Short range repulsive interatomic interactions in energetic processes in solids

The repulsive interaction between two atoms at short distances is studied in order to explore the range of validity of standard first-principles simulation techniques and improve the available short-range potentials for the description of energetic collision cascades in solids. Pseudopotentials represent the weakest approximation, given their lack of explicit Pauli repulsion in the core-core interactions. The energy (distance) scale realistically accessible is studied by comparison with all-electron reference calculations in some binary systems. Reference calculations are performed with no approximations related to either core (frozen core, augmentation spheres) or basis set. This is important since the validity of such approximations, even in all-electron calculations, rely on the small core perturbation usual in low-energy studies. The expected importance of semicore states is quantified. We propose a scheme for improving the electronic screening given by pseudopotentials for very short distances. The results of this study are applied to the assessment and improvement of existing repulsive empirical potentials.Comment: 10 pages, 7 figure

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Correspondence in Natur

Combined photo- and electroreflectance of multijunction solar cells enabled by subcell electric coupling

Electric coupling between subcells of a monolithically grown multijunction solar cell in short circuit allows their simultaneous and independent characterization by means of photo- and electroreflectance. The photovoltage generated by selective absorption of the pump beam in a given subcell during photoreflectance measurements results in reverse biasing the complementary subunits at the modulation frequency set on the pump illumination. Such voltage bias modulation acts then as external perturbation on the complementary subcells. The spectral separation of the different subcell absorption ranges permits the probe beam to record in a single spectrum the response of the complete device as a combination of photo- and electroreflectance, thereby providing access for diagnosis of subcells on an individual basis. This form of modulation spectroscopy is demonstrated on a GaInP/GaAs tandem solar cell.Comment: 5 pages, 4 figures. This article has been accepted by Appl. Phys. Lett. After it is published, it will be found at https://doi.org/10.1063/1.506260

Continuous melting through a hexatic phase in confined bilayer water

Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to understand this behavior, we have analyzed the melting exhibited by a bilayer of nanoconfined water by means of molecular dynamics simulations. At high density we observe the continuous melting to be related to the phase change of the oxygens only, with the hydrogens remaining liquidlike throughout. Moreover, we find an intermediate hexatic phase for the oxygens between the liquid and a triangular solid ice phase, following the Kosterlitz-Thouless-Halperin-Nelson-Young theory for two-dimensional melting. The liquid itself tends to maintain the local structure of the triangular ice, with its two layers being strongly correlated yet with very slow exchange of matter. The decoupling in the behavior of the oxygens and hydrogens gives rise to a regime in which the complexity of water seems to disappear, resulting in what resembles a simple monoatomic liquid. This intrinsic tendency of our simulated water may be useful for understanding novel behaviors in other confined and interfacial water systems

Floquet theory for the electronic stopping of projectiles in solids

A general theoretical framework for the study of electronic stopping of particle projectiles in crystalline solids is proposed. It neither relies on perturbative or linear response approximations, nor on an ideal metal host. Instead, it exploits the discrete translational symmetries in a space-time diagonal determined by a projectile with constant velocity moving along a trajectory with crystalline periodicity. This allows for the characterisation of (stroboscopically) stationary solutions, by means of Floquet theory for time-periodic systems. Previous perturbative and non-linear jellium models are recovered from this general theory. An analysis of the threshold velocity effect in insulators is presented based on Floquet quasi-energy conservation

Ab initio calculation of the shock Hugoniot of bulk silicon

We describe how ab initio molecular dynamics can be used to determine the Hugoniot locus (states accessible by a shock wave) for materials with a number of stable phases, and with an approximate treatment of plasticity and yield, without having to simulate these phenomena directly. We consider the case of bulk silicon, with forces from density-functional theory, up to 70 GPa. The fact that shock waves can split into multiple waves due to phase transitions or yielding is taken into account here by specifying the strength of any preceding waves explicitly based on their yield strain. Points corresponding to uniaxial elastic compression along three crystal axes and a number of postshock phases are given, including a plastically yielded state, approximated by an isotropic stress configuration following an elastic wave of predetermined strength. The results compare well to existing experimental data for shocked silicon.We thank Alan Minchinton, Richard Needs, Nikos Nikiforakis, Stephen Walley and David Williamson for useful input and discussions.This research was supported with funding from Orica Ltd. and the following grants: MINECO-Spain’s Plan Nacional Grant No. FIS2012-37549-C05-01, Basque Government Grant No. PI2014-105 CIC07 2014-2016, and EU Grant “ElectronStopping” in the Marie Curie CIG Program. Part of this work was performed using the Darwin Supercomputer of the University of Cambridge High Performance Computing Service [41], provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council

