1,721,393 research outputs found
Spin and charge density waves in the extended Hubbard model: a slave-boson approach
No full textWe use the extended Hubbard model to investigate the properties of the charge- and spin-density-wave phases in the presence of a nearest-neighbors repulsion term in the framework of the slave-boson technique. We show that, contrary to Hartree-Fock results, an instablity may occur for sufficiently high values of the Hubbard repulsion, both in the spin- and charge-density-wave phase, which makes the system discontinuously jump to a phase with a smaller or zero wave amplitude. The limits of applicability of our approach are discussed and our results are compared with previous numerical analysis. The phase diagram of the model at half-filling is determined
Relativistic GW plus BSE study of the optical properties of Ruddlesden-Popper iridates
No full textWe study the optical properties of the Ruddlesden-Popper series of iridates Sr(n+1)Ir(n)O3(n+1) (n = 1, 2, and infinity) by solving the Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and screened interactions W are obtained by the G W approximation including spin-orbit coupling. The computed optical conductivity spectra show strong excitonic effects and reproduce very well the experimentally observed double-peak structure, in particular for the spin-orbital Mott insulators Sr-2 IrO4 and Sr3Ir2O7. However, G W does not account well for the correlated metallic state of SrIrO3 owing to a much too small band renormalization, and this affects the overall quality of the optical conductivity. Our analysis describes well the progressive redshift of the main optical peaks as a function of dimensionality (n), which is correlated with the gradual decrease of the electronic correlation (quantified by the constrained random phase approximation) towards the metallic n = infinity limit. We have also assessed the quality of a computationally cheaper BSE approach that is based on a model dielectric function and conducted on top of DFT+U one-electron energies. Unfortunately, this model BSE approach does not accurately reproduce the outcome of the full GW+BSE method and leads to larger deviations to the measured spectra
Metal to insulator transition and local structure in Sr2FeMo1-xWxO6 ,
No full textSr2FeMoO6 oxides1 and several related compounds of general formula A2B’B”O6 (A: alkaline earth, B’, B”: heterovalent transition metals) exhibit half-metallic magnetism and high spin polarization at the Fermi level. They have recently stimulated a large interest in the field of heavily correlated electron systems while they attracted the applied research as promising innovative materials in magnetic tunnel junctions and spintronic devices. Nevertheless a theory completely understanding these special properties is still lacking. The Sr2FeMoO6, for example, is half metallic ferromagnetic (FM) with elevated Curie temperature (Tc > 400 K) and presents large magnetoresistance at room temperature2. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been proposed to explain the metallic state and the magnetoresistance. The strangeness of this model is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply a huge exchange interaction despite the very long distance among Fe ions (~8 Å)3. Half metallic properties on Sr2FeMoO6 sensitively depends on Fe-Mo cation order on the B site of the A2B’B”O6 double perovskite structure. Doping with W substituting for Mo ions in Sr2FeMo1-xWxO6 improves the B-site Fe-Mo/W ordering4. The Sr2FeWO6 is an antiferromagnetic (AFM) insulator, thus the doped system undergoes to a metal to insulator (MI) transition and AFM-FM transition as a function of composition. The doped compounds remain (half-) metallic with high Tc >> RT (Room temperature) on a wide range of W concentrations (0<x<0.7). The nature of the transition is still matter of debate and different possible scenarios are proposed: i) a collective valence transition from Fe3+-(W,Mo)5+ in the metallic state to Fe2+-(W,Mo)6+ in the insulating state; ii) a percolation process in which the nominal valence for W and Mo remain 6+ and 5+ respectively for every compositions and conductivity is given by metallic Mo-rich clusters embedded in the insulating W-rich regions. Finally a combination of the two effects has been suggested5.Detailed knowledge of the microstructure across the metal to insulator transition must help in understanding the origin of magnetotransport properties of these compounds. This study reports the evolution of Sr2FeMo1-xWxO6 local structure as a function of composition as resulting from accurate analysis of Sr, Fe, Mo, and W x-ray absorption fine structure (XAFS) data. XAFS experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. The comparative analysis of near edge regions (XANES) of Fe (K-edge), Mo (K-edge) demonstrates that the local structure around the Fe and Mo ions changes steeply across the M-I transition. On the contrary the local structure around W remains almost unchanged as a function of x. The analysis of the extended regions of the spectra (EXAFS) was performed taking into account for single and multiple scattering contributions till about 5 Å from the absorber. Accurate microstructure evolution as a function of composition is reported. The structural results suggest a step-like structural transition accompanying the MI transition as a function of W doping
Local structure and magnetotransport in Sr2FeMoO6 double perovskite compounds: an EXAFS study
No full textSr2FeMoO6 oxides with double perovskite structure1 are half metallic ferromagnetic with elevated Curie temperature (Tc > 400 K) and present large magnetoresistance at room temperature2. In the last few years these compounds stimulated large interest as potential electrodes in magnetic tunnel junction and innovative spintronic materials. On the other hand these peculiar magnetic and electronic properties attracted the fundamental research in the field of heavily correlated electron systems. Their crystallographic structure derives from the ideal cubic perovskites: it is made of weakly tetragonal distorted cubic units with the Sr ions located at the center and the Fe(Mo)O6 sharing the cube edges. The regular alternation of Fe and Mo along the lattice edges (chemical order) strengthens the magnetoresistance while increasing the miss-site defects (chemical disorder) weakens the magnetoresistance. A kind of double exchange interaction (DE) between Fe ions along the Fe-O-Mo-O-Fe chains mediated by Mo, has been suggested to explain the metallic state. Chemical disorder reduces the DE coupling and gives rise to tunnel type conductibility. The most striking features in this picture is the high Curie temperature, well above that observed