We investigate the structural, electronic and magnetic properties of the newly synthesized mineral barlowite Cu4(OH)6FBr which contains Cu 2+ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ZnCu3(OH)6Cl2, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by F − and Br − compared to Cl − . We perform density functional theory calculations to obtain the Heisenberg Hamiltonian parameters of Cu4(OH)6FBr which has a Cu 2+ site coupling the kagome layers. The 3D network of exchange couplings together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound at TN = 15 K as observed by magnetic susceptibility measurements on single crystals.
The theory describing energy losses of charged non-relativistic projectiles crossing a planar interface is derived on the basis of the Maxwell equations, outlining the physical assumptions of the model in great detail. The employed approach is very general in that various common models for surface excitations (such as the specular reflection model) can be obtained by an appropriate choice of parameter values. The dynamics of charged projectiles near surfaces is examined by calculations of the induced surface charge and the depth- and direction-dependent differential inelastic inverse mean free path (DIIMFP) and stopping power. The effect of several simplifications frequently encountered in the literature is investigated: differences of up to 100% are found in heights, widths, and positions of peaks in the DIIMFP. The presented model is implemented in a Monte Carlo algorithm for the simulation of the electron transport relevant for surface electron spectroscopy. Simulated reflection electron energy loss spectra are in good agreement with experiment on an absolute scale. Copyright © 2012 John Wiley & Sons, Ltd.
Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spinstate transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S = 5 2 mineral hauerite (MnS 2 ) undergoes an unprecedented (ΔV ∼ 22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S = Magnetism plays an important role in determining the properties of the minerals that make up Earth's crust and mantle (1, 2). This is due to the ubiquity of transition metals (notably Fe) with unpaired electrons. Under geological pressures, these may undergo abrupt volume collapses due to spin-state transitions. These occur when the crystal field splitting Δ, competes with Hund's rule and electronic Coulomb repulsion to determine the size of the magnetic moment. High pressure can thus make a high-spin (e.g., S = 5 2 ) state unstable with respect to a low-spin state (1-5), because Δ increases as the metal-ligand bond distances decrease. For simple materials, volume collapses of ΔV ' 5 % are regarded as notable (3-9). Understanding the mechanisms (5) of such magnetically driven transitions, and predicting the high-pressure structures, is ultimately of great importance for modeling the mantle in particular.The simple pyrite-structured mineral hauerite (10) (MnS 2 ) was reported nearly 30 years ago to undergo a pressure-induced transition (11, 12). The pyrite structure (13) (Fig. 1A) is cubic, with molecular disulfide S 2− 2 groups, which octahedrally coordinate an fcc lattice of Mn 2+ sites. Under ambient conditions, a high-spin t 3 2g e 2 g S = 5 2 moment is stabilized by Hund's rule coupling, and long-range magnetic order results (10). Early work on MnS 2 , which used laboratory energy-dispersive diffraction (11), detected a structural change at 11 GPa. These results were interpreted as a spin-state transition to a t 5 2g e 0 g S = 1 2 state in the marcasite structure. This was supported by later calculations (14). However, when the experiment was repeated using synchrotron radiation (12), an unidentified disordered phase was observed. Together with the large estimated volume collapse (∼15%), this little-remarked-upon finding was one of the motivations for the present work.We used the same natural sample as previous investigations (11, 12), and confirmed its purity by diffraction and X-ray fluorescence measurements (15) (Fig. S1). Pressure was applied using gas-loaded diamond anvil cells. Up to 11.7 GPa, X-ray diffra...
EBSD has evolved into an effective tool for microstructure investigations in the scanning electron microscope. The purpose of this contribution is to give an overview of various simulation approaches for EBSD Kikuchi patterns and to discuss some of the underlying physical mechanisms.
