11,140 research outputs found
Isomorphs in model molecular liquids
Isomorphs are curves in the phase diagram along which a number of static and
dynamic quantities are invariant in reduced units. A liquid has good isomorphs
if and only if it is strongly correlating, i.e., the equilibrium
virial/potential energy fluctuations are more than 90% correlated in the NVT
ensemble. This paper generalizes isomorphs to liquids composed of rigid
molecules and study the isomorphs of two systems of small rigid molecules, the
asymmetric dumbbell model and the Lewis-Wahnstrom OTP model. In particular, for
both systems we find that the isochoric heat capacity, the excess entropy, the
reduced molecular center-of-mass self part of the intermediate scattering
function, the reduced molecular center-of-mass radial distribution function to
a good approximation are invariant along an isomorph. In agreement with theory,
we also find that an instantaneous change of temperature and density from an
equilibrated state point to another isomorphic state point leads to no
relaxation. The isomorphs of the Lewis-Wahnstrom OTP model were found to be
more approximative than those of the asymmetric dumbbell model, which is
consistent with the OTP model being less strongly correlating. For both models
we find "master isomorphs", i.e., isomorphs have identical shape in the
virial/potential energy phase diagram.Comment: 20 page
Spin-orbit interaction and the 'metal-insulator' transition observed in two-dimensional hole systems
We present calculations of the spin and phase relaxation rates in GaAs/AlGaAs
-type quantum wells. These rates are used to derive the temperature
dependence of the weak-localization correction to the conductivity. In -type
quantum wells both weak localization and weak anti-localization are present due
to the strong spin-orbit interaction. When determining the total conductivity
correction one also have to include the term due to hole-hole interaction. The
magnitude of the latter depends on the ratio between the thermal energy and the
Fermi energy, and whether the system can be considered
as ballistic or diffusive (). We argue that due to the relatively low Fermi energy
and the moderate mobilities, in the -type systems in question, the
conductivity correction arising from hole-hole interactions is negligible at
the highest temperatures accessible in the experiments. Hence the
'metal-insulator' transition observed at these relatively high temperatures
could be caused by interference effects. We compare our calculations of the
weak anti-localization correction with the experimental results from different
independent groups with special emphasis on the experiments by Simmons et al.
We find good agreement between predicted and observed transistion density
.Comment: 6 pages, 4 figures. Accepted to PRB (15 June, 2002
Clar Sextet Analysis of Triangular, Rectangular and Honeycomb Graphene Antidot Lattices
Pristine graphene is a semimetal and thus does not have a band gap. By making
a nanometer scale periodic array of holes in the graphene sheet a band gap may
form; the size of the gap is controllable by adjusting the parameters of the
lattice. The hole diameter, hole geometry, lattice geometry and the separation
of the holes are parameters that all play an important role in determining the
size of the band gap, which, for technological applications, should be at least
of the order of tenths of an eV. We investigate four different hole
configurations: the rectangular, the triangular, the rotated triangular and the
honeycomb lattice. It is found that the lattice geometry plays a crucial role
for size of the band gap: the triangular arrangement displays always a sizable
gap, while for the other types only particular hole separations lead to a large
gap. This observation is explained using Clar sextet theory, and we find that a
sufficient condition for a large gap is that the number of sextets exceeds one
third of the total number of hexagons in the unit cell. Furthermore, we
investigate non-isosceles triangular structures to probe the sensitivity of the
gap in triangular lattices to small changes in geometry
Feasibility of single-order parameter description of equilibrium viscous liquid dynamics
Molecular dynamics results for the dynamic Prigogine-Defay ratio are
presented for two glass-forming liquids, thus evaluating the experimentally
relevant quantity for testing whether metastable-equilibrium liquid dynamics to
a good approximation are described by a single parameter. For the Kob-Andersen
binary Lennard-Jones mixture as well as for an asymmetric dumbbell model liquid
a single-parameter description works quite well. This is confirmed by
time-domain results where it is found that energy and pressure fluctuations are
strongly correlated on the alpha-time scale in the NVT ensemble; in the NpT
ensemble energy and volume fluctuations similarly correlate strongly.Comment: Phys. Rev. E, in pres
Volume-energy correlations in the slow degrees of freedom of computer-simulated phospholipid membranes
Constant-pressure molecular-dynamics simulations of phospholipid membranes in
the fluid phase reveal strong correlations between equilibrium fluctuations of
volume and energy on the nanosecond time-scale. The existence of strong
volume-energy correlations was previously deduced indirectly by Heimburg from
experiments focusing on the phase transition between the fluid and the ordered
gel phases. The correlations, which are reported here for three different
membranes (DMPC, DMPS-Na, and DMPSH), have volume-energy correlation
coefficients ranging from 0.81 to 0.89. The DMPC membrane was studied at two
temperatures showing that the correlation coefficient increases as the phase
transition is approached
Phase transitions in two dimensions - the case of Sn adsorbed on Ge(111) surfaces
Accurate atomic coordinates of the room-temperature (root3xroot3)R30degree
and low-temperature (3x3) phases of 1/3 ML Sn on Ge(111) have been established
by grazing-incidence x-ray diffraction with synchrotron radiation. The Sn atoms
are located solely at T4-sites in the (root3xroot3)R30degree structure. In the
low temperature phase one of the three Sn atoms per (3x3) unit cell is
displaced outwards by 0.26 +/- 0.04 A relative to the other two. This
displacement is accompanied by an increase in the first to second double-layer
spacing in the Ge substrate.Comment: RevTeX, 5 pages including 2 figure
Massive and massless monopoles with nonabelian magnetic charges
We use the multimonopole moduli space as a tool for studying the properties
