1,720,976 research outputs found
Isolated Minkowski vacua, and stability analysis for an extended brane in the rugby ball
No full textWe study a recently proposed model, where a codimension one brane is wrapped around the axis of symmetry of an internal two-dimensional space compactified by a flux. This construction is free from the problems which plague delta-like, codimension two branes, where only tension can be present. In contrast, arbitrary fields can be localized on this extended brane, and their gravitational interaction is standard 4d gravity at large distances. In the first part of this work, we study the de Sitter (dS) vacua of the model. The landscape of these vacua is characterized by discrete points labeled by two integer numbers, related to the flux responsible for the compactification and to the current of a brane field. A Minkowski external space emerges only for a special ratio between these two integers, and it is therefore (topologically) isolated from the nearby dS solutions. In the second part, we show that the Minkowski vacua are stable under the most generic axially-symmetric perturbations, and we argue that this is sufficient to ensure the overall stability
Instability of anisotropic cosmological solutions supported by vector fields
No full textModels with vector fields acquiring a nonvanishing vacuum expectation value along one spatial direction have been proposed to sustain a prolonged stage of anisotropic accelerated expansion. Such models have been used for realizations of early time inflation, with a possible relation to the large scale cosmic microwave background anomalies, or of the late time dark energy. We show that, quite generally, the concrete realizations proposed so far are plagued by instabilities (either ghosts or unstable growth of the linearized perturbations) which can be ultimately related to the longitudinal vector polarization present in them. Phenomenological results based on these models are therefore unreliable
Instability of the Ackerman-Carroll-Wise model, and problems with massive vectors during inflation
No full textWe prove that the anisotropic inflationary background of the Ackerman-Carroll-Wise model, characterized by a fixed-norm vector field, is unstable. We found the instability by explicitly solving the linearized equations for the most general set of perturbations around this background, and by noticing that the solutions diverge close to horizon crossing. This happens because one perturbation becomes a ghost at that moment. A simplified computation, with only the perturbations of the vector field included, shows the same instability, clarifying the origin of the problem. We then discuss several other models, with a particular emphasis on the case of a nonminimal coupling to the curvature, in which vector fields are used either to support an anisotropic expansion, or to generate cosmological perturbations on an isotropic background. In many cases, the mass squared of the vector needs to be negative; we show that, as a consequence, the longitudinal vector mode is a ghost (a field with negative kinetic term, and negative energy, and not simply a tachyon). We comment on problems that arise at the quantum level. In particular, the presence of a ghost can be a serious difficulty for the UV completion that such models require in the subhorizon regime
Origin of magnetic interactions and their influence on the structural properties of Ni2MnGa and related compounds
No full textIn this work, we perform first-principles DFT calculations to investigate the interplay between magnetic and structural properties in Ni2MnGa. We demonstrate that the relative stability of austenite (cubic) and non-modulated martensite (tetragonal) phases depends critically on the magnetic interactions between Mn atoms. While standard approximate DFT functionals stabilize the latter phase, a more accurate treatment of electronic localization and magnetism, obtained with DFT + U, suppresses the non-modulated tetragonal structure for the stoichiometric compound, in better agreement with experiments. We show that the Anderson impurity model, with Mn atoms treated as magnetic impurities, can explain this observation and that the fine balance between super-exchange RKKY type interactions mediated by Ni d and Ga p orbitals determines the equilibrium structure of the crystal. The Anderson model is also demonstrated to capture the effect of the number of valence electrons per unit cell on the structural properties, often used as an empirical parameter to tune the behavior of Ni2MnGa based alloys. Finally, we show that off-stoichiometric compositions with excess Mn promote transitions to a non-modulated tetragonal structure, in agreement with experiments
Ghost instabilities of cosmological models with vector fields nonminimally coupled to the curvature
Full text linkWe prove that many cosmological models characterized by vectors nonminimally coupled to the curvature (such as the Turner-Widrow mechanism for the production of magnetic fields during inflation, and models of vector inflation or vector curvaton) contain ghosts. The ghosts are associated with the longitudinal vector polarization present in these models and are found from studying the sign of the eigenvalues of the kinetic matrix for the physical perturbations. Ghosts introduce two main problems: (1) they make the theories ill defined at the quantum level in the high energy/subhorizon regime (and create serious problems for finding a well-behaved UV completion), and (2) they create an instability already at the linearized level. This happens because the eigenvalue corresponding to the ghost crosses zero during the cosmological evolution. At this point the linearized equations for the perturbations become singular (we show that this happens for all the models mentioned above). We explicitly solve the equations in the simplest cases of a vector without a vacuum expectation value in a Friedmann-Robertson-Walker geometry, and of a vector with a vacuum expectation value plus a cosmological constant, and we show that indeed the solutions of the linearized equations diverge when these equations become singular
