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[en] In the last three years, various theoretical interpretations have been proposed on the nature of the ground state in the antiferromagnetic (AFI) phase of V₂O₃. There are two main results on this subject: the idea of a correlated vertical molecule and that of a complex ground state. In spite of the results achieved, we show, with the present work, that the pictures proposed are still lacking and more work is needed for a unified description of the AFI phase of V₂O₃. (author)

[en] The present paper is based on our graduate lectures in condensed-matter physics. We found that the mean-field solution of the Hubbard model is an excellent tool to stimulate students’ reflections towards the treatment of realistic magnetic interactions. We show by detailed analytical and numerical calculations how to find the mean-field solution of the model on a square lattice. We then interpret the physical implications of the ground-state magnetic phase diagram in terms of the electron density and the ratio between the Coulomb repulsion and the electron-structure bandwidth. (paper)

[en] We here report a combined experimental and theoretical resonant x-ray scattering study in two isomorphous compounds, LaMnO₃ and TbMnO₃. The two samples show an orthorhombic Pbnm structure with four Mn³⁺ atoms at equivalent positions. The Mn sites have a local inversion symmetry and they are surrounded by nearly tetragonal-distorted oxygen octahedron. The tetragonal distortion is larger in TbMnO₃ than in LaMnO₃. Resonances were measured at the Mn K-edge of the (h,0,0), (0,k,0) and (0,0,1) (h,k,1=3,5) reflections. All the reflections show a resonance only in the σ-π' polarization channel. The observed resonant intensities exhibit a sin²φ dependence on the azimuth angle φ. The (h,0,0) and (0,k,0) reflections have identical energy dependence but markedly different than the (0,0,1) reflections. Differences in the energy line shape and width of these resonant reflections between La and Tb samples are explained in terms of the different tetragonal distortion and the different ionic radius of the rare-earth. The origin of the two types of reflections (related to two different nonzero non-diagonal elements of the scattering tensor) is explained within the multiple scattering theory considering long-range order of structural distortions in a cluster up to 63 atoms without invoking any d-orbital order.

[en] We present a unitary cluster approach to the calculation of several electron and photon spectroscopies, ranging from core and valence level photoelectron diffraction and absorption to electron, Auger and anomalous diffraction. Electron energy loss and Auger-photoelectron coincidence spectroscopies can also be treated in the same frame. This approach is based on multiple-scattering theory with a complex optical potential of the Hedin-Lundqvist type and is valid for all electron kinetic energies. Similarities and differences between these diffraction techniques are examined and cluster size convergence is discussed in connection with the electron mean free path. Applications to selected problems are presented to illustrate the method, both for structural and electronic analysis. (topical review)

[en] The research field of Resonant X-ray Scattering (RXS) has achieved tremendous progress in the last years. Nowadays RXS is rapidly becoming the crucial technique for investigating the subtleties of microscopic magnetism in systems where the ground state properties reflect a delicate balance between several different correlated processes. The aim of this workshop is to discuss present and future possibilities for RXS investigations of electronic order, including studies of charge, magnetic, and multipolar ordered states. The sessions will cover experimental and theoretical aspects of hard and soft X-ray resonant scattering from single crystals and thin films. This document gathers the summaries of the presentations

[en] The VIP (Violation of the Pauli Exclusion Principle) experiment is dedicated to check the validity of one of the basic principles of modern physics. This investigation is done searching for anomalous X-rays emitted by copper atoms in a conductor: any detection of these anomalous X-rays would mark a Pauli forbidden transition. VIP is currently taking data at the Gran Saso underground laboratories, and its scientific goal is to improve by three-four orders of magnitude the previous limit on the probability of Pauli violating transitions, bringing it into the 10^-29 to ^-30 region. The new experimental results and future plans are presented. The PDF file for this article was corrected and the names and affiliations were amended on 4 September 2009.

[en] The Pauli Exclusion Principle is one of the most fundamental rules of nature and represents a pillar of modern physics. According to many observations the Pauli Exclusion Principle must be extremely well fulfilled. Nevertheless, numerous experimental investigations were performed to search for a small violation of this principle. The VIP experiment at the Gran Sasso underground laboratory searched for Pauli-forbidden X-ray transitions in copper atoms using the Ramberg-Snow method and obtained the best limit so far. The follow-up experiment VIP2 is designed to reach even higher sensitivity. It aims to improve the limit by VIP by orders of magnitude. The experimental method, comparison of different PEP tests based on different assumptions and the developments for VIP2 are presented. (paper)

[en] The Pauli Principle represents one of the most important rules in physics and explains numerous phenomena as well as characteristic properties of matter, like its stability. Testing the validity of this principle at the highest possible sensitivity is a challenging experimental task - the VIP experiment at the Gran Sasso underground laboratory aims at a limit for the violation probability of the order of 10^-29 to 10^-30. The method is based on the search for Pauli-forbidden x-ray transitions in a pure copper conductor using silicon x-ray detectors with high resolution in energy. The experimental setup, results obtained so far and new ideas to further enhance the sensitivity will be presented.

[en] One of the fundamental rules of nature and a pillar in the foundation of quantum theory and thus of modern physics is represented by the Pauli Exclusion Principle. We know that this principle is extremely well fulfilled due to many observations. Numerous experiments were performed to search for tiny violation of this rule in various systems. The experiment VIP at the Gran Sasso underground laboratory is searching for possible small violations of the Pauli Exclusion Principle for electrons leading to forbidden X-ray transitions in copper atoms. VIP is aiming at a test of the Pauli Exclusion Principle for electrons with high accuracy, down to the level of 10⁻²⁹ – 10⁻³⁰, thus improving the previous limit by 3–4 orders of magnitude. The experimental method, results obtained so far and new developments within VIP2 (follow-up experiment at Gran Sasso, in preparation) to further increase the precision by 2 orders of magnitude will be presented