Borophene (two-dimensional boron sheet) is a new type of two-dimensional material, which was recently grown successfully on single crystal Ag substrates. In this paper, we investigate the electronic structure and bonding characteristics of borophene by first-principle calculations. The band structure of borophene shows highly anisotropic metallic behaviour.The obtained optical properties of borophene exhibit strong anisotropy as well. The combination of high optical transparency and high electrical conductivity in borophene makes it a promising candidate for future design of transparent conductors used in photovoltaics.Finally, the thermodynamic properties are investigated based on the phonon properties.arXiv:1601.00140v3 [cond-mat.mes-hall] 3 Apr 2016 2

Polycrystalline materials are composites of crystalline particles or ''grains'' separated by thin ''amorphous'' grain boundaries (GBs). Although GBs have been exhaustively investigated at low temperatures, at which these regions are relatively ordered, much less is known about them at higher temperatures, where they exhibit significant mobility and structural disorder and characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GBs at elevated temperatures, and we exploit MD to explore basic aspects of GB dynamics as a function of temperature. It has long been hypothesized that GBs have features in common with glass-forming liquids based on the processing characteristics of polycrystalline materials. We find remarkable support for this suggestion, as evidenced by string-like collective atomic motion and transient caging of atomic motion, and a non-Arrhenius GB mobility describing the average rate of large-scale GB displacement.glass formation ͉ grain-boundary mobility ͉ molecular dynamics ͉ polycrystalline materials ͉ string-like collective motion M ost technologically important materials are polycrystalline in nature (1), and it is appreciated that the grain boundaries (GBs) of these materials, the interfacial region separating the crystal grains (see Fig. 1A), significantly influence the properties of this broad class of materials (2). In particular, the dynamical properties of GBs, such as the GB mobility (M), play an important role in the plastic deformation and evolution of microstructure during material processing and service (3). † The atomic organization in the GBs represents a compromise between the ordering effects of adjacent grains, and ''packing frustration'' [or reduced packing efficiency (6)] is also characteristic of glass-forming (GF) fluids, in which particle ordering is likewise limited in range (7). This simple observation leads us to expect similarities between the dynamics of GBs and GF fluids, and below we provide evidence for this relationship. By implication, GB migration should then be sensitive to impurities, geometrical confinement, and applied stresses-basically any factor that affects particle-packing efficiency (8-10). To illustrate this point and test our perspective of GB dynamics, we quantitatively interpret differences in the effect of large tensile and compressive deformations on M in terms of measures of cooperative atomic motion drawn from the physics of GF fluids.Nearly 100 years ago, Rosenhain and Ewen (11) suggested that metal grains in cast iron were ''cemented'' together by a thin layer of ''amorphous'' (i.e., noncrystalline) material ''identical with or at least closely analogous to the condition of a greatly undercooled liquid.'' Although this conceptual model was able to rationalize processing characteristics of ferritic materials (11), it was not possible to validate it at the time through direct observation or simulation. Sixty year...

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We test the localization model (LM) prediction of a parameter-free relationship between the α-structural relaxation time τ α and the Debye–Waller factor  〈u 2 〉  for a series of simulated glass-forming Cu–Zr metallic liquids having a range of alloy compositions. After validating this relationship between the picosecond (‘fast’) and long-time relaxation dynamics over the full range of temperatures and alloy compositions investigated in our simulations, we show that it is also possible to estimate the self-diffusion coefficients of the individual atomic species (D Cu, D  Zr) and the average diffusion coefficient D using the LM, in conjunction with the empirical fractional Stokes–Einstein (FSE) relation linking these diffusion coefficients to τ α . We further observe that the fragility and extent of decoupling between D and τ α strongly correlate with  〈u 2 〉  at the onset temperature of glass-formation T A where particle caging and the breakdown of Arrhenius relaxation first emerge.

Despite the fundamental nature and practical importance of melting, there is still no generally accepted theory of this ubiquitous phenomenon. Even the earliest simulations of melting of hard discs by Alder and Wainwright indicated the active role of collective atomic motion in melting and here we utilize molecular dynamics simulation to determine whether these correlated motions are similar to those found in recent studies of glass-forming (GF) liquids and other condensed, strongly interacting, particle systems. We indeed find string-like collective atomic motion in our simulations of "superheated" Ni crystals, but other observations indicate significant differences from GF liquids. For example, we observe neither stretched exponential structural relaxation, nor any decoupling phenomenon, while we do find a boson peak, findings that have strong implications for understanding the physical origin of these universal properties of GF liquids. Our simulations also provide a novel view of "homogeneous" melting in which a small concentration of interstitial defects exerts a powerful effect on the crystal stability through their initiation and propagation of collective atomic motion. These relatively rare point defects are found to propagate down the strings like solitons, driving the collective motion. Crystal integrity remains preserved when the permutational atomic motions take the form of ring-like atomic exchanges, but a topological transition occurs at higher temperatures where the rings open to form linear chains similar in geometrical form and length distribution to the strings of GF liquids. The local symmetry breaking effect of the open strings apparently destabilizes the local lattice structure and precipitates crystal melting. The crystal defects are thus not static entities under dynamic conditions, such as elevated temperatures or material loading, but rather are active agents exhibiting a rich nonlinear dynamics that is not addressed in conventional "static" defect melting models.

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