6,051 research outputs found
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
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
An International Dynamic Term Structure Model with Economic Restrictions and Unspanned Risks
We construct a multi-country affine term structure model that contains unspanned macroeconomic and foreign exchange risks. The canonical version of the model is derived and is shown to be easy to estimate. We show that it is important to impose restrictions (including global asset pricing, carry trade fundamentals and maximal Sharpe ratios) on the prices of risk to obtain plausible decompositions of forward curves. The forecasts of interest rates and exchange rates from the restricted model match those from international survey data. Unspanned macroeconomic variables are important drivers of international term and foreign exchange risk premia as well as expected exchange rate changes.Asset Pricing; Exchange rates; Interest rates
Slow energy relaxation of macromolecules and nano-clusters in solution
Many systems in the realm of nanophysics from both the living and inorganic
world display slow relaxation kinetics of energy fluctuations. In this paper we
propose a general explanation for such phenomenon, based on the effects of
interactions with the solvent. Within a simple harmonic model of the system
fluctuations, we demonstrate that the inhomogeneity of coupling to the solvent
of the bulk and surface atoms suffices to generate a complex spectrum of decay
rates. We show for Myoglobin and for a metal nano-cluster that the result is a
complex, non-exponential relaxation dynamics.Comment: 5 pages, 3 figure
Melting temperature of graphene
We present an approach to the melting of graphene based on nucleation theory
for a first order phase transition from the 2D solid to the 3D liquid via an
intermediate quasi-2D liquid.
The applicability of nucleation theory, supported by the results of
systematic atomistic Monte Carlo simulations, provides an intrinsic definition
of the melting temperature of graphene, , and allows us to determine it.
We find K, about 250 K higher than that of graphite using the
same interatomic interaction model. The found melting temperature is shown to
be in good agreement with the asymptotic results of melting simulations for
finite disks and ribbons of graphene. Our results strongly suggest that
graphene is the most refractory of all known materials
Spiral graphone and one sided fluorographene nano-ribbons
The instability of a free-standing one sided hydrogenated/fluorinated
graphene nano-ribbon, i.e. graphone/fluorographene, is studied using ab-initio,
semiempirical and large scale molecular dynamics simulations. Free standing
semi-infinite arm-chair like hydrogenated/fluorinated graphene (AC-GO/AC-GF)
and boat like hydrogenated/fluorinated graphene (B-GO/B-GF) (nano-ribbons which
are periodic along the zig-zag direction) are unstable and spontaneously
transform into spiral structures. We find that rolled, spiral B-GO and B-GF are
energetically more favorable than spiral AC-GO and AC-GF which is opposite to
the double sided flat hydrogenated/fluorinated graphene, i.e.
graphane/fluorographene. We found that the packed, spiral structures exhibit
unexpected localized HOMO-LUMO at the edges with increasing energy gap during
rolling. These rolled hydrocarbon structures are stable beyond room temperature
up to at least =1000\,K.Comment: Phys. Rev. B 87, 075448 (2013
The role of the Berry Phase in Dynamical Jahn-Teller Systems
The presence/absence of a Berry phase depends on the topology of the manifold
of dynamical Jahn-Teller potential minima. We describe in detail the relation
between these topological properties and the way the lowest two adiabatic
potential surfaces get locally degenerate. We illustrate our arguments through
spherical generalizations of the linear T x h and H x h cases, relevant for the
physics of fullerene ions. Our analysis allows us to classify all the spherical
Jahn-Teller systems with respect to the Berry phase. Its absence can, but does
not necessarily, lead to a nondegenerate ground state.Comment: revtex 7 pages, 2 eps figures include
Atomistic simulations of structural and thermodynamic properties of bilayer graphene
We study the structural and thermodynamic properties of bilayer graphene, a
prototype two-layer membrane, by means of Monte Carlo simulations based on the
empirical bond order potential LCBOPII. We present the temperature dependence
of lattice parameter, bending rigidity and high temperature heat capacity as
well as the correlation function of out-of-plane atomic displacements. The
thermal expansion coefficient changes sign from negative to positive above
K, which is lower than previously found for single layer graphene
and close to the experimental value of bulk graphite. The bending rigidity is
twice as large than for single layer graphene, making the out-of-plane
fluctuations smaller. The crossover from correlated to uncorrelated
out-of-plane fluctuations of the two carbon planes occurs for wavevectors
shorter than nmComment: 6 pages, 7 figures
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