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In an attempt to explain flash sintering experiments, it had been proposed that the electron–phonon coupling leads to a proliferation of short wave-length lattice vibrations in an electric field. In this paper we investigate this by solving two coupled Boltzmann equations, describing a free electron gas in an electric field scattering from a crystal lattice coupled to a heat bath. The electric field imposes cylindrical symmetry and drives the electrons and the phonons into a non-equilibrium steady state. We find that the phonon distribution shows a strong excess population at the Brillouin zone edge in the direction of the electric field. We argue analytically, that this can be traced back to the shifted Fermi sphere for the electrons. Furthermore, not only energy but also momentum is exchanged in the electron–phonon system, which defies any attempt at describing the system by a two-temperature model.

Surfaces possess mechanical features on smaller scales that stand out against bulk phases, e.g., scaling of stiffness, curvature‐dependence, surfactant control and anchoring‐induced anisotropy. Continuum properties for the respective scales are often derived from ab initio simulations. This scale‐bridging however bears conceptual challenges and we highlight three aspects for the example of pure copper. First, free surface atoms relax and alter the boundary region in terms of interatomistic distances and resulting initital stresses. Second, eliminating the influence of finite thickness on the two‐dimensional continuum surface can be achieved by different averages or limit definitions, not all being physically consistent. Third, the continuum model of the surface is usually coupled to a continuum model of the bulk, which causes an approximation error itself. However, the bulk phase can not be eliminated direclty from the examination and simple averaging may even mask the aforementioned influences on the surface mechanics. A thermodynamically sound parameter identification across the scales is hence required. We present a Ritz‐type modeling approach for surfaces that ensures energy equivalence between atmostic and continuum simulations. The influences of relaxation, finite thickness and bulk approximation are identified by a mismatch in the energy contributions and accounted for by using appropriate homogenization limits., Proceedings in applied mathematics & mechanics;Vol. 20. 2021, Issue 1, e202000193

We study the relaxation time of granular media in simple shear by means of DEM simulations (the methods being Molecular Dynamics as well as Contact Dynamics) in two and three dimensions with rough and with smooth frictional walls. While the system with rough walls behaves according to its steady state constitutive laws, the systems with smooth walls show a much stronger increase of the relaxation time with driving strength than to be expected. Employing a dynamic non-local rheology model[1] allows for a stronger increase but not sufficiently so. OA gold

The thermal boundary resistance (Kapitza resistance) of (001) twist boundaries in silicon is investigated by nonequilibrium molecular dynamics simulations. In order to enable continuous adjustment of the mismatch angle, a cylindrical geometry with fixed atomic positions at the boundaries is devised. The influence of the boundary conditions on the Kapitza resistance is removed by means of a finite size analysis. Due to the diamond structure of silicon, twist boundaries with mismatch angles ϕ and 90 ° − ϕ are not equivalent, whereas those with ± ϕ or with 90 ° ± ϕ are. The Kapitza resistance increases with mismatch angle up to 45 ° , where it reaches a plateau around 1.56 ± 0.05 K m 2 / GW . Between 80 ° and the 90 ° Σ 1 grain boundary it drops by about 30%. Surprisingly, lattice coincidence at other angles ( Σ 5 , Σ 13 , Σ 27 , Σ 25 ) has no noticable effect on the Kapitza resistance. However, there is a clear correlation between the Kapitza resistance and the width of a non-crystalline layer at the twist boundaries.

Simulation results for (a) interfacial alloying and (b) the Villain instability are presented. Both phenomena are important for the interface structure of epitaxial heterolayers and involve new scaling laws. For (a) a mechanism is discussed, which in a limiting case leads to a concentration profile decaying like (distance from the interface)−2. In (b) the critical film thickness is estimated, at which the Villain instability becomes important. Its scaling with the diffusion constant and the Schwoebel length can be combined with results on the strain dependence of these parameters in order to predict what kind of strains enhance or suppress the instability.