1,239 research outputs found
Hadamard States and Adiabatic Vacua
Reversing a slight detrimental effect of the mailer related to TeXabilityComment: 10pages, LaTeX (RevTeX-preprint style
Ultra-strong Adhesion of Graphene Membranes
As mechanical structures enter the nanoscale regime, the influence of van der
Waals forces increases. Graphene is attractive for nanomechanical systems
because its Young's modulus and strength are both intrinsically high, but the
mechanical behavior of graphene is also strongly influenced by the van der
Waals force. For example, this force clamps graphene samples to substrates, and
also holds together the individual graphene sheets in multilayer samples. Here
we use a pressurized blister test to directly measure the adhesion energy of
graphene sheets with a silicon oxide substrate. We find an adhesion energy of
0.45 \pm 0.02 J/m2 for monolayer graphene and 0.31 \pm 0.03 J/m2 for samples
containing 2-5 graphene sheets. These values are larger than the adhesion
energies measured in typical micromechanical structures and are comparable to
solid/liquid adhesion energies. We attribute this to the extreme flexibility of
graphene, which allows it to conform to the topography of even the smoothest
substrates, thus making its interaction with the substrate more liquid-like
than solid-like.Comment: to appear in Nature Nanotechnolog
Leveraging Skype in the Classroom for Science Communication: A Streaming Science – Scientist Online Approach
A growing need exists to identify, implement, and research alternative methods to communicate with, educate, and engage youth about science, in order to increase science literacy and knowledge of future societal decision-makers. Electronic field trips (EFTs) are one channel of non-formal communication and education that have been introduced in agricultural and natural resources to reach youth audiences with science-based information in real-time. EFTs can be conducted in several different ways due to the proliferation of video production and web-streaming technologies. The following professional development article offers science communication professionals and scientists a detailed model and specific steps to develop and host an EFT via the Skype in the Classroom platform. The outlined model builds off of prior application and research from the Streaming Science online science communication platform and offers a secondary model for effective EFT implementation and research. The authors describe the establishment of an online science communication network, the development of the Streaming Science: Scientist Online format, content creation, the production team structure, and mobile production hardware and software. Scientist Online EFT program outcomes in terms of participation are noted, as well as student outcomes in the form of excerpts to demonstrate student engagement are shared
Determination of the Bending Rigidity of Graphene via Electrostatic Actuation of Buckled Membranes
The small mass and atomic-scale thickness of graphene membranes make them
highly suitable for nanoelectromechanical devices such as e.g. mass sensors,
high frequency resonators or memory elements. Although only atomically thick,
many of the mechanical properties of graphene membranes can be described by
classical continuum mechanics. An important parameter for predicting the
performance and linearity of graphene nanoelectromechanical devices as well as
for describing ripple formation and other properties such as electron
scattering mechanisms, is the bending rigidity, {\kappa}. In spite of the
importance of this parameter it has so far only been estimated indirectly for
monolayer graphene from the phonon spectrum of graphite, estimated from AFM
measurements or predicted from ab initio calculations or bond-order potential
models. Here, we employ a new approach to the experimental determination of
{\kappa} by exploiting the snap-through instability in pre-buckled graphene
membranes. We demonstrate the reproducible fabrication of convex buckled
graphene membranes by controlling the thermal stress during the fabrication
procedure and show the abrupt switching from convex to concave geometry that
occurs when electrostatic pressure is applied via an underlying gate electrode.
The bending rigidity of bilayer graphene membranes under ambient conditions was
determined to be eV. Monolayers have significantly lower
{\kappa} than bilayers
Not all adiabatic vacua are physical states
Adiabatic vacua are known to be Hadamard states. We show, however that the
energy-momentum tensor of a linear Klein-Gordon field on Robertson-Walker
spaces developes a generic singularity on the initial hypersurface if the
adiabatic vacuum is of order less than four. Therefore, adiabatic vacua are
physically reasonable only if their order is at least four.
