[en] The purpose of the Workshop was to discuss the up-to-date theoretical understanding of Synchrotron Radiation Spectroscopies through a strong interaction between theorists and experimentalists and to highlight the advances in the fundamental aspects of the theory as well as developments of efficient theoretical methods to calculate and simulate experiments

[en] Short note

[en] The effect of the metal-insulator phase transition on the X-ray K-absorption spectrum of V in V₂O₃ was measured by synchrotron radiation. A different behavior of the transition to 3d vs 4p states in the two phases was observed. These results have been correlated with the change in the screening of the 1s hole due to the variation of the dielectric constant epsilon(q,ω) in the two phases. (author)

[en] A many-body description of the photoemission and photoabsorption processes is outlined that incorporates the multichannel treatment of the atomic dynamical excitations into the framework of the multiple scattering theory. In this context the interplay between excitation dynamics and electronic and geometrical structure of the ground state is elucidated. A new multiple scattering expansion is derived that takes into account interchannel transitions as well. An application to the analysis of photoabsorption spectra of mixed valence compounds is outlined. The same approach is shown to provide a theoretical model for the study of the evolution from the adiabatic to the sudden regime. Limiting, asymptotic cases are discussed. Finally the unifying approach provided by the multiple scattering theory in the description of photoemission and photoabsorption processes in condensed and gaseous phase matter is illustrated

[en] We outline a many-body description of the photoemission and photoabsorption processes that incorporates the multichannel treatment of the atomic dynamical excitations into the framework of the multiple scattering theory. In this way the interplay between excitation dynamics and electronic and geometrical structure of the ground state is elucidated. At the same time this approach provides a theoretical model for the study of the evolution from the adiabetic to the sudden regime. We derive a new multiple scattering expansion that takes into account interchannel transitions as well. As an application we discuss the homogeneous and inhomogeneous mixed-valent compounds, where the theory provides a clue at resolution of the ''threshold puzzle'' and sheds light onto the relation between photoemission and photoabsorption spectroscopy

[en] The aim of the paper is to get more knowledge about many-body systems and their properties, about many-body content of quantum gauge theories and its connection with mass generation mechanisms. The way to achieve this is to perform the galilean limit of the relativistic theory by sending the speed of light c to infinity. This limiting process exposes the low energy behaviour of the relativistic theory

[en] X-Ray absorption spectra of Na K-edge in NaCl and NaF crystalline compounds calculated according to the Full Multiple Scattering (FMS) approach in the framework of the one particle approximation. A 7-shell cluster of 93 atoms has been utilized for the calculation and good agreement with experimental spectra has been achieved

[en] The problem of the highly correlated electron gas V₂O₃ consisting of a filled a/sub 1g/ and a quarterly full e/sub g/ band is treated on the basis of a Hartree-Fock calculation with spin and orbit unrestriction. The values of the effective hopping integrals which include covalency effects (due to the overlap of the 2p/sub π/ orbitals of the oxygens with the 3d wave functions of the vanadium atoms) are assessed on the bases of available band-structure calculations and experimental results measuring covalency contributions. For reasonable values of the Hubbard parameters U/sub m/m approx. = 2 eV, U/sub m/n approx. = 1.6 eV, and J/sub m/n approx. = 0.2 eV [the interatomic Coulomb repulsion of electrons on the same orbit (m, m) on different orbits (m, n) and the exchange integral J/sub m/n] it is found that the observed spin structure of V₂O₃ together with an antiferromagnetic orbital order gives the lowest Hartree-Fock ground-state energy amongst a large class of solutions which we considered and shows a gap in the density of states of the order of 0.2--0.3 eV. Since this gap appears already in the trigonal phase, we feel confident that the monoclinic distortion in the low-temperature phase is of magnetostrictive origin and not a primary cause of the metal-insulator transition. The peculiar value of 1.2μ/sub B/ per V atom as observed by neutron scattering is interpreted as a strongly covalency-enhanced moment on the V atom. The atomic limit value of 1μ/sub B/ due to one magnetic e/sub g/ electron per V atom is reduced to approx. = 0.75μ/sub B/ in an itinerant picture. The covalency mechanism providing the extra 0.4μ/sub B/ is known as back-bonding effect and leads at the same time to a negative spin density on the oxygen ions which are therefore no longer diamagnetic. Negative ¹⁷O NMR shift in the insulating antiferromagnetic phase should be able to verify this conjecture

[en] On the basis of our theoretical examination of the insulating state of V₂O₃ reported in the preceding two papers and the experimental results of NMR and susceptibility measurements of the metallic phase, we conjecture the highly correlated electron-gas character in this latter phase of V₂O₃. We present arguments for the first-order metal-insulator transition which we consider to be entropy driven passing from the insulating state to a paramagnetic metallic one of nearly equal inner energy but considerably different entropy due to the breakdown of the magnetic and orbital long-range order present in the insulating phase. We believe that the origin of the highly correlated electron gas in the paramagnetic metallic phase lies in the stability of the electronic molecular state of the V pairs along the c axis which persist through the metallic phase, a picture which estimates extremely well the observed entropy in this phase. The lattice distortion observed in the insulating phase is believed to be purely magnetostrictive and of no direct importance to the transition mechanism

[en] A phase diagram for all the possible collinear spin arrangements for V₂O₃ is derived within the atomic limit. Due to the fact that the a/sub 1g/ electrons of the V atoms form a diamagnetic bond for the vertical pairs of V atoms, the magnetic structure of V₂O₃ can be considered to be essentially determined by the remaining one electron per V atom in a twofold degenerate e/sub g/ level. Depending on only two parameters: t¹¹/sub i/j/t²²/sub i/j, the ratio of the hopping integrals within the two orbital states 1 and 2 and between a certain pair (i, j) of V atoms in the basal plane, and J/U, the ratio of the exchange constant to Hubbard's U, the regions of stability for a particular magnetic and orbital order are determined. The experimentally observed magnetic order falls into a region of values of these parameters which are expected for V₂O₃