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We used polarization-dependent angle-resolved photoemission spectroscopy (ARPES) to study the high-energy anomaly (HEA) in the dispersion of Nd2-xCexCuO4, (x=0.123). We have found that at particular photon energies the anomalous, waterfalllike dispersion gives way to a broad, continuous band. This suggests that the HEA is a matrix element effect: it arises due to a suppression of the intensity of the broadened quasi-particle band in a narrow momentum range. We confirm this interpretation experimentally, by showing that the HEA appears when the matrix element is suppressed deliberately by changing the light polarization. Calculations of the matrix element using atomic wave functions and simulation of the ARPES intensity with one-step model calculations provide further proof for this scenario. The possibility to detect the full quasi-particle dispersion further allows us to extract the high-energy self-energy function near the center and at the edge of the Brillouin zone., Comment: 5 pages 2 figures, corrected typos

The high-energy kink or the waterfall effect seen in the photoemission spectra of the cuprates is suggestive of the coupling of the quasiparticles to a high energy bosonic mode with implications for the mechanism of superconductivity. Recent experiments however indicate that this effect may be an artifact produced entirely by the matrix element effects, i.e. by the way the photoemitted electron couples to the incident photons in the emission process. In order to address this issue directly, we have carried out realistic computations of the photo-intensity in ${\rm Bi_2Sr_2CaCu_2O_8}$ (Bi2212) where the effects of the matrix element are included together with those of the corrections to the self-energy resulting from electronic excitations. Our results demonstrate that while the photoemission matrix element plays an important role in shaping the spectra, the waterfall effect is a clear signature of the presence of strong coupling of quasiparticles to electronic excitations., Comment: 5 pages, 4 figures

We derive photoelectron selection rules along the glide plane in orthorhombic Bi_2Sr_2CaCu_2O_{8+\delta} (Bi2212). These selection rules explain the reversed intensity behavior of the shadow and the main band of the material as a natural consequence of the variating representation of the final state as a function of k_\parallel. Our one-step simulations strongly support the structural origin of the shadow band but we also introduce a scenario for detecting antiferromagnetic signatures in low doping., Comment: AMS-LaTeX, 5 pages, 4 figures

By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ with structural information from LEED and {\it ab initio} one-step photoemission simulations, we prove that the shadow Fermi surface in these systems has structural origin, being due to orthorhombic distortions from tetragonal symmetry present in both surface and bulk. Consequently, one of the longest standing open issues in the fermiology of these widely studied systems finally meets its resolution., Comment: v2. Marginal changes. Accepted for publication in PRL, Jan. 2006

We discuss the effects of interlayer hopping and the resulting $k_z$-dispersion in the cuprates within the framework of the one-band tight binding (TB) model Hamiltonian. Specific forms of the dispersion relations in terms of the in-plane hopping parameters $t$, $t'$, $t''$ and $t'''$ and the effective interlayer hopping $t_z$ in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO) and the added intracell hopping $t_{bi}$ between the CuO$_2$ bilayers in Bi$_2$Sr$_2$CaCu$_2$O$_{8}$ (Bi2212) are presented. The values of the `bare' parameters are obtained via fits with the first principles LDA-based band structures in LSCO, NCCO and Bi2212. The corresponding `dressed' parameter sets which account for correlation effects beyond the LDA are derived by fitting experimental FS maps and dispersions near the Fermi energy in optimally doped and overdoped systems. The interlayer couplings $t_z$ and $t_{bi}$ are found generally to be a substantial fraction of the in-plane hopping $t$, although the value of $t_z$ in NCCO is anomalously small, reflecting absence of apical O atoms in the crystal structure. Our results provide some insight into the issues of the determination of doping from experimental FS maps in Bi2212, the role of intercell coupling in c-axis transport, and the possible correlations between the doping dependencies of the binding energies of the Van Hove singularities (VHSs) and various prominent features observed in the angle-resolved photoemission (ARPES) and tunneling spectra of the cuprates., Comment: 14 pages, 14 figures

We have carried out extensive first principles doping-dependent computations of angle-resolved photoemission (ARPES) intensities in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over a wide range of binding energies. Intercell hopping and the associated 3-dimensionality, which is usually neglected in discussing cuprate physics, is shown to play a key role in shaping the ARPES spectra. Despite the obvious importance of strong coupling effects (e.g. the presence of a lower Hubbard band coexisting with mid-gap states in the doped insulator), we show that a number of salient features of the experimental ARPES spectra are captured to a surprisingly large extent when effects of $k_z$-dispersion are properly included in the analysis., Comment: 5 pages, 4 figures

