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The layer-dependent structural, electronic, and vibrational properties of SnS2 and SnSe2 are investigated using first-principles density functional theory (DFT). The in-plane lattice constants, interlayer distances and binding energies are found to be layer-independent. Bulk SnS2 and SnSe2 are both indirect band gap semiconductors with Eg=2.18 and 1.07eV, respectively. Few-layer and monolayer 2D systems also possess an indirect band gap, which is increased to 2.41 and 1.69eV for single layers of SnS2 and SnSe2. The effective mass theory of 2D excitons, which takes into account the combined effect of the anisotropy, nonlocal 2D screening and layer-dependent 3D screening, predicts strong excitonic effects. The binding energy of indirect excitons in monolayer samples, Ex∼0.9eV, is substantially reduced to Ex=0.14eV in bulk SnS2 and Ex=0.09eV in bulk SnSe2. The layer-dependent Raman spectra display a strong decrease of intensities of the Raman active A1g mode upon decreasing the number of layers down to a monolayer, by a factor of 7 in the case of SnS2 and a factor of 20 in the case of SnSe2, which can be used to identify the number of layers in a 2D sample. [ABSTRACT FROM AUTHOR]

The development of condensed-phase detonation instabilities is simulated using moving window molecular dynamics and a generic AB model of a high explosive. It is found that an initially planar detonation front with one-dimensional flow can become unstable through development of transverse perturbations resulting in highly inhomogeneous and complex two- and three-dimensional distributions of pressure and other variables within the detonation front. Chemical reactions are initiated in localized transverse shock fronts and Mach stems with a pressure and temperature higher than those predicted by classic Zel'dovich, von Neumann, and Doering detonation theory. The two-dimensional cellular and transverse and three-dimensional pulsating detonation structures are found by varying the physico-chemical properties of AB energetic material, sample geometry, and boundary conditions. The different regimes of condensed-phase detonation that can develop from instabilities within a planar detonation front exhibit structures, although at a much smaller scale, that are similar to those observed in gases and diluted liquids. [ABSTRACT FROM AUTHOR]

A screened environment-dependent reactive empirical bond-order (SED-REBO) potential has been developed for large-scale molecular dynamics (MD) simulations of carbon materials. Based on the second-generation REBO potential developed by Brenner and co-workers [J. Phys.: Condens. Matter 14, 783 (2002)], the SED-REBO potential overcomes the deficiencies of the REBO potential, which arise from a short range of interatomic interactions and their abrupt switching off at the cutoff distance, by increasing the range of interatomic interactions and eliminating the explicit switching function while introducing a simple yet efficient screening function. The increased cutoff distance allows the inclusion of interactions critically important for the physically correct description of bond breaking and bond remaking. An analytic form of the attractive and repulsive pairwise terms was devised to automatically become zero at distances above the cutoff, thus, eliminating the need for the switching function. The screening function effectively screens off the second- and further-nearest-neighbor interactions for calculation of energy and forces in a smooth and continuous way for both compression and expansion. The pairwise attractive and repulsive terms were refitted within a wide range of interatomic distances to properly describe large compressions and expansions of diamond and graphene as well as their behavior near equilibrium. Good performances of the SED-REBO potential to describe bond-breaking processes at extreme tensile stresses are demonstrated in large-scale MD simulations of the nanoindentation of graphene membranes. A computationally efficient version of the SED-REBO potential is introduced for large-scale MD simulations of shock-wave compression in carbon materials. The SED-REBO potential is implemented as a module in the Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and is freely available. [ABSTRACT FROM AUTHOR]

The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses in single-crystal nickel films are investigated by molecular dynamics simulations. Ultrafast laser heating is isochoric, leading to pressurization of a 100-nm-thick layer below the irradiated surface. For low-intensity laser pulses, the highly pressurized subsurface layer breaks into a single elastic shock wave having a combined loading and unloading time ≈10-20 ps. Owing to the time-dependent nature of elastic-plastic transformations, an elastic response is maintained for shock amplitudes exceeding the Hugoniot elastic limit determined from simulations of steady shock waves. However, for high-intensity laser pulses (absorbed laser fluence >0.6 J/cm²), both elastic and plastic shock waves are formed independentiy from the initial high-pressure state. Acoustic pulses emitted by the plastic front support the motion of the elastic precursor resulting in a fluence-independent elastic amplitude; whereas the unsupported plastic front undergoes significant attenuation during propagation and may fully decay within the metal film. [ABSTRACT FROM AUTHOR]

The evolution of orientation-dependent metastable states during shock-induced solid-liquid phase transitions in crystalline A1 is followed using moving window molecular dynamics simulations. The orientation-dependent transition pathways towards an orientation-independent final state Hugoniot include both "cold melting" followed by recrystallization in [110]- and [11 l]-oriented shock waves and crystal overheating followed by melting in [100] shock waves. The orientation-dependent dynamics take place within a zone that can extend up to hundreds of nanometers behind the shock front. [ABSTRACT FROM AUTHOR]

Structural, electronic, and magnetic properties of simple interfaces (graphene on top of a metallic substrate) and complex interfaces (a single metallic adlayer on a simple graphene/metal system, either on top or between the graphene and metallic substrate) have been studied using density functional theory. Two types of simple interface with strong (Ni/graphene) and weak (Cu/graphene) bonding were considered. In addition to binding energies and interface distances, which are used to quantify the strength of graphene-substrate interactions, the bonding in simple and complex interfaces was analyzed using charge density distributions and bond orders. Substantial enhancement of the metallic substrate/graphene binding was observed in complex interfaces, consisting of a Ni monolayer on top of a simple {Ni or Cu}/graphene interface. The increase of substrate-graphene bonding in such complex interfaces is accompanied by weakening of in-plane C-C bonds in graphene, as quantified by the bond orders. A weak ferrimagnetism in graphene, i.e., unequal magnetic moments --0.04jLtfl and -|-0.06/XB on the C atoms, is induced by a ferromagnetic Ni substrate. The strength of graphene-substrate interactions is also reflected in simulated scanning tunneling microscopy images. [ABSTRACT FROM AUTHOR]

By decoupling time and length scales in moving window molecular dynamics shock-wave simulations, a new regime of shock-wave propagation is uncovered characterized by a two-zone elastic-plastic shock- wave structure consisting of a leading elastic front followed by a plastic front, both moving with the same average speed and having a fixed net thickness that can extend to microns. The material in the elastic zone is in a metastable state that supports a pressure that can substantially exceed the critical pressure characteristic of the onset of the well-known split-elastic-plastic, two-wave propagation. The two-zone elastic-plastic wave is a general phenomenon observed in simulations of a broad class of crystalline materials and is within the reach of current experimental techniques. [ABSTRACT FROM AUTHOR]

Force distribution in a granular medium subjected to an impulse loading is investigated in experiment and computer simulations. An experimental technique is developed to measure forces acting on individual grains at the bottom of the granular sample consisting of steel balls. Discrete element method simulation also is performed under conditions mimicking those in experiment. Both theory and experiment display exponentially decaying maximum force distributions at the bottom of the sample in the range of large forces. In addition, the simulations also reveal exponential force distribution throughout the sample and uncover correlation properties of the interparticle forces during dynamic loading of the granular samples. Simulated time dependence of coordination number, orientational order parameter, correlation radius, and force distribution clearly demonstrates the nonequilibrium character of the deformation process in a granular medium under impulse loading. [ABSTRACT FROM AUTHOR]