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

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

Sectional symmetry of solutions of elliptic systems in cylindrical domains

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

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

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

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

Superconducting ground state of the two-dimensional Hubbard model: a variational study

A trial wave function is proposed for studying the instability of the two-dimensional Hubbard model with respect to d-wave superconductivity. Double occupancy is reduced in a similar way as in previous variational studies, but in addition our wave function both enhances the delocalization of holes and induces a kinetic exchange between the electron spins. These refinements lead to a large energy gain, while the pairing appears to be weakly affected by the additional term in the variational wave function.Comment: 2 pages, 1 figure, Proceedings of the M2S-HTSC-VII

Low temperature ellipsometry of NaV2O5

The dielectric function of alpha'NaV2O5 was measured with electric field along the a and b axes in the photon energy range 0.8-4.5 eV for temperatures down to 4K. We observe a pronounced decrease of the intensity of the 1 eV peak upon increasing temperature with an activation energy of about 25meV, indicating that a finite fraction of the rungs becomes occupied with two electrons while others are emptied as temperature increases. No appreciable shifts of peaks were found s in the valence state of individual V atoms at the phase transition is very small. A remarkable inflection of this temperature dependence at the phase transition at 34 K indicates that charge ordering is associated with the low temperature phase.Comment: Revisions in style and order of presentation. One new figure. In press in Physical Review B. REVTeX, 4 pages with 4 postscript figure

Deconstruction of Resolution Effects in Angle-Resolved Photoemission

We study how the energy and momentum resolution of angle-resolved photoemission spectroscopy (ARPES) affects the linewidth, Fermi crossing, velocity, and curvature of the measured band structure. Based on the fact that the resolution smooths out the spectra, acting as a low-pass filter, we develop an iterative simulation scheme that compensates for resolution effects and allows the fundamental physical parameters to be accurately extracted. By simulating a parabolic band structure of Fermi-liquid quasiparticles, we show that this method works for an energy resolution up to 100 meV and a momentum resolution equal to twice the energy resolution scaled by the Fermi velocity. Our analysis acquires particular relevance in the hard and soft x-ray regimes, where a degraded resolution limits the accuracy of the extracted physical parameters, making it possible to study how the electronic excitations are modified when the ARPES probing depth increases beyond the surface.Comment: A high-resolution version can be found at: http://www.phas.ubc.ca/~quantmat/ARPES/PUBLICATIONS/Articles/ARPES_resolution.pd