Enhanced Configurational Entropy in High-Density Nanoconfined Bilayer Ice

A novel kind of crystal order in high-density nanoconfined bilayer ice is proposed from molecular dynamics and density-functional theory simulations. A first-order transition is observed between a low-temperature proton-ordered solid and a high-temperature proton-disordered solid. The latter is shown to possess crystalline order for the oxygen positions, arranged on a close-packed triangular lattice with AA stacking. Uniquely among the ice phases, the triangular bilayer is characterized by two levels of disorder (for the bonding network and for the protons) which results in a configurational entropy twice that of bulk ice.This work was partly funded by Grants No. FIS2012- 37549-C05 from the Spanish Ministry of Science, and Exp. 97/14 (Wet Nanoscopy) from the Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación, Diputación Foral de Gipuzkoa. We thank José M. Soler and M.-V. Fernández-Serra for useful discussions. The calculations were performed on the arina HPC cluster (Universidad del País Vasco/Euskal Herriko Unibertsitatea, Spain). SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) support is gratefully acknowledged

Structural and configurational properties of nanoconfined monolayer ice from first principles

Understanding the structural tendencies of nanoconfined water is of great interest for nanoscience and biology, where nano/micro-sized objects may be separated by very few layers of water. Here we investigate the properties of ice confined to a quasi-2D monolayer by a featureless, chemically neutral potential, in order to characterize its intrinsic behaviour. We use density-functional theory simulations with a non-local van der Waals density functional. An ab initio random structure search reveals all the energetically competitive monolayer configurations to belong to only two of the previously-identified families, characterized by a square or honeycomb hydrogen-bonding network, respectively. We discuss the modified ice rules needed for each network, and propose a simple point dipole 2D lattice model that successfully explains the energe tics of the square configurations. All identified stable phases for both networks are found to be non-polar (but with a topologically non-trivial texture for the square) and, hence, non-ferroelectric, in contrast to previous predictions from a five-site empirical force-field model. Our results are in good agreement with very recently reported experimental observations.This work was partly funded by grants FIS2012-37549-C05 from the Spanish Ministry of Science, and Exp. 97/14 (Wet Nanoscopy) from the Programa Red Guipuzcoana de Ciencia, Tecnología e Innovación, Diputación Foral de Gipuzkoa. We thank Richard Korytár and Javier Junquera for their work on the SIESTA interface to Wannier90, and Raffaele Resta and M.-V. Fernández-Serra for useful discussions. The calculations were performed on the arina HPC cluster (Universidad del País Vasco/Euskal Herriko Unibertsitatea, Spain). SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) support is gratefully acknowledged

Intrinsic point defects and volume swelling in ZrSiO4 under irradiation

The effects of high concentration of point defects in crystalline ZrSiO4 as originated by exposure to radiation, have been simulated using first principles density functional calculations. Structural relaxation and vibrational studies were performed for a catalogue of intrinsic point defects, with different charge states and concentrations. The experimental evidence of a large anisotropic volume swelling in natural and artificially irradiated samples is used to select the subset of defects that give similar lattice swelling for the concentrations studied, namely interstitials of O and Si, and the anti-site Zr(Si), Calculated vibrational spectra for the interstitials show additional evidence for the presence of high concentrations of some of these defects in irradiated zircon.Comment: 9 pages, 7 (color) figure

Van der Waals interaction in magnetic bilayer graphene nanoribbons

We study the interaction energy between two graphene nanoribbons by first-principles calculations, including van der Waals interactions and spin polarization. For ultranarrow zigzag nanoribbons, the direct stacking is even more stable than the Bernal stacking, competing in energy for wider ribbons. This behavior is due to the magnetic interaction between edge states. We relate the reduction of the magnetization in zigzag nanoribbons with increasing ribbon width to the structural changes produced by the magnetic interaction, and we show that when deposited on a substrate, zigzag bilayer ribbons remain magnetic for larger widths. © 2012 American Physical Society