Kinetic Monte Carlo simulations inspired by epitaxial graphene growth

Graphene, a flat monolayer of carbon atoms packed tightly into a two dimensional hexagonal lattice, has unusual electronic properties which have many promising nanoelectronic applications. Recent Low Energy Electron Microscopy (LEEM) experiments show that the step edge velocity of epitaxially grown 2D graphene islands on Ru(0001) varies with the fifth power of the supersaturation of carbon adatoms. This suggests that graphene islands grow by the addition of clusters of five atoms rather than by the usual mechanism of single adatom attachment. We have carried out Kinetic Monte Carlo (KMC) simulations in order to further investigate the general scenario of epitaxial growth by the attachment of mobile clusters of atoms. We did not seek to directly replicate the Gr/Ru(0001) system but instead considered a model involving mobile tetramers of atoms on a square lattice. Our results show that the energy barrier for tetramer break up and the number of tetramers that must collide in order to nucleate an immobile island are the important parameters for determining whether, as in the Gr/Ru(0001) system, the adatom density at the onset of island nucleation is an increasing function of temperature. A relatively large energy barrier for adatom attachment to islands is required in order for our model to produce an equilibrium adatom density that is a large fraction of the nucleation density. A large energy barrier for tetramer attachment to islands is also needed for the island density to dramatically decrease with increasing temperature. We show that islands grow with a velocity that varies with the fourth power of the supersaturation of adatoms when tetramer attachment is the dominant process for island growth

Giant electron-phonon interactions in molecular crystals and the importance of nonquadratic coupling

We investigate electron-phonon coupling in the molecular crystals CH$_4$, NH$_3$, H$_2$O, and HF, using first-principles quantum mechanical calculations. We find vibrational corrections to the electronic band gaps at zero temperature of -1.97 eV, -1.01 eV, -1.52 eV, and -1.62 eV, respectively, which are comparable in magnitude to those from electron-electron correlation effects. Microscopically, the strong electron-phonon coupling arises in roughly equal measure from the almost dispersionless high-frequency molecular modes and from the lower frequency lattice modes. We also highlight the limitations of the widely used Allen-Heine-Cardona theory, which gives significant discrepancies compared to our more accurate treatment.B.M. acknowledges Robinson College, Cambridge, and the Cambridge Philosophical Society for a Henslow Research Fellowship. E.A.E. and R.J.N. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the UK [EP/K013688/1]. The calculations were performed on the Cambridge High Performance Computing Service facility and the Archer facility of the UK's national high-performance computing service (for which access was obtained via the UKCP consortium [EP/K013564/1]).This is the author accepted manuscript. The final version is available from APS via http://dx.doi.org/10.1103/PhysRevB.92.14030

Anharmonic nuclear motion and the relative stability of hexagonal and cubic ice

We use extensive first-principles quantum mechanical calculations to show that, although the static lattice and harmonic vibrational energies are almost identical, the anharmonic vibrational energy of hexagonal ice is significantly lower than that of cubic ice. This difference in anharmonicity is crucial, stabilising hexagonal ice compared with cubic ice by at least 1.4 meV/H2O, in agreement with experimental estimates. The difference in anharmonicity arises predominantly from molecular O-H bond stretching vibrational modes and is related to the different stacking of atomic layers.We acknowledge financial support from the Engineering and Physical Sciences Research Council of the UK [EP/J017639/1]. B. M. also acknowledges Robinson College, Cambridge, and the Cambridge Philosophical Society for a Henslow Research Fellowship. The calculations were performed on the Cambridge High Performance Computing Service facility and the HECToR and Archer facilities of the UK’s national high-performance computing service (for which access was obtained via the UKCP consortium [EP/K013564/1]).This is the final version of the article. It first appeared from APS via http://dx.doi.org/http://dx.doi.org/10.1103/PhysRevX.5.02103

Anàlisi de les causes accidentals de mortalitat de rapinyaires a Mallorca des del 2004 al 2016