in the well known Mn-perovskites (La1-xCaxMnO3, La1-xSrxMnO3 and related systems), which imply huge exchange interactions despite the very long distance among Fe ions (~8 Å)3. This work proposes a detailed EXAFS study on two Sr2FeMoO6, a fully chemically ordered and a fully chemically disordered sample, in order to get insights on the micro-structural origin of their magnetotransport properties. X-ray absorption experiments were performed at the Italian beamline (GILDA-BM8) at the ESRF. In order to have a complete and exhaustive description of local order Fe K-edge (~7112 eV), Mo-K edge (~20000 eV) and W-LIII (~10200 eV) edge EXAFS data were analysed. The complete EXAFS spectra were refined taking into account for single as well multiple scattering contributions till about 6 Å. This permitted to probe the relative arrangement of Fe and Mo ions. The coherence of the structural models, as resulting from the three edges analysis strengthens and gives confidence on the results. The main result is that either ordered and disordered compounds show very similar chemically ordered local structure, i.e. the presence of Fe-Fe and Mo-Mo coordination is weak (almost negligible) in both the samples. These finding contrasts with the information from X-ray diffraction analysis, i.e. a high ordered (order parameter ~ 90%) and a completely disordered sample. Our results suggest that the long-range chemically disordered sample is made of small (20-30 Å) chemically ordered clusters. Such a small size would produce weak and broad superlattice reflections easily confused with the background in standard diffraction patterns
Mn-K edge XANES structural refinement in manganese perovskites
No full textManganese oxides with perovskite structure play a special role in the field of strongly correlated systems since their peculiar magnetic transport properties. These are due to ferromagnetic double-exchange (DE) and antiferromagnetic super-exchange (SE) interaction among Mn ions, and strong electron-phonon interaction (small and/or large polarons). The balance among these interactions is mainly determined by local atomic structure surrounding Mn ions, characterized by Jahn-Teller (JT) distortions of MnO6 octahedra. Therefore the sensitivity to the short-range order of x-ray absorption spectroscopy (XAS) makes this technique suited to study these compounds as demonstrated by the wide recent literature. In particular the analysis of the XANES region could provide topological information useful to deeply understand the physics of manganites. This work reports the first attempt to a quantitative analysis of Mn K edge XANES on CaMnO3 and LaMnO3 perovskite samples. The local structures obtained by fitting the XANES data are compared with structural models derived by standard EXAFS analysis and diffraction (x-ray and neutron) data analysis. In CaMnO3 the average structure derived from XANES results in good agreement with EXAFS analysis and with the structure found by diffraction. This strengthens the confidence on the MXAN method. In LaMnO3 XAS results reproduce the main structural features as given by XRD, but both EXAFS and XANES suggest reduced Mn-O apical distance, resulting in a Jahn-Teller distortion 15-20 % reduced compared with the coherent distortion obtained by neutron diffraction
Nature of ‘‘Disorder’’ in the Ordered Double Perovskite Sr2FeMoO6
No full textThe degree of B/B' alternate cation order is known to heavily influence the magnetic properties of
A2BB'O6 double perovskites although the nature of such disorder has never been critically studied. Our
detailed x-ray absorption fine structure studies in conjunction with synchrotron radiation x-ray diffraction
experiments on polycrystalline Sr2FeMoO6 samples with various degrees of disorder reveal that a very
high degree of short range order is preserved even in samples with highly reduced long range chemical
order. Based on these experimental results and with the help of detailed structural simulations, we are able
to model the nature of the disorder in this important class of materials and discuss the consequent
implications on its physical properties
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Manganese oxide perovskites: quantitative structural refinement of Mn-K- edge XANES data
No full textRare hearth doped manganites with perovskite structure show peculiar magnetic and electronic transport properties, due to a balance of ferromagnetic double-exchange (DE) and antiferromagnetic superexchange (SE) interaction among Mn ions, and strong electron-phonon interaction (small and/or large polarons)(1). These features are largely determined by the local atomic structure surrounding Mn ions, characterized by Jahn-Teller (JT) distortions of MnO6 octahedra and deeply affected by temperature, composition, pressure and magnetic .elds. Therefore the sensitivity to the short range order of x-ray absorption spectroscopy (XAS) makes this technique largely favorite in studying these compounds. Despite recent scientific literature reports several accurate XAS based studies on these compounds(2), it mainly concerns the analysis of extended region of the XAS spectra, the so called EXAFS region. On the contrary the interpretation of the near edge region of the spectra (XANES) remains qualitative or semi-quantitative(3). Nevertheless structural information in the EXAFS and XANES are complementary. The former mainly probes the atomic radial distribution functions around the absorber, whereas the latter, as it results from the full multiple scattering (FMS) processes and from the details of electronic density of states near the Fermi level, directly probes the topology of local environment surrounding the absorber and its electronic nature. This work reports the .rst attempt to a quantitative analysis of Mn K edge XANES on CaMnO3 and LaMnO3 perovskite samples. The original MXAN program has been used, a recently implemented code(4) which allows ab-initio structural refinement of XANES data in FMS approach. The topology of Mn local atomic environment on CaMnO3 and LaMnO3 samples are reported as derived by the quantitative Mn- K edge XANES fitting. The 3D structures obtained by .tting the XANES data are compared with structural models derived by EXAFS and diffraction (x-ray and neutron). While Mn local structure in CaMnO3 is consistent with the shape and size of MnO6 octahedra obtained by diffraction experiments, in LaMnO3 sample the short and long range information appear quite different. In LaMnO3, in fact, the MnO6 JT distortion determined by XANES is about 15 % reduced with respect to the one determinedby diffraction experiments, confirming and strengthening previous EXAFS results (5)
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