A certain non-local large momentum expansion of the mode functions has
recently been suggested to yield the subtraction terms needed to remove the
ultraviolet divergences in the energy-momentum tensor. We find that this scheme
fails to reproduce the trace anomaly and therefore is not equivalent to
adiabatic regularisation.Comment: 13 pages, LaTex2
On the scalar sector of the covariant graviton two-point function in de Sitter spacetime
We examine the scalar sector of the covariant graviton two-point function in
de Sitter spacetime. This sector consists of the pure-trace part and another
part described by a scalar field. We show that it does not contribute to
two-point functions of gauge-invariant quantities. We also demonstrate that the
long-distance growth present in some gauges is absent in this sector for a wide
range of gauge parameters.Comment: 15 pages, no figures, LaTeX, considerably shortene
Quantum Inequalities on the Energy Density in Static Robertson-Walker Spacetimes
Quantum inequality restrictions on the stress-energy tensor for negative
energy are developed for three and four-dimensional static spacetimes. We
derive a general inequality in terms of a sum of mode functions which
constrains the magnitude and duration of negative energy seen by an observer at
rest in a static spacetime. This inequality is evaluated explicitly for a
minimally coupled scalar field in three and four-dimensional static
Robertson-Walker universes. In the limit of vanishing curvature, the flat
spacetime inequalities are recovered. More generally, these inequalities
contain the effects of spacetime curvature. In the limit of short sampling
times, they take the flat space form plus subdominant curvature-dependent
corrections.Comment: 18 pages, plain LATEX, with 3 figures, uses eps
Short distance and initial state effects in inflation: stress tensor and decoherence
We present a consistent low energy effective field theory framework for
parameterizing the effects of novel short distance physics in inflation, and
their possible observational signatures in the Cosmic Microwave Background. We
consider the class of general homogeneous, isotropic initial states for quantum
scalar fields in Robertson-Walker (RW) spacetimes, subject to the requirement
that their ultraviolet behavior be consistent with renormalizability of the
covariantly conserved stress tensor which couples to gravity. In the functional
Schr\"odinger picture such states are coherent, squeezed, mixed states
characterized by a Gaussian density matrix. This Gaussian has parameters which
approach those of the adiabatic vacuum at large wave number, and evolve in time
according to an effective classical Hamiltonian. The one complex parameter
family of squeezed states in de Sitter spacetime does not fall into
this UV allowed class, except for the special value of the parameter
corresponding to the Bunch-Davies state. We determine the finite contributions
to the inflationary power spectrum and stress tensor expectation value of
general UV allowed adiabatic states, and obtain quantitative limits on the
observability and backreaction effects of some recently proposed models of
short distance modifications of the initial state of inflation. For all UV
allowed states, the second order adiabatic basis provides a good description of
particles created in the expanding RW universe. Due to the absence of particle
creation for the massless, minimally coupled scalar field in de Sitter space,
there is no phase decoherence in the simplest free field inflationary models.
We apply adiabatic regularization to the renormalization of the decoherence
functional in cosmology to corroborate this result.Comment: 83 pages, 2 figures, minor changes in content and styl
Black Hole Evaporation in an Expanding Universe
We calculate the quantum radiation power of black holes which are asymptotic
to the Einstein-de Sitter universe at spatial and null infinities. We consider
two limiting mass accretion scenarios, no accretion and significant accretion.
We find that the radiation power strongly depends on not only the asymptotic
condition but also the mass accretion scenario. For the no accretion case, we
consider the Einstein-Straus solution, where a black hole of constant mass
resides in the dust Friedmann universe. We find negative cosmological
correction besides the expected redshift factor. This is given in terms of the
cubic root of ratio in size of the black hole to the cosmological horizon, so
that it is currently of order but could have been significant at the formation epoch of
primordial black holes. Due to the cosmological effects, this black hole has
not settled down to an equilibrium state. This cosmological correction may be
interpreted in an analogy with the radiation from a moving mirror in a flat
spacetime. For the significant accretion case, we consider the Sultana-Dyer
solution, where a black hole tends to increase its mass in proportion to the
cosmological scale factor. In this model, we find that the radiation power is
apparently the same as the Hawking radiation from the Schwarzschild black hole
of which mass is that of the growing mass at each moment. Hence, the energy
loss rate decreases and tends to vanish as time proceeds. Consequently, the
energy loss due to evaporation is insignificant compared to huge mass accretion
onto the black hole. Based on this model, we propose a definition of
quasi-equilibrium temperature for general conformal stationary black holes.Comment: Accepted for publication in Class.Quant.Grav., 18 pages and 3 figure
On Infrared Effects in de~Sitter Background
We have estimated higher order quantum gravity corrections to de~Sitter
spacetime. Our results suggest that, while the classical spacetime metric may
be distorted by the graviton self-interactions, the corrections are relatively
weaker than previously thought, possibly growing like a power rather than
exponentially in time.Comment: 17, UM-TH-94-11, (1 postscript fig. at end
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