First principles calculation of the electronic properties and lattice dynamics of Cu 2ZnSn(S 1-xSe x) 4
No full textUsing density functional theory, we calculated the electronic structure, the lattice dynamics, and the Raman spectra of Cu2ZnSn(S1-xSex)(4), (CZTSSe) an emerging photovoltaic material for thinfilm solar cells. In particular, we investigated the effects of the local arrangement of S and Se within the unit cell on the electronic properties of these materials. We find that the S-to-Se ratio (e. g., x) and the spatial distribution of the anions in the unit cell can significantly alter the band structure. In particular, the S-to-Se ratio and anion distribution determine the energy splitting between the electronic states at the top of the valence band and the hole mobility in CZTSSe alloys and solar cells. Moreover, we find that x-ray diffraction patterns and phonon dispersion curves are sensitive to the local anion ordering. The predicted Raman scattering frequencies and their variation with x agree with experimentally determined values and trends
First-principles study of electronic and structural properties of CuO
No full textWe investigate the electronic and structural properties of CuO, which shows significant deviations from the trends obeyed by other transition-metal monoxides. Using an extended Hubbard-based corrective functional, we uncover an orbitally ordered insulating ground state for the cubic phase of this material, which was expected but, to the best of our knowledge, was not found in the literature. This insulating state results from a fine balance between the tendency of Cu to complete its d-shell and Hund's rule magnetism. Starting from the ground state for the cubic phase, we also study tetragonal distortions of the unit cell (recently reported in experiments) and identify the equilibrium structure. Our calculations reveal an unexpected richness of possible magnetic and orbital orders, relatively close in energy to the ground state, whose stability depends on the sign and nature of distortion
Atomic bonding effects in annular dark field scanning transmission electron microscopy. I. Computational predictions
No full textAnnular dark field scanning transmission electron microscopy (ADF-STEM) image simulations
were performed for zone-axis-oriented light-element single crystals, using a multislice method
adapted to include charge redistribution due to chemical bonding. Examination of these image simulations
alongside calculations of the propagation of the focused electron probe reveal that the evolution
of the probe intensity with thickness exhibits significant sensitivity to interatomic charge
transfer, accounting for observed thickness-dependent bonding sensitivity of contrast in all ADFSTEM
imaging conditions. Because changes in image contrast relative to conventional neutral
atom simulations scale directly with the net interatomic charge transfer, the strongest effects are
seen in crystals with highly polar bonding, while no effects are seen for nonpolar bonding.
Although the bonding dependence of ADF-STEM image contrast varies with detector geometry,
imaging parameters, and material temperature, these simulations predict the bonding effects to be
experimentally measureable
Scalar-scalar, scalar-tensor, and tensor-tensor correlators from anisotropic inflation
No full textWe compute the phenomenological signatures of a model [Watanabe et al. Phys. Rev. Lett. 102, 191302 (2009)] of anisotropic inflation driven by a scalar and a vector field. The action for the vector is U(1) invariant, and the model is free of ghost instabilities. A suitable coupling of the scalar to the kinetic term of the vector allows for a slow roll evolution of the vector vacuum expectation value, and hence for a prolonged anisotropic expansion; this provides a counter example to the cosmic no hair conjecture. We compute the nonvanishing two point correlation functions between physical modes of the system, and express them in terms of power spectra with angular dependence. The anisotropy parameter g∗ for the scalar-scalar spectrum (defined as in the Ackerman et al. [Phys. Rev. D 75, 0835002 (2007)] parametrization) turns out to be negative in the simplest realization of the model, which, therefore, cannot account for the angular dependence emerged in some analyses of the Wilkinson Microwave Anisotropy Probe data. A g∗ of order −0.1 is achieved when the energy of the vector is about 6–7 orders of magnitude smaller than that of the scalar during inflation. For such values of the parameters, the scalar-tensor correlation (which is in principle a distinctive signature of anisotropic spaces) is smaller than the tensor-tensor correlation
DFT calculation and experimental investigation of Mn doping effect in Fe16N2
No full textAn effective dopant to improve the thermal stability of a Fe16N2 permanent magnet is proposed in this paper. It is demonstrated both theoretically and experimentally that manganese is a promising candidate as dopant in Fe16N2 magnet to improve the thermal stability. Firstly, the atomic moments of the Fe ions with respect to N is investigated by using first-principles DFT calculation. Two possible candidates of elements, including Co and Mn, are compared in terms of its preferred position and magnetic coupling mode. It is found that Mn prefers Fe1 position and ferromagnetic coupling in the Fe16N2 lattice. So Mn is considered as a promising dopant in Fe16N2 magnet to improve its thermal stability. Based on theoretical results, experiments are conducted by a cold-crucible method to prepare (Fe1−xMnx) N bulk samples. The samples are thermal treated at different temperatures to observe their thermal stabilities. X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) are characterized on the samples
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