1,116 research outputs found
Nonstatistical dynamics on potentials exhibiting reaction path bifurcations and valley-ridge inflection points
We study reaction dynamics on a model potential energy surface exhibiting
post-transition state bifurcation in the vicinity of a valley ridge inflection
point. We compute fractional yields of products reached after the VRI region is
traversed, both with and without dissipation. It is found that apparently minor
variations in the potential lead to significant changes in the reaction
dynamics. Moreover, when dissipative effects are incorporated, the product
ratio depends in a complicated and highly non-monotonic fashion on the
dissipation parameter. Dynamics in the vicinity of the VRI point itself play
essentially no role in determining the product ratio, except in the highly
dissipative regime.Comment: 33 pages, 10 figures, corrected the author name in reference [6
Isomerization dynamics of a buckled nanobeam
We analyze the dynamics of a model of a nanobeam under compression. The model
is a two mode truncation of the Euler-Bernoulli beam equation subject to
compressive stress. We consider parameter regimes where the first mode is
unstable and the second mode can be either stable or unstable, and the
remaining modes (neglected) are always stable. Material parameters used
correspond to silicon. The two mode model Hamiltonian is the sum of a
(diagonal) kinetic energy term and a potential energy term. The form of the
potential energy function suggests an analogy with isomerisation reactions in
chemistry. We therefore study the dynamics of the buckled beam using the
conceptual framework established for the theory of isomerisation reactions.
When the second mode is stable the potential energy surface has an index one
saddle and when the second mode is unstable the potential energy surface has an
index two saddle and two index one saddles. Symmetry of the system allows us to
construct a phase space dividing surface between the two "isomers" (buckled
states). The energy range is sufficiently wide that we can treat the effects of
the index one and index two saddles in a unified fashion. We have computed
reactive fluxes, mean gap times and reactant phase space volumes for three
stress values at several different energies. In all cases the phase space
volume swept out by isomerizing trajectories is considerably less than the
reactant density of states, proving that the dynamics is highly nonergodic. The
associated gap time distributions consist of one or more `pulses' of
trajectories. Computation of the reactive flux correlation function shows no
sign of a plateau region; rather, the flux exhibits oscillatory decay,
indicating that, for the 2-mode model in the physical regime considered, a rate
constant for isomerization does not exist.Comment: 42 pages, 6 figure
Book Reviews
It was peculiarly fortunate for the cause of the American Revolution that the sympathies of the French people and the policies of the French foreign office which knew no diplomatic methods save those of secret diplomacy, were for once heartily in accord in support of the American revolutionists. Professor Corwin in this book deals entirely with the complicated and obscure political plots and counter-plots which eventually led France to espouse openly the cause of the revolting colonies. The whole question of the timely aid France gave to America has, of course, a very particular value at the present time when this country is preparing to repay the debt we have owed France for so long. That the.author does not deal with the social conditions and the sympathic temper of the French people, which in a real sense underlay the action of France, but confines himself wholly to hante politique does not in any sense take away from the value of his work. He reveals a new side to this very interesting phase in our contest for independence, at least one that will be new to those who are not deeply read in American history. The tendency heretofore has been to emphasize the popular and moral support of the French people and to overlook the political reasons which led to the action of the king and his ministers. The author has throughout relied upon the monumental researches of Doniol and has carefully fortified every point by copious references and quotations
Thermocapillary actuation of liquid flow on chemically patterned surfaces
We have investigated the thermocapillary flow of a Newtonian liquid on hydrophilic microstripes which are lithographically defined on a hydrophobic surface. The speed of the microstreams is studied as a function of the stripe width w, the applied thermal gradient |dT/dx| and the liquid volume V deposited on a connecting reservoir pad. Numerical solutions of the flow speed as a function of downstream position show excellent agreement with experiment. The only adjustable parameter is the inlet film height, which is controlled by the ratio of the reservoir pressure to the shear stress applied to the liquid stream. In the limiting cases where this ratio is either much smaller or much larger than unity, the rivulet speed shows a power law dependency on w, |dT/dx| and V. In this study we demonstrate that thermocapillary driven flow on chemically patterned surfaces can provide an elegant and tunable method for the transport of ultrasmall liquid volumes in emerging microfluidic technologies
Towards an effective potential for the monomer, dimer, hexamer, solid and liquid forms of hydrogen fluoride
We present an attempt to build up a new two-body effective potential for