Angle-resolved photoemission spectroscopy (ARPES) presents significant simplications in analyzing strictly two-dimensional (2D) materials, but even the most anisotropic physical systems display some residual three-dimensionality. Here we demonstrate how this third dimension manifests itself in ARPES spectra of quasi-2D materials by considering the example of the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8}$ (Bi2212). The intercell, interlayer hopping, which is responsible for $k_z$-dispersion of the bands, is found to induce an irreducible broadening to the ARPES lineshapes with a characteristic dependence on the in-plane momentum $k_\parallel$. Our study suggests that ARPES lineshapes can provide a direct spectroscopic window for establishing the existence of coherent c-axis conductivity in a material via the detection of this new broadening mechanism, and bears on the understanding of 2D to 3D crossover and pseudogap and stripe physics in novel materials through ARPES experiments., Comment: 5 pages, 4 figures

We discuss selected results from our recent work concerning the ARPES (angle-resolved photoemission) spectra from the cuprates. Our focus is on developing an understanding of the effects of the ARPES matrix element and those of strong electron correlations in analyzing photointensities. With simulations on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212), we show that the ARPES matrix element possesses remarkable selectivity properties, such that by tuning the photon energy and polarization, emission from the bonding or the antibonding states can be enhanced. Moreover, at low photon energies (below 25 eV), the Fermi surface (FS) emission is dominated by transitions from just the O-atoms in the CuO$_2$ planes. In connection with strong correlation effects, we consider the evolution with doping of the FS of Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ (NCCO) in terms of the $t$-$t'$-$U$ Hubbard model Hamiltonian. We thus delineate how the FS evolves on electron doping from the insulating state in NCCO. The Mott pseudogap is found to collapse around optimal doping suggesting the existence of an associated quantum critical point., Comment: 5 pages, 4 figures, accepted to be published in Journal of Physics and Chemistry of Solids

We analyze doping dependent spectral intensities and Fermi surface maps obtained recently in Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ (NCCO) via high resolution ARPES measurements, and show that the behavior of this electron-doped compound can be understood as the closing of a Mott (pseudo) gap, leading to a quantum critical point just above optimal doping. The doping dependence of the effective Hubbard $U$ adduced by comparing theoretical and experimental spectra is in resonable accord with various estimates and a simple screening calculation., Comment: 12 pages, 6 figures

We report angle-resolved photoemission spectra (ARPES) from the Fermi energy ($E_F$) over a large area of the ($k_x,k_y$) plane using 21.2 eV and 32 eV photons in two distinct polarizations from an optimally doped single crystal of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212), together with extensive first-principles simulations of the ARPES intensities. The results display a wide-ranging level of accord between theory and experiment and clarify how myriad Fermi surface (FS) maps emerge in ARPES under various experimental conditions. The energy and polarization dependences of the ARPES matrix element help disentangle primary contributions to the spectrum due to the pristine lattice from those arising from modulations of the underlying tetragonal symmetry and provide a route for separating closely placed FS sheets in low dimensional materials., Comment: submitted to PRL

We have carried out extensive simulations of the angle-resolved photoemission (ARPES) intensity in Bi2212 within the one-step and three-step type models using a first-principles band theory framework. The focus is on understanding the behavior of emissions from the antibonding and bonding bands arising from the $CuO_2$ bilayers around the $\overline{M}(\pi,0)$ symmetry point. The specific issues addressed include: Dependencies of the photointensity on the energy and polarization of the incident light; character of the initial and final states involved as well as the spectrum of the relevant final states; and, changes in the spectral intensity as a function of the perpendicular component, $k_\perp$, of the momentum of the photoelectron. Considerable insight into the nature of individual transitions is adduced by examining the momentum matrix element for bulk transitions within the solid and by further decomposing this matrix element into contributions from various atomic sites and angular momentum channels. These results indicate that, via remarkable interference effects, the ARPES matrix element can in particular cases help zoom in on the properties of electrons excited from specific sites and/or angular momentum channels even in a complex material., Comment: 11 pages, 9 figures, submitted to PRB

We have carried out extensive first-principles angle-resolved photointensity (ARPES) simulations in Bi2212 wherein the photoemission process is modelled realistically by taking into account the full crystal wavefunctions of the initial and final states in the presence of the surface. The spectral weight of the ARPES feature associated with the CuO_2 plane bands is found to undergo large and systematic variations with $k_\parallel$ as well as the energy and polarization of the incident photons. These theoretical predictions are in good accord with the corresponding measurements, indicating that the remarkable observed changes in the spectral weights in Bi2212 are essentially a matrix element effect and that the importance of matrix elements should be kept in mind in analyzing the ARPES spectra in the high-Tc's., Comment: 4 pages, 4 figures: To appear in Phys. Rev. Lett. Nov. 1999