""Spectra of secondary electrons (SE) emitted from a polycrystalline Al surface have been measured in coincidence with 500 eV-electrons for energy losses between 10 and 155 eV. The spectra for a given energy loss are qualitatively similar, consisting of surface and volume plasmon decay and a contribution attributable to direct electron-electron scattering. The similarity of the contribution of surface and volume plasmon decay in the SE spectra proves directly that electron multiple scattering is governed by a Markov-type process. The average value of the surface plasmon decay contribution to the SE spectrum amounts to similar to 25%. (C) 2011 American Institute of Physics. [doi:10.1063\\\/1.3658455]"
The Fortran subroutine package pengeom provides a complete set of tools to handle quadric geometries in Monte Carlo simulations of radiation transport. The material structure where radiation propagates is assumed to consist of homogeneous bodies limited by quadric surfaces. The pengeom subroutines (a subset of the penelope code) track particles through the material structure, independently of the details of the physics models adopted to describe the interactions. Although these subroutines are designed for detailed simulations of photon and electron transport, where all individual interactions are simulated sequentially, they can also be used in mixed (class II) schemes for simulating the transport of high-energy charged particles, where the effect of soft interactions is described by the random-hinge method. The definition of the geometry and the details of the tracking algorithm are tailored to optimize simulation speed. The use of fuzzy quadric surfaces minimizes the impact of round-off errors. The provided software includes a Java graphical * Corresponding author. E-mail address: francesc.salvat@ub.edu Preprint submitted to Computer Physics CommunicationsMay 21, 2015 Manuscript 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 user interface for editing and debugging the geometry definition file and for visualizing the material structure. Images of the structure are generated by using the tracking subroutines and, hence, they describe the geometry actually passed to the simulation code.Keywords: Constructive quadric geometry; Monte Carlo particle transport; Ray tracing; Geometry visualization The Fortran subroutines perform all geometry operations in Monte Carlo simulations of radiation transport with arbitrary interaction models. They track particles through material systems consisting of homogeneous bodies limited by quadric surfaces. Particles are moved in steps (free flights) of a given length, which is dictated by the simulation program, and are halted when they cross an interface between media of different compositions or when they enter selected bodies. Solution method: The pengeom subroutines are tailored to optimize simulation speed and accuracy. Fast tracking is accomplished by the use of quadric surfaces, which facilitate the calculation of ray intersections, and of modules (connected volumes limited by quadric surfaces) organized in a hierarchical structure. Optimal accuracy is obtained by considering fuzzy surfaces, with the aid of a simple algorithm that keeps control of multiple intersections of a ray and a surface. The Java GUI PenGeomJar provides a geometry toolbox; it allows building and debugging 2 1 PROGRAM SUMMARY
The double-differential spectrum of coincidences between backscattered electrons and secondary electrons (SEs) emitted from a polycrystalline Al surface bombarded with 100-eV electrons was measured. For energy losses of the scattered electron in between the work function of Al and the bulk plasmon energy, a sharp peak is observed in the SE spectra, corresponding to ejection of a single electron near the Fermi edge receiving the full energy loss and momentum of the primary electron. This process predominantly takes place when the primary electron suffers a surface energy loss in vacuum, and leads to SE ejection from the very surface. At energy losses just above the bulk plasmon energy, a sharp transition is observed, corresponding to a sudden increase in the depth of ejection. The latter is a direct consequence of the complementarity of surface and bulk plasmons, the so-called Begrenzungs effect.When an energetic electron strikes a solid surface it may transfer part of its energy to the solid-state electrons. If the energy transfer exceeds the work function of the solid, an electron near the Fermi edge may escape over the surface barrier as a secondary electron (SE) or may in turn suffer energy losses, giving rise to the formation of a cascade of slow electrons. 1-4 The above phenomenon of electroninduced SE emission is highly relevant in a broad variety of applications but, due to a lack of spectral features in the cascade, SE emission is still far from being quantitatively understood. 5 While the different excitation channels can be easily discriminated with electron-energy-loss spectroscopy, the mechanism by which the deposited energy is dissipated away over the degrees of freedom of the solid is not easily resolvable by experiment. It is obvious that this requires the detection of correlated electron pairs, where one electron carries the signature of the involved excitation, while the other provides information on its decay. [6][7][8][9][10][11][12][13][14] This Rapid Communication presents the double-differential secondary-electron electron-energy-loss coincidence spectrum (SE2ELCS) of Al bombarded with 100-eV primary electrons. In the coincidence data, events can be distinguished in which the primary electron experiences a surface energy loss in vacuum, leading to ejection of a solid-state electron from the very surface (less than half an angstrom below the surface) that reaches the detector without traversing the solid at all. The choice of Al as the material for these investigations is motivated by the fact that the electron-solid interaction is Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.characterized by a sharp surface and bulk plasmon, making it possible to distinguish the single-and multiple-scattering regime in experimental data with the bare eye, considerably simplifying the identification of relevant ...
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