555 research outputs found
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 . 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
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
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
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
Mechanics of thermally fluctuating membranes
Besides having unique electronic properties, graphene is claimed to be the
strongest material in nature. In the press release of the Nobel committee it is
claimed that a hammock made of a squared meter of one-atom thick graphene could
sustain the wight of a 4 kg cat. More practically important are applications of
graphene like scaffolds and sensors which are crucially dependent on the
mechanical strength. Meter-sized graphene is even being considered for the
lightsails in the starshot project to reach the star alpha centaury. The
predicted strength of graphene is based on its very large Young modulus which
is, per atomic layer, much larger than that of steel. This reasoning however
would apply to conventional thin plates but does not take into account the
peculiar properties of graphene as a thermally fluctuating crystalline
membrane. It was shown recently both experimentally and theoretically that
thermal fluctuations lead to a dramatic reduction of the Young modulus and
increase of the bending rigidity for micron-sized graphene samples in
comparison with atomic scale values. This makes the use of the standard
F\"oppl-von Karman elasticity (FvK) theory for thin plates not directly
applicable to graphene and other single atomic layer membranes. This fact is
important because the current interpretation of experimental results is based
on the FvK theory. In particular, we show that the FvK-derived Schwerin
equation, routinely used to derive the Young modulus from indentation
experiments has to be essentially modified for graphene at room temperature and
for micron sized samples. Based on scaling analysis and atomistic simulation we
investigate the mechanics of graphene under transverse load up to breaking. We
determine the limits of applicability of the FvK theory and provide
quantitative estimates for the different regimes.Comment: to appear in npj 2D Materials and Application
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
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.
Zero modes in magnetic systems: general theory and an efficient computational scheme
The presence of topological defects in magnetic media often leads to normal
modes with zero frequency (zero modes). Such modes are crucial for long-time
behavior, describing, for example, the motion of a domain wall as a whole.
Conventional numerical methods to calculate the spin-wave spectrum in magnetic
media are either inefficient or they fail for systems with zero modes. We
present a new efficient computational scheme that reduces the magnetic
normal-mode problem to a generalized Hermitian eigenvalue problem also in the
presence of zero modes. We apply our scheme to several examples, including
two-dimensional domain walls and Skyrmions, and show how the effective masses
that determine the dynamics can be calculated directly. These systems highlight
the fundamental distinction between the two types of zero modes that can occur
in spin systems, which we call special and inertial zero modes. Our method is
suitable for both conservative and dissipative systems. For the latter case, we
present a perturbative scheme to take into account damping, which can also be
used to calculate dynamical susceptibilities.Comment: 64 pages, 15 figure
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 as a power law with , 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
- …