of BPS monopoles carrying nonabelian magnetic charges. For configurations whose
total magnetic charge is purely abelian, the moduli space for nonabelian
breaking can be obtained as a smooth limit of that for a purely abelian
breaking. As the asymptotic Higgs field is varied toward one of the special
values for which the unbroken symmetry is enlarged to a nonabelian group, some
of the fundamental monopoles of unit topological charge remain massive but
acquire nonabelian magnetic charges. The BPS mass formula indicates that others
should become massless in this limit. We find that these do not correspond to
distinct solitons but instead manifest themselves as ``nonabelian clouds''
surrounding the massive monopoles. The moduli space coordinates describing the
position and phase of these massless monopoles are transformed into an
equal number of nonabelian global gauge orientation and gauge-invariant
structure parameters characterizing the nonabelian cloud. We illustrate this
explicitly in a class of examples for which the full family of
monopole solutions is known. We show in detail how the unbroken symmetries of
the theory are manifested as isometries of the moduli space metric. We discuss
the connection of these results to the Montonen-Olive duality conjecture,
arguing in particular that the massless monopoles should be understood as the
duals to the massless gauge bosons that appear as the mediators of the
nonabelian forces in the perturbative sector.Comment: 45pp, LaTeX, two .eps figures include
Health effects and wind turbines: A review of the literature
<p>Abstract</p> <p>Background</p> <p>Wind power has been harnessed as a source of power around the world. Debate is ongoing with respect to the relationship between reported health effects and wind turbines, specifically in terms of audible and inaudible noise. As a result, minimum setback distances have been established world-wide to reduce or avoid potential complaints from, or potential effects to, people living in proximity to wind turbines. People interested in this debate turn to two sources of information to make informed decisions: scientific peer-reviewed studies published in scientific journals and the popular literature and internet.</p> <p>Methods</p> <p>The purpose of this paper is to review the peer-reviewed scientific literature, government agency reports, and the most prominent information found in the popular literature. Combinations of key words were entered into the Thomson Reuters Web of Knowledge<sup>SM </sup>and the internet search engine Google. The review was conducted in the spirit of the evaluation process outlined in the Cochrane Handbook for Systematic Reviews of Interventions.</p> <p>Results</p> <p>Conclusions of the peer reviewed literature differ in some ways from those in the popular literature. In peer reviewed studies, wind turbine annoyance has been statistically associated with wind turbine noise, but found to be more strongly related to visual impact, attitude to wind turbines and sensitivity to noise. To date, no peer reviewed articles demonstrate a direct causal link between people living in proximity to modern wind turbines, the noise they emit and resulting physiological health effects. If anything, reported health effects are likely attributed to a number of environmental stressors that result in an annoyed/stressed state in a segment of the population. In the popular literature, self-reported health outcomes are related to distance from turbines and the claim is made that infrasound is the causative factor for the reported effects, even though sound pressure levels are not measured.</p> <p>Conclusions</p> <p>What both types of studies have in common is the conclusion that wind turbines can be a source of annoyance for some people. The difference between both types is the reason for annoyance. While it is acknowledged that noise from wind turbines can be annoying to some and associated with some reported health effects (e.g., sleep disturbance), especially when found at sound pressure levels greater than 40 db(A), given that annoyance appears to be more strongly related to visual cues and attitude than to noise itself, self reported health effects of people living near wind turbines are more likely attributed to physical manifestation from an annoyed state than from wind turbines themselves. In other words, it appears that it is the change in the environment that is associated with reported health effects and not a turbine-specific variable like audible noise or infrasound. Regardless of its cause, a certain level of annoyance in a population can be expected (as with any number of projects that change the local environment) and the acceptable level is a policy decision to be made by elected officials and their government representatives where the benefits of wind power are weighted against their cons. Assessing the effects of wind turbines on human health is an emerging field and conducting further research into the effects of wind turbines (and environmental changes) on human health, emotional and physical, is warranted.</p
Macroscopic Quantum Tunneling of a Fluxon in a Long Josephson Junction
Macroscopic quantum tunneling (MQT) for a single fluxon moving along a long
Josephson junction is studied theoretically. To introduce a fluxon-pinning
force, we consider inhomogeneities made by modifying thickness of an insulating
layer locally. Two different situations are studied: one is the quantum
tunneling from a metastable state caused by a single inhomogeneity, and the
other is the quantum tunneling in a two-state system made by two
inhomogeneities. In the quantum tunneling from a metastable state, the decay
rate is estimated within the WKB approximation. Dissipation effects on a fluxon
dynamics are taken into account by the Caldeira-Leggett theory. We propose a
device to observe quantum tunneling of a fluxon experimentally. Required
experimental resolutions to observe MQT of a fluxon seem attainable within the
presently available micro-fabrication technique. For the two-state system, we
study quantum resonance between two stable states, i.e., macroscopic quantum
coherence (MQC). From the estimate for dissipation coefficients due to
quasiparticle tunneling, the observation of MQC appears to be possible within
the Caldeira-Leggett theory.Comment: 30 pages LaTeX including 11 PS figures, using jpsj.sty. To be
published on J. Phys. Soc. Jpn. Overestimates for damping amplitude is
correcte
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