Nonlinear dynamics and surface diffusion of diatomic molecules

The motion of molecules on solid surfaces is of interest for technological applications, but it is also a theoretical challenge. We study the deterministic and thermal diffusive dynamics of a dimer moving on a periodic substrate. The deterministic motion of the dimer displays strongly nonlinear features and chaotic behavior. The dimer thermal diffusive dynamics deviates from simple Arrhenius behavior, due to the coupling between vibrational and translational degrees of freedom. In the low-temperature limit the dimer diffusion can become orders of magnitude larger than that of a single atom, as also found experimentally. The relation between chaotic deterministic dynamics and stochastic thermal diffusion is discussed.Comment: 4 pages, 4 figure

Power law load dependence of atomic friction

We present a theoretical study of the dynamics of a tip scanning a graphite surface as a function of the applied load. From the analysis of the lateral forces, we extract the friction force and the corrugation of the effective tip-surface interaction potential. We find both the friction force and potential amplitude to have a power law dependence on applied load with exponent $\sim 1.6$. We interpret these results as characteristic of sharp undeformable tips in contrast to the case of macroscopic and elastic microscopic contacts.Comment: 4 pages, 4 figure

Slow dynamics in a model of the cellulose network

We present numerical simulations of a model of cellulose consisting of long stiff rods, representing cellulose microfibrils, connected by stretchable crosslinks, representing xyloglucan molecules, hydrogen bonded to the microfibrils. Within a broad range of temperature the competing interactions in the resulting network give rise to a slow glassy dynamics. In particular, the structural relaxation described by orientational correlation functions shows a logarithmic time dependence. The glassy dynamics is found to be due to the frustration introduced by the network of xyloglucan molecules. Weakening of interactions between rod and xyloglucan molecules results in a more marked reorientation of cellulose microfibrils, suggesting a possible mechanism to modify the dynamics of the plant cell wall.Comment: 13 pages, 7 figures, accepted in Polyme

Scaling behavior and strain dependence of in-plane elastic properties of graphene

We show by atomistic simulations that, in the thermodynamic limit, the in-plane elastic moduli of graphene at finite temperature vanish with system size $L$ as a power law $~ L^{-\eta_u}$ with $\eta_u \simeq 0.325$, in agreement with the membrane theory. Our simulations clearly reveal the size and strain dependence of graphene's elastic moduli, allowing comparison to experimental data. Although the recently measured difference of a factor 2 between the asymptotic value of the Young modulus for tensilely strained systems and the value from {\it ab initio} calculations remains unsolved, our results do explain the experimentally observed increase of more than a factor 2 for a tensile strain of only a few permille. We also discuss the scaling of the Poisson ratio, for which our simulations disagree with the predictions of the self-consistent screening approximation.Comment: 5 figure

Solvent Driven Formation of Bolaamphiphilic Vesicles

We show that a spontaneous bending of single layer bolaamphiphiles results from the frustration due to the competition between core-core and tail-solvent interactions. We find that spherical vesicles are stable under rather general assumptions on these interactions described within the Flory-Huggins theory. We consider also the deformation of the vesicles in an external magnetic field that has been recently experimentally observed.Comment: J. Phys. Chem. B, accepte

Motion of domain walls and the dynamics of kinks in the magnetic Peierls potential

We study the dynamics of magnetic domain walls in the Peierls potential due to the discreteness of the crystal lattice. The propagation of a narrow domain wall (comparable to the lattice parameter) under the effect of a magnetic field proceeds through the formation of kinks in its profile. We predict that, despite the discreteness of the system, such kinks can behave like sine-Gordon solitons in thin films of materials such as yttrium iron garnets, and we derive general conditions for other materials. In our simulations we also observe long-lived breathers. We provide analytical expressions for the effective mass and limiting velocity of the kink in excellent agreement with our numerical results.Comment: 12 pages, 9 figures (incl. supp. mat.

Moir{\'e} patterns as a probe of interplanar interactions: graphene on h-BN

By atomistic modeling of moir{\'e} patterns of graphene on a substrate with a small lattice mismatch, we find qualitatively different strain distributions for small and large misorientation angles, corresponding to the commensurate-incommensurate transition recently observed in graphene on hexagonal BN. We find that the ratio of C-N and C-B interactions is the main parameter determining the different bond lengths in the center and edges of the moir{\'e} pattern. Agreement with experimental data is obtained only by assuming that the C-B interactions are at least twice weaker than the C-N interactions. The correspondence between the strain distribution in the nanoscale moir{\'e} pattern and the potential energy surface at the atomic scale found in our calculations, makes the moir{\'e} pattern a tool to study details of dispersive forces in van der Waals heterostructures.Comment: 5 pages, 3 figure

Minimal graphene thickness for wear protection of diamond

We show by means of molecular dynamics simulations that graphene is an excellent coating for diamond. The transformation of diamond to amorphous carbon while sliding under pressure can be prevented by having at least two graphene layers between the diamond slabs, making this combination of materials suitable for new coatings and micro- and nanoelectromechanical devices. Grain boundaries, vacancies and adatoms on the diamond surface do not change this picture whereas reactive adsorbates between the graphene layers may have detrimental effects. Our findings can be explained by the properties of layered materials where the weak interlayer bonding evolves to a strong interlayer repulsion under pressure

Gap opening in ultrathin Si layers: Role of confined and interface states

We present first principle calculations of ultrathin silicon (111) layers embedded in CaF2, a lattice matched insulator. Our all electron calculation allows a check of the quantum confinement hypothesis for the Si band gap opening as a function of thickness. We find that the gap opening is mostly due to the valence band while the lowest conduction band states shift very modestly due to their pronounced interface character. The latter states are very sensitive to the sample design. We suggest that a quasidirect band gap can be achieved by stacking Si layers of different thickness

Strained Semidconductor clusters in sodalite.

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