sharp transition from a state formed by two weakly bound light polarons to a heavy, strongly bound on-site bipolaron. Aside from this rather conventional bipolaron a new bipolaron state is found for large U at intermediate and large electron-phonon coupling, corresponding to two polarons bound on nearest-neighbor sites. We discuss both the properties of the different bipolaron states and the transition from one state to another. We present a phase diagram in parameter space defined by the electron-phonon coupling and U . Our numerical method does not use any artificial approximation and can be easily modified to other bipolaron models with longer range electron-phonon and/or electron-electron interaction.

Employing dynamical cluster quantum Monte Carlo calculations we show that the single particle spectral weight A(k, ω) of the one-band two-dimensional Hubbard model displays a high energy kink in the quasiparticle dispersion followed by a steep dispersion of a broad peak similar to recent ARPES results reported for the cuprates. Based on the agreement between the Monte Carlo results and a simple calculation which couples the quasiparticle to spin fluctuations, we conclude that the kink and the broad spectral feature in the Hubbard model spectra is due to scattering with damped high energy spin fluctuations. Introduction.Angle-resolved photoemission spectroscopy (ARPES) has revealed much about the cuprates, including the energy scales associated with the d-wave gap[1] and a low energy kink presumably associated with strong electron-phonon coupling [2]. Recent ARPES experiments have revealed a high-energy (HE) kink and a waterfall structure [3,4,5,6,7,8], in which the band dispersion broadens and falls abruptly at binding energies below ≈ 0.35 eV. The origin of this kink has been attributed to a crossover from the quasiparticle (QP) to the Mott-Hubbard band[4, 9] the settlement of spincharge separation [3], or interaction of the quasiparticles (QP) with spin fluctuation excitations [5,10,11,12,13].In this Letter, we study the single particle spectral weight A(k, ω) of the one-band 2D Hubbard model with near-neighbor hopping t and Coulomb interaction U in the regime where U is comparable to the bandwidth W = 8t and in the doping range relevant for cuprate superconductors. The single-band Hubbard model is believed to describe the low-energy physics of the cuprates down to energies of ≈ 2t below Fermi surface (FS). Surprisingly, the calculated spectra of the single band model are remarkably similar to the experimental ones down to binding energies of ≈ 4t − 5t. They reveal a sharp QP feature down to a kink energy E kink , followed by a steep dispersion of a broad waterfall structure. We find that these features are accurately captured by a renormalized second order (RSO) approximation to the self-energy in which the QP couple only to spin fluctuations. A careful inspection of the different contributions to the RSO self energy shows that the HE kink and the waterfall structure is due to the coupling to damped high energy spin excitations.