1,721,009 research outputs found
Berry-phase calculation of magnetic screening and rotational g factor in molecules and solids
No full textThe orbital magnetic moment due to rotation or pseudorotation in a molecule or a solid and the corresponding rotational g factor are formulated using the Berry-phase technique and standard density functional plane wave methods. Among the simplest molecules, H-2(+), H-2, C2H2, CH4, and CF4, with known rotational g factors, are used as test cases with excellent results. Alternative, faster localized orbital calculations including the magnetic coupling through heuristic Peierls phase factors are also tested and found to be viable, though less accurate. Application to pseudorotations is exemplified in benzene. It is proposed that these methods will be suited for application to pseudorotations in solids
Peak effect versus skating in high-temperature nanofriction
No full textThe physics of sliding nanofriction at high temperature near the substrate melting point, TM, is so far unexplored. We conducted simulations of hard tips sliding on a prototype non-melting surface, NaCl(100), revealing two distinct and opposite phenomena for ploughing and for grazing friction in this regime. We found a frictional drop close to TM for deep ploughing and wear, but on the contrary a frictional rise for grazing, wearless sliding. For both phenomena, we obtain a fresh microscopic understanding, relating the former to ‘skating’ through a local liquid cloud, and the latter to linear response properties of the free substrate surface. We argue that both phenomena occur more generally on surfaces other than NaCl and should be pursued experimentally. Most metals, in particular those possessing one or more close-packed non-melting surfaces, such as Pb, Al or Au(111), are likely to behave similarly
Melting Behavior of CaO at High Temperature and Pressure: A Molecular Dynamics Study
No full textThe thermodynamic behavior of calcium oxide (CaO) under high temperature and pressure conditions is critical for understanding the physics of planetary interiors. This study employs molecular dynamics (MD) simulations, including both classical and ab initio approaches, to investigate the melting behavior of CaO. We calculate the melting temperature of CaO by the void-nucleated melting and two-phase coexistence techniques, aiming to resolve discrepancies in experimental data on the melting point, which range from 2843 to 3223 K in different studies due to the high reactivity and vapor pressure of the substance. The obtained results are Tf = 3066 ± 12 K and Tf = 2940 ± 65 K using the void-nucleated melting and the two-phase coexistence method, respectively. Additionally, we calculate the enthalpy of fusion and the high-pressure melting curve for the first time without making any assumption on the Clapeyron slope. This is extremely important since in experiments, the Clapeyron slope of the melting curve is estimated from low pressure measurements and the overheating ratio [i.e., η = (Ts/Tf) -1, where Ts represents the thermal instability limit corresponding to the homogeneous melting temperature of the solid] is often assumed to be constant in simulations. Our MD results show that Ts increases more rapidly with pressure than Tf and, thus, that the overheating ratio sensibly depends upon pressure. These findings contribute to accurate modeling of the CaO phase diagram, which is essential for geochemistry, cosmochemistry, and materials science
Comparative Analysis of DFT+U, ACBN0, and Hybrid Functionals on the Spin Density of YTiO<sub>3</sub> and SrRuO<sub>3</sub>
Full text linkWe present a quantitative analysis of the theoretical spin density map of two ferromagnetic perovskites, YTiO3 and SrRuO3. We calculated the spin density using the standard density functional theory (DFT)+U method, where the Hubbard U correction is applied to the Ti and Ru ions, and with the pseudo-hybrid ACBN0 method, where the Hubbard U parameters are determined self-consistently. The ACBN0 calculations yielded a large value of the Hubbard U of the oxygen 2p orbitals. We also used the screened hybrid HSE06 functional, which is widely used to describe the electronic structure of oxides. We used the Quantum Theory of Atoms in Molecules (QTAIM) theory and integrated the spin density in the atomic basins instead of projecting on atomic orbitals. This way, our results can be compared to experimental reports as well as to other DFT calculations
Why are alkali halide surfaces not wetted by their own melt?