[cat] Al present treball s’avaluen les entrades de rapinyaires al COFIB (Consorci per a la Recuperació de la Fauna de les Illes Balears) en el període 2004/2016. Les dades recopilades de tots els rapinyaires ferits o morts durant aquest període en dona una mostra significativa amb l’afectació de 4.603 individus. Així, s’analitzen les set principals causes d’entrada: atropellaments; electrocucions/impactes amb esteses elèctriques; tirotejats; enverinats; trampejats; molèsties als nius i confiscats (rapinyaires que es tenien en captivitat de manera il·legal). De cada una d’aquestes causes d’entrada s’analitzen les espècies que han estat més afectades, els municipis on se n’han trobat més i si el fenomen va a l’alça o a la baixa al llarg del temps. Es pretén determinar els principals factors que delimiten el correcte desenvolupament d’aquestes espècies d’aus.[eng] The present work evaluates the entries of raptors to the COFIB (Consortium for the Recovery of the Fauna of the Balearic Islands) in the period 2004/2016. The data collected from all the injured or killed raptors during this period gives a significant sample with the involvement of 4603 individuals. Thus, the seven main causes of entry are analyzed: runoffs; electrocutions / impacts with electrical extensions; shot; poisoned; trampled; annoyances to the nests and confiscated (birds of prey that were captivated illegally). Each of these causes of entry analyzes the most affected species, the municipalities where they have been found most and if the phenomenon is rising or falling over time. It is intended to determine the main factors that delimit the correct development of these bird species

Optical absorption driven by dynamical symmetry breaking in indium oxide

Input files for the density functional theory calculations reported in the associated publication

Finite temperature optoelectronic properties of BAs from first principles

© 2019 American Physical Society. The high thermal conductivity of boron arsenide (BAs) makes it a promising material for optoelectronic applications in which thermal management is central. In this work, we study the finite temperature optoelectronic properties of BAs by considering both electron-phonon coupling and thermal expansion. The inclusion of electron-phonon coupling proves imperative to capture the temperature dependence of the optoelectronic properties of the material, while thermal expansion makes a negligible contribution due to the highly covalent bonding character of BAs. We predict that with increasing temperature the optical absorption onset is subject to a red-shift, the absorption peaks become smoother, and the phonon-assisted absorption at energies below those of the optical gap has a coefficient that lies in the range 10-3-10-4cm-1. We also show that good agreement with the measured indirect band gap of BAs is only obtained if exact exchange, electron-phonon coupling, and spin-orbit coupling effects are all included in the calculations

Theory of phonon-assisted luminescence in solids: Application to hexagonal boron nitride

International audienceIn this manuscript we study luminescence of hexagonal boron nitride (hBN) by means of non-equilibrium Green's functions plus time-dependent perturbation theory. We derive a formula for light emission in solids in the limit of a weak excitation that includes perturbatively the contribution of electron-phonon coupling at the first order. This formula is applied to study luminescence in bulk hBN. This material has attracted interest due to its strong luminescence in the ultraviolet [Watanabe et al., Nature Mat. 3, 404(2004)]. The origin of this luminescence has been widely discussed, but only recently has a clear signature of phonon mediated light emission emerged in the experiments [Cassabois et al., Nature Phot. 10, 262(2016)]. By means of our new theoretical approach we provide a clear and full explanation of light emission in hBN

O(N) methods in electronic structure calculations

Linear scaling methods, or O(N) methods, have computational and memory requirements which scale linearly with the number of atoms in the system, N, in contrast to standard approaches which scale with the cube of the number of atoms. These methods, which rely on the short-ranged nature of electronic structure, will allow accurate, ab initio simulations of systems of unprecedented size. The theory behind the locality of electronic structure is described and related to physical properties of systems to be modelled, along with a survey of recent developments in real-space methods which are important for efficient use of high performance computers. The linear scaling methods proposed to date can be divided into seven different areas, and the applicability, efficiency and advantages of the methods proposed in these areas is then discussed. The applications of linear scaling methods, as well as the implementations available as computer programs, are considered. Finally, the prospects for and the challenges facing linear scaling methods are discussed.Comment: 85 pages, 15 figures, 488 references. Resubmitted to Rep. Prog. Phys (small changes