688 research outputs found
Plasma deposited diamondlike carbon on GaAs and InP
The properties of diamond like carbon films grown by RF flow discharge 30 kHz plasma using methane are reported. The Cls XPS line shape of films showed localized hybrid carbon bonds as low as 40 to as high as 95 percent. Infrared spectroscopy and N(15) nuclear reaction profiling data indicated 35 to 42 percent hydrogen, depending inversely on deposition temperature. The deposition rate of films on Si falls off exponentially with substrate temperature, and nucleation does not occur above 200 C on GaAs and InP. Optical data of the films showed bandgap values of 2.0 to 2.4 eV increasing monotonically with CH4 flow rate
Optical properties of hydrogenated amorphous carbon films grown from methane plasma
A 30 kHz ac glow discharge formed from methane gas was used to grow carbon films on InP substrates. Both the growth rate, and the realitive Ar ion sputtering rate at 3 keV varied monotonically with deposition power. Results from the N-15 nuclear reaction profile experiments indicated a slight drop in the hydrogen concentration as more energy was dissipated in the ac discharge. Values for the index of refraction and extinction coefficient ranged from 1.721 to 1.910 and 0 to -0.188, respectively. Optical bandgaps as high as 2.34 eV were determined
The Boltzmann Entropy for Dense Fluids Not in Local Equilibrium
We investigate, via computer simulations, the time evolution of the
(Boltzmann) entropy of a dense fluid not in local equilibrium. The
macrovariables describing the system are the (empirical) particle density
f=\{f(\un{x},\un{v})\} and the total energy . We find that is
monotone increasing in time even when its kinetic part is decreasing. We argue
that for isolated Hamiltonian systems monotonicity of
should hold generally for ``typical'' (the overwhelming majority of) initial
microstates (phase-points) belonging to the initial macrostate ,
satisfying . This is a direct consequence of Liouville's theorem
when evolves autonomously.Comment: 8 pages, 5 figures. Submitted to PR
Sampling rare fluctuations of height in the Oslo ricepile model
We have studied large deviations of the height of the pile from its mean
value in the Oslo ricepile model. We sampled these very rare events with
probabilities of order by Monte Carlo simulations using importance
sampling. These simulations check our qualitative arguement [Phys. Rev. E, {\bf
73}, 021303, 2006] that in steady state of the Oslo ricepile model, the
probability of large negative height fluctuations about
the mean varies as as with
held fixed, and .Comment: 7 pages, 8 figure
A dynamical classification of the range of pair interactions
We formalize a classification of pair interactions based on the convergence
properties of the {\it forces} acting on particles as a function of system
size. We do so by considering the behavior of the probability distribution
function (PDF) P(F) of the force field F in a particle distribution in the
limit that the size of the system is taken to infinity at constant particle
density, i.e., in the "usual" thermodynamic limit. For a pair interaction
potential V(r) with V(r) \rightarrow \infty) \sim 1/r^a defining a {\it
bounded} pair force, we show that P(F) converges continuously to a well-defined
and rapidly decreasing PDF if and only if the {\it pair force} is absolutely
integrable, i.e., for a > d-1, where d is the spatial dimension. We refer to
this case as {\it dynamically short-range}, because the dominant contribution
to the force on a typical particle in this limit arises from particles in a
finite neighborhood around it. For the {\it dynamically long-range} case, i.e.,
a \leq d-1, on the other hand, the dominant contribution to the force comes
from the mean field due to the bulk, which becomes undefined in this limit. We
discuss also how, for a \leq d-1 (and notably, for the case of gravity, a=d-2)
P(F) may, in some cases, be defined in a weaker sense. This involves a
regularization of the force summation which is generalization of the procedure
employed to define gravitational forces in an infinite static homogeneous
universe. We explain that the relevant classification in this context is,
however, that which divides pair forces with a > d-2 (or a < d-2), for which
the PDF of the {\it difference in forces} is defined (or not defined) in the
infinite system limit, without any regularization. In the former case dynamics
can, as for the (marginal) case of gravity, be defined consistently in an
infinite uniform system.Comment: 12 pages, 1 figure; significantly shortened and focussed, additional
references, version to appear in J. Stat. Phy
Rigorous Analysis of Singularities and Absence of Analytic Continuation at First Order Phase Transition Points in Lattice Spin Models
We report about two new rigorous results on the non-analytic properties of
thermodynamic potentials at first order phase transition. The first one is
valid for lattice models () with arbitrary finite state space, and
finite-range interactions which have two ground states. Under the only
assumption that the Peierls Condition is satisfied for the ground states and
that the temperature is sufficiently low, we prove that the pressure has no
analytic continuation at the first order phase transition point. The second
result concerns Ising spins with Kac potentials
, where is a small scaling
parameter, and a fixed finite range potential. In this framework, we
relate the non-analytic behaviour of the pressure at the transition point to