hydrogen fluoride, fitted to theoretical and experimental data relevant not
only to the gas and liquid phases, but also to the crystal. The model is simple
enough to be used in Molecular Dynamics and Monte Carlo simulations. The
potential consists of: a) an intra-molecular contribution, allowing for
variations of the molecular length, plus b) an inter-molecular part, with three
charged sites on each monomer and a Buckingham "exp-6" interaction between
fluorines. The model is able to reproduce a significant number of observables
on the monomer, dimer, hexamer, solid and liquid forms of HF. The shortcomings
of the model are pointed out and possible improvements are finally discussed.Comment: LaTeX, 24 pages, 2 figures. For related papers see also
http://www.chim.unifi.it:8080/~valle
Reionization: Characteristic Scales, Topology and Observability
Recently the numerical simulations of the process of reionization of the
universe at z>6 have made a qualitative leap forward, reaching sufficient sizes
and dynamic range to determine the characteristic scales of this process. This
allowed making the first realistic predictions for a variety of observational
signatures. We discuss recent results from large-scale radiative transfer and
structure formation simulations on the observability of high-redshift Ly-alpha
sources. We also briefly discuss the dependence of the characteristic scales
and topology of the ionized and neutral patches on the reionization parameters.Comment: 4 pages, 5 figures (4 in color), to appear in Astronomy and Space
Science special issue "Space Astronomy: The UV window to the Universe",
proceedings of 1st NUVA Conference ``Space Astronomy: The UV window to the
Universe'' in El Escorial (Spain
Experimental determination of the complete spin structure for anti-proton + proton -> anti-\Lambda + \Lambda at anti-proton beam momentum of 1.637 GeV/c
The reaction anti-proton + proton -> anti-\Lambda + \Lambda -> anti-proton +
\pi^+ + proton + \pi^- has been measured with high statistics at anti-proton
beam momentum of 1.637 GeV/c. The use of a transversely-polarized frozen-spin
target combined with the self-analyzing property of \Lambda/anti-\Lambda decay
allows access to unprecedented information on the spin structure of the
interaction. The most general spin-scattering matrix can be written in terms of
eleven real parameters for each bin of scattering angle, each of these
parameters is determined with reasonable precision. From these results all
conceivable spin-correlations are determined with inherent self-consistency.
Good agreement is found with the few previously existing measurements of spin
observables in anti-proton + proton -> anti-\Lambda + \Lambda near this energy.
Existing theoretical models do not give good predictions for those
spin-observables that had not been previously measured.Comment: To be published in Phys. Rev. C. Tables of results (i.e. Ref. 24) are
available at http://www-meg.phys.cmu.edu/~bquinn/ps185_pub/results.tab 24
pages, 16 figure
Low-threshold analysis of CDMS shallow-site data
Data taken during the final shallow-site run of the first tower of the
Cryogenic Dark Matter Search (CDMS II) detectors have been reanalyzed with
improved sensitivity to small energy depositions. Four ~224 g germanium and two
~105 g silicon detectors were operated at the Stanford Underground Facility
(SUF) between December 2001 and June 2002, yielding 118 live days of raw
exposure. Three of the germanium and both silicon detectors were analyzed with
a new low-threshold technique, making it possible to lower the germanium and
silicon analysis thresholds down to the actual trigger thresholds of ~1 keV and
~2 keV, respectively. Limits on the spin-independent cross section for weakly
interacting massive particles (WIMPs) to elastically scatter from nuclei based
on these data exclude interesting parameter space for WIMPs with masses below 9
GeV/c^2. Under standard halo assumptions, these data partially exclude
parameter space favored by interpretations of the DAMA/LIBRA and CoGeNT
experiments' data as WIMP signals, and exclude new parameter space for WIMP
masses between 3 GeV/c^2 and 4 GeV/c^2.Comment: 18 pages, 12 figures, 5 table
Recommended from our members
Comparison of 5-year progression of retinitis pigmentosa involving the posterior pole among siblings by means of SD-OCT: a retrospective study
The blockchain technology promises to transform finance, money and evengovernments. However, analyses of blockchain applicability and robustness typicallyfocus on isolated systems whose actors contribute mainly by running the consensusalgorithm. Here, we highlight the importance of considering trustless platformswithin the broader ecosystem that includes social and communication networks. Asan example, we analyse the flash-crash observed on 21st June 2017 in the Ethereumplatform and show that a major phenomenon of social coordination led to acatastrophic cascade of events across several interconnected systems. We proposethe concept of “emergent centralisation” to describe situations where a single systembecomes critically important for the functioning of the whole ecosystem, and arguethat such situations are likely to become more and more frequent in interconnectedsocio-technical systems. We anticipate that the systemic approach we propose willhave implications for future assessments of trustless systems and call for the attentionof policy-makers on the fragility of our interconnected and rapidly changing world
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