1,227 research outputs found

    Probing the Low-Energy Electronic Structure of Complex Systems by ARPES

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    Angle-resolved photoemission spectroscopy (ARPES) is one of the most direct methods of studying the electronic structure of solids. By measuring the kinetic energy and angular distribution of the electrons photoemitted from a sample illuminated with sufficiently high-energy radiation, one can gain information on both the energy and momentum of the electrons propagating inside a material. This is of vital importance in elucidating the connection between electronic, magnetic, and chemical structure of solids, in particular for those complex systems which cannot be appropriately described within the independent-particle picture. The last decade witnessed significant progress in this technique and its applications, thus ushering in a new era in photoelectron spectroscopy; today, ARPES experiments with 2 meV energy resolution and 0.2 degree angular resolution are a reality even for photoemission on solids. In this paper we will review the fundamentals of the technique and present some illustrative experimental results; we will show how ARPES can probe the momentum-dependent electronic structure of solids providing detailed information on band dispersion and Fermi surface, as well as on the strength and nature of those many-body correlations which may profoundly affect the one-electron excitation spectrum and, in turn, determine the macroscopic physical properties.Comment: Lecture notes for the 2003 Exciting Summer School (http://www.fysik4.fysik.uu.se/~thor/school.html). A HIGH-RESOLUTION pdf file is available at http://www.physics.ubc.ca/~damascel/ARPES_Intro.pdf, and related viewgraphs at http://www.physics.ubc.ca/~damascel/Exciting2003.pd

    A priori estimates for some elliptic equations involving the pp-Laplacian

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    We consider the Dirichlet problem for positive solutions of the equation Δp(u)=f(u)-\Delta_p (u) = f(u) in a convex, bounded, smooth domain ΩRN\Omega \subset\R^N, with ff locally Lipschitz continuous. \par We provide sufficient conditions guarantying LL^{\infty} a priori bounds for positive solutions of some elliptic equations involving the pp-Laplacian and extend the class of known nonlinearities for which the solutions are LL^{\infty} a priori bounded. As a consequence we prove the existence of positive solutions in convex bounded domains

    Sectional symmetry of solutions of elliptic systems in cylindrical domains

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    In this paper we prove a kind of rotational symmetry for solutions of semilinear elliptic systems in some bounded cylindrical domains. The symmetry theorems obtained hold for low-Morse index solutions whenever the nonlinearities satisfy some convexity assumptions. These results extend and improve those obtained in \cite{DaPaSys, DaGlPa1, Pa, PaWe}.Comment: arXiv admin note: text overlap with arXiv:1209.5581, arXiv:1206.392

    Symmetry results for cooperative elliptic systems via linearization

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    In this paper we prove symmetry results for classical solutions of nonlinear cooperative elliptic systems in a ball or in annulus in \RN, N2N \geq 2 . More precisely we prove that solutions having Morse index jNj \leq N are foliated Schwarz symmetric if the nonlinearity is convex and a full coupling condition is satisfied along the solution

    Optical spectroscopy of quantum spin systems

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    This thesis work illustrates the results obtained on a number of solid crystalline materials using optical spectroscopy. This experimental technique consists of shing light of different frequencies onto the sample under investigation, and of observing which frequencies are absorbed by the material itself. The experiments were performed in the frequency range extending from the far infrared to the ultra violet (i.e., from 4 meV to 4 eV). ... Zie: Summary