1,721,076 research outputs found
From antiferromagnetism to d-wave superconductivity in the two-dimensional t-J model
No full textWe have found that the two dimensional t-J model, for the physical parameter range J/t= 0.4 reproduces the main experimental qualitative features of high-ir, copper oxide superconductors: d-wave superconducting correlations are strongly enhanced upon small doping and clear evidence of off-diagonal long-range order is found at the optimal doping delta approximate to 0.15. On the other hand, antiferromagnetic long-range order, clearly present at zero hole doping, is suppressed at small hole density with clear absence of antiferromagnetism at delta>-0.1
Charge density wave in single-layer Pb/Ge(111) driven by Pb-substrate exchange interaction
No full textSingle layer Pb on top of (111) surfaces of group IV semiconductors hosts charge density wave and superconductivity depending on the coverage and on the substrate. These systems are normally considered to be experimental realizations of single band Hubbard models and their properties are mostly investigated using lattice models with frozen structural degrees of freedom, although the reliability of this approximation is unclear. Here, we consider the case of Pb/Ge(111) at coverage, for which surface x-ray diffraction and angle-resolved photoemission spectroscopy data are available. By performing first-principles calculations, we demonstrate that the nonlocal exchange between Pb and the substrate drives the system into a charge density wave. The electronic structure of this charge ordered phase is mainly determined by two effects: The magnitude of the Pb distortion and the large spin-orbit coupling. Finally, we show that the effect applies also to the phase of Pb/Si(111) where the Pb-substrate exchange interaction increases the bandwidth by more than a factor with respect to density functional theory , in better agreement with scanning tunneling spectroscopy data. The delicate interplay between substrate, structural, and electronic degrees of freedom invalidates the widespread interpretation available in literature considering these compounds as physical realizations of single band Hubbard models
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No full textElectronic structure and magnetic properties of few-layer Cr2Ge2Te6: The key role of nonlocal electron-electron interaction effects
No full textAtomically-thin magnetic crystals have been recently isolated experimentally, greatly expanding the family of two-dimensional materials. In this Article we present an extensive comparative analysis of the electronic and magnetic properties of Cr2Ge2Te6, based on density functional theory (DFT). We first show that the often-used DFT + U approaches fail in predicting the ground-state properties of this material in both its monolayer and bilayer forms, and even more spectacularly in its bulk form. In the latter case, the fundamental gap decreases by increasing the Hubbard-U parameter, eventually leading to a metallic ground state for physically relevant values of U, in stark contrast with experimental data. On the contrary, the use of hybrid functionals, which naturally take into account nonlocal exchange interactions between all orbitals, yields good account of the electronic gap as measured by ARPES. We then calculate all the relevant exchange couplings (and the magneto-crystalline anisotropy energy) for monolayer, bilayer, and bulk Cr2Ge2Te6 with a hybrid functional, with super-cells containing up to 270 atoms, commenting on existing calculations with much smaller super-cell sizes. In the case of bilayer Cr2Ge2Te6, we show that two distinct intra-layer second-neighbor exchange couplings emerge, a result which, to the best of our knowledge, has not been noticed in the literature
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
Charge fluctuations close to phase separation in the two-dimensional t-j model
No full textWe have studied the t-J model using the Green function Monte Carlo technique. We have obtained accurate energies well converged in the thermodynamic limit by performing simulations for up to 242 lattice sites. By studying the energy as a function of hole doping we conclude that there is no phase separation in the physical region relevant for high- Tc superconductors. This finding is further supported by the hole-hole correlation function calculation. Remarkably, near the phase separation instability, for Jc/t ∼ 0.5 this function displays enhanced fluctuations at incommensurate wave vectors that scale linearly with the doping, in agreement with experimental findings
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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