No full textAlkali halide (100) crystal surfaces are anomalous, being very poorly wetted by their own melt at the triple point. We present extensive simulations for NaCl, followed by calculations of the solid-vapor, solid-liquid, and liquid-vapor free energies showing that solid NaCl(100) is a nonmelting surface, and that its full behavior can quantitatively be accounted for within a simple Born-Meyer-Huggins-Fumi-Tosi model potential. The incomplete wetting is traced to the conspiracy of three factors: surface anharmonicities stabilizing the solid surface; a large density jump causing bad liquid-solid adhesion; incipient NaCl molecular correlations destabilizing the liquid surface. The latter is pursued in detail, and it is shown that surface short-range charge order acts to raise the surface tension because incipient NaCl molecular formation anomalously reduces the surface entropy of liquid NaCl much below that of solid NaCl(100). © 2005 The American Physical Society
High pressure structure studies of three SrGeO3 polymorphs – Amorphization under pressure
No full textWe report on the synthesis and high pressure behavior of three polymorphs of SrGeO3. At ambient pressure, SrGeO3 crystallizes in the monoclinic structure pseudo-wollastonite. Two high pressure polymorphs, triclinic walstromite, and cubic perovskite were synthesized using a large volume multi-anvil press. The crystal structures of the three polymorphs were investigated with powder X-ray diffraction as a function of pressure using diamond anvil cells. It was found that the pseudo-wollastonite polymorph becomes amorphous at 10 GPa and equation of state fitting of the volume data yielded a bulk modulus of K0 = 47(4) GPa, reported for the first time. Compression of the walstromite structure showed the structure to be very compressible with two distinct phase transitions at around 10–12 GPa and 35–38 GPa. The data suggest that the structure then becomes amorphous although it retains a small degree of long-range order to the highest pressure studied. The perovskite polymorph was very incompressible and equation of state fitting of the volume data yielded a high bulk modulus of K0 = 194(3) GPa. All the experimental data was compared to density functional theory calculations, which were observed to fit well with the experiments
The neurobiology of moral sense: facts or hypotheses?
No full textOne of the most intriguing frontiers of current neuroscientific research is represented by the investigation of the possible neural substrates of morality. The assumption is that in humans an innate moral sense would exist. If this is true, with no doubt it should be regulated by specific brain mechanisms selected over the course of evolution, as they would promote our species' survival. In the last decade, an increasing number of studies have been carried out to explore the neural bases of human morality.The aim of this paper is to present a comprehensive review of the data regarding the neurobiological origin of the moral sense, through a Medline search of English-language articles from 1980 to February 2012.The available findings would suggest that there might be a main integrative centre for the innate morality, in particular the ventromedial prefrontal cortex, with its multiple connections with the limbic lobe, thalamus and brainstem. The subjective moral sense would be the result of an integration of multiple automatic responses, mainly associated with social emotions and interpretation of others' behaviours and intentions.Since converging observations outline how lesions of the proposed neural networks may underlie some personality changes and criminal behaviours, the implications of the studies in this field encompass many areas of the scientific domain
Rare earth doped ceria: The complex connection between structure and properties
No full textThe need for high efficiency energy production, conversion, storage and transport is serving as a robust guide for the development of new materials. Materials with physical-chemical properties matching specific functions in devices are produced by suitably tuning the crystallographic- defect- and micro-structure of the involved phases. In this review, we discuss the case of Rare Earth doped Ceria. Due to their high oxygen diffusion coefficient at temperatures higher than ~500°C, they are very promising materials for several applications such as electrolytes for Solid Oxide Fuel and Electrolytic Cells (SOFC and SOEC, respectively). Defects are integral part of the conduction process, hence of the final application. As the fluorite structure of ceria is capable of accommodating a high concentration of lattice defects, the characterization and comprehension of such complex and highly defective materials involve expertise spanning from computational chemistry, physical chemistry, catalysis, electrochemistry, microscopy, spectroscopy, and crystallography. Results coming from different experimental and computational techniques will be reviewed, showing that structure determination (at different scale length) plays a pivotal role bridging theoretical calculation and physical properties of these complex materials
High-Pressure Computational Search of Trivalent Lanthanide Dinitrides
Full text linkTransition-metal nitrides have attracted much interest of the scientific community for their intriguing properties and technological applications. Here, we focus on yttrium dinitride (YN2) and its formation and structural transition under pressure. We employed a fixed composition USPEX search to find the most stable polymorphs. We choose yttrium as a proxy for the lanthanide series because it has only +3 oxidation state, contrary to most transition metals. We then computed the thermodynamic and dynamical stabilities of these structures compared to the decomposition reactions, and we found that the compound undergoes two structural transitions, the latter showing the formation of N-4 chains. A closer look into the nature of the nitrogen bonding showed that in the first two structures, where nitrogen forms dimers, the bond length is intermediate between that of a single bond and that of a double bond, making it hard to rationalize the proper oxidation state configuration for YN2. In the latter structure, where there is formation of N-4 chains, the bond lengths increase significantly up to a value that can be justified as a single bond. Finally, we also studied the electronic structure and dynamical stability of the structures we found
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