139 research outputs found

    The end points in the dispersion of Holstein polarons

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    We investigate the existence of end points in the dispersion of Holstein polarons in various dimensions, using the Momentum Average approximation which has proved to be very accurate for this model. An end point separates momenta for which the lowest-energy state is a discrete level, i.e., an infinitely-lived polaron, from those where the lowest-energy feature is a continuum in which the "polaron'" is signalled by a resonance with a finite lifetime. While such end points are known to not appear in 1D, we show here that they are generic in 3D if the particle-boson coupling is not too strong. The 2D case is "critical": a pure 2D Holstein model has no end points, like the 1D case. However, any amount of interlayer hopping leads to 3D-like behavior. As a result, such end points are expected to appear in the spectra of layered, quasi-2D systems described by Holstein models. Generalizations to other models are also briefly discussed.Comment: 6 pages, 6 figure

    The Green function variational approximation: Significance of physical constraints

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    We present a calculation of the spectral properties of a single charge doped at a Cu(3d3d) site of the Cu-F plane in KCuF3_{3}. The problem is treated by generating the equations of motion for the Green's function by means of subsequent Dyson expansions and solving the resulting set of equations. This method, dubbed the variational approximation, is both very dependable and flexible, since it is a systematic expansion with precise control over elementary physical processes. It allows for deep insight into the underlying physics of polaron formation as well as for inclusion of many physical constraints, such as excluding crossing diagrams and double occupation constraint, which are not included in the Self-Consistent Born Approximation. Here we examine the role and importance of such constraints by analyzing various spectral functions obtained in second order VA.Comment: 5 pages, 1 figur

    First-Principles Study of Integer Quantum Hall Transitions in Mesoscopic Samples

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    We perform first principles numerical simulations to investigate resistance fluctuations in mesoscopic samples, near the transition between consecutive Quantum Hall plateaus. We use six-terminal geometry and sample sizes similar to those of real devices. The Hall and longitudinal resistances extracted from the generalized Landauer formula reproduce all the experimental features uncovered recently. We then use a simple generalization of the Landauer-B\"uttiker model, based on the interplay between tunneling and chiral currents -- the co-existing mechanisms for transport -- to explain the three distinct types of fluctuations observed, and identify the central region as the critical region.Comment: changes to acknowledgements onl

    Hidden Quasiparticles and Incoherent Photoemission Spectra in Na2IrO3

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    We study two Heisenberg-Kitaev t-J-like models on a honeycomb lattice, focusing on the zigzag magnetic phase of Na2_2IrO3_3, and investigate hole motion by exact diagonalization and variational methods. The spectral functions are quite distinct from those of cuprates and are dominated by large incoherent spectral weight at high energy, almost independent of the microscopic parameters --- a universal and generic feature for zigzag magnetic correlations. We explain why quasiparticles at low energy are strongly suppressed in the photoemission spectra and determine an analog of a pseudogap. We point out that the qualitative features of the predominantly incoherent spectra obtained within the two different models for the zigzag phase are similar, and they have remarkable similarity to recently reported angular resolved photoemission spectra for Na2_2IrO3_3.Comment: 5 pages, 5 figures, and appendi

    Mechanism of d_{x^2-y^2}-wave superconductivity based on doped hole induced spin texture in high T_c cuprates

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    A mechanism of d_{x^2-y^2}-wave superconductivity is proposed for the high-T_c cuprates based on a spin texture with non-zero topological density induced by doped holes through Zhang-Rice singlet formation. The pairing interaction arises from the magnetic Lorentz force like interaction between the holes and the spin textures. The stability of the pairing state against the vortex-vortex interaction and the Coulomb repulsion is examined. The mechanism suggests appearance of a p-wave pairing component by introducing anisotropy in the CuO_2 plane.Comment: 9 pages, 3 figures; added references, corrected minor error

    Elusive electron-phonon coupling in quantitative analyses of the spectral function

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    We examine multiple techniques for extracting information from angle-resolved photoemission spectroscopy (ARPES) data, and test them against simulated spectral functions for electron-phonon coupling. We find that, in the low-coupling regime, it is possible to extract self-energy and bare-band parameters through a self-consistent Kramers-Kronig bare-band fitting routine. We also show that the effective coupling parameters deduced from the renormalization of quasiparticle mass, velocity, and spectral weight are momentum dependent and, in general, distinct from the true microscopic coupling; the latter is thus not readily accessible in the quasiparticle dispersion revealed by ARPES.Comment: A high-resolution version can be found at http://www.physics.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/KKBF.pd
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