the range of interaction, which equals . Our analysis exhibits a
crossover between the non-analytic behaviour of finite range models
() and analyticity in the mean field limit (). In
general, the basic mechanism responsible for the appearance of a singularity
blocking the analytic continuation is that arbitrarily large droplets of the
other phase become stable at the transition point.Comment: 4 pages, 2 figure
Klamath Tribal response to the pandemic of COVID-19 among Klamath Tribal Community in Oregon, USA
Introduction Socially-disadvantaged populations are more at risk of contracting COVID-19 than those with access to better medical facilities. We looked at responses of Klamath Tribes in Oregon, USA to mitigate spread of COVID-19 in a community with a higher incidence of obesity, diabetes and coronary heart disease, compared to the general US population. This study reports on Klamath Tribes response to COVID-19 March -September 2020. Methods Klamath Tribes Tribal Health and Family Services established a COVID-19 Incident Management Team (IMT), instituting creative programs including a Walk-In Testing Center, implementing strict infection control protocols and regular sharing of information on the pandemic and prevalence of COVID-19 amongst Klamath Tribes. All COVID-19 tests were documented with positive cases isolated and people with high risk exposures quarantined and provided with wrap-around medical and social services until recovered or past quarantine time period. Results A total of 888 (12%) tribal members were tested for COVID1-19 between March to September 2020; 50 were found positive for COVID-19, giving a test positivity rate of 5.6% (Male – 6.3%; Female – 5.2%). No deaths have been reported amongst the local Klamath Tribes and other American Indians/Alaska Native (AI/AN) population served by the tribe. Conclusion Despite the fact that structural inequities including income disparities have shaped racial and ethnic impact of epidemics around the world, the timely response, establishment of partnerships and proactive control of the epidemic resulted in minimal impact among the Klamath Tribal and other AI/AN populations served by the tribal facilities
Chaotic properties of quantum many-body systems in the thermodynamic limit
By using numerical simulations, we investigate the dynamics of a quantum
system of interacting bosons. We find an increase of properly defined mixing
properties when the number of particles increases at constant density or the
interaction strength drives the system away from integrability. A
correspondence with the dynamical chaoticity of an associated -number system
is then used to infer properties of the quantum system in the thermodynamic
limit.Comment: 4 pages RevTeX, 4 postscript figures included with psfig; Completely
restructured version with new results on mixing properties added
Theory of Circle Maps and the Problem of One-Dimensional Optical Resonator with a Periodically Moving Wall
We consider the electromagnetic field in a cavity with a periodically
oscillating perfectly reflecting boundary and show that the mathematical theory
of circle maps leads to several physical predictions. Notably, well-known
results in the theory of circle maps (which we review briefly) imply that there
are intervals of parameters where the waves in the cavity get concentrated in
wave packets whose energy grows exponentially. Even if these intervals are
dense for typical motions of the reflecting boundary, in the complement there
is a positive measure set of parameters where the energy remains bounded.Comment: 34 pages LaTeX (revtex) with eps figures, PACS: 02.30.Jr, 42.15.-i,
42.60.Da, 42.65.Y
Phase transitions and configuration space topology
Equilibrium phase transitions may be defined as nonanalytic points of
thermodynamic functions, e.g., of the canonical free energy. Given a certain
physical system, it is of interest to understand which properties of the system
account for the presence of a phase transition, and an understanding of these
properties may lead to a deeper understanding of the physical phenomenon. One
possible approach of this issue, reviewed and discussed in the present paper,
is the study of topology changes in configuration space which, remarkably, are
found to be related to equilibrium phase transitions in classical statistical
mechanical systems. For the study of configuration space topology, one
considers the subsets M_v, consisting of all points from configuration space
with a potential energy per particle equal to or less than a given v. For
finite systems, topology changes of M_v are intimately related to nonanalytic
points of the microcanonical entropy (which, as a surprise to many, do exist).
In the thermodynamic limit, a more complex relation between nonanalytic points
of thermodynamic functions (i.e., phase transitions) and topology changes is
observed. For some class of short-range systems, a topology change of the M_v
at v=v_t was proved to be necessary for a phase transition to take place at a
potential energy v_t. In contrast, phase transitions in systems with long-range
interactions or in systems with non-confining potentials need not be
accompanied by such a topology change. Instead, for such systems the
nonanalytic point in a thermodynamic function is found to have some
maximization procedure at its origin. These results may foster insight into the
mechanisms which lead to the occurrence of a phase transition, and thus may
help to explore the origin of this physical phenomenon.Comment: 22 pages, 6 figure
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