Matrix element effects in angle-resolved photoemission from Bi2212: Energy and polarization dependencies, final state spectrum, spectral signatures of specific transitions and related issues

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 PR

Matrix Element and Strong Electron Correlation Effects in ARPES from Cuprates

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 Solid

Evolution of Mid-gap States and Residual 3-Dimensionality in La2−x_{2-x}Srx_xCuO4_4

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 figure

Mismatch between the PSB and CPS due to the present vertical recombination scheme

The production of the nominal LHC beam will deamand optimum emittance preservation between individual machines in the injection chain. The edge effects at the entry and exit of the bending magnets used for the vertical recombination of the four PS booster rings to the level of the CPS results in a small uncompensated, and for each ring different, mismatch. We present recent measurements of the mismatch done in the PSB measurement line

"Intelligent" Automatic Beam Steering and Shaping

The strategy for Automated Beam Steering and Shaping (ABS) in the PS complex is to use theoretical response matrices calculated from an optics database. The main reason for this is that it enforces a certain understanding of the machine optics. A drawback is that the validation of such a matrix can be a lengthy process. However, every time a correction is made using an ABS program, a partial measurement of the response matrix is effectively performed. Since the ABS programs are very frequently used, the full matrices could thus be measured on an almost daily basis, provided this information is retained. The information can be used in two ways. Either the program passively logs the data to be analysed off­line, or the information is directly fed back to the matrix, which makes the program 'learn' as it executes. The data logging provides a powerful machine debugging tool, since deviations between the measured and theoretical matrices can be traced back to incorrect optical parameters. The 'learning' mode ensures that the correction will always converge. Implementation details and simulation results are discussed

Physics opportunities with future proton accelerators at CERN

We analyze the physics opportunities that would be made possible by upgrades of CERN's proton accelerator complex. These include the new physics possible with luminosity or energy upgrades of the LHC, options for a possible future neutrino complex at CERN, and opportunities in other physics including rare kaon decays, other fixed-target experiments, nuclear physics and antiproton physics, among other possibilities. We stress the importance of inputs from initial LHC running and planned neutrino experiments, and summarize the principal detector R&D issues.Comment: 39 page, word document, full resolution version available from http://cern.ch/pofpa/POFPA-arXive.pd

Tomographic reconstruction of transverse phase space from turn-by-turn profile data

Tomographic methods have the potential for useful application in beam diagnostics. The tomographic reconstruction of transverse phase space density from turn-by-turn profile data has been studied with particular attention to the effects of dispersion and chromaticity. It is shown that the modified Algebraic Reconstruction Technique (ART) that deals successfully with the problem of non-linear motion in the longitudinal plane cannot, in general, be extended to cover the transverse case. Instead, an approach is proposed in which the effect of dispersion is deconvoluted from the measured profiles before the phase space picture is reconstructed using either the modified ART algorithm or the inverse Radon Transform. This requires an accurate knowledge of the momentum distribution of the beam and the modified ART reconstruction of longitudinal phase space density yields just such information. The method has been tested extensively with simulated data