20,710 research outputs found
Zero-variance zero-bias quantum Monte Carlo estimators of the spherically and system-averaged pair density
We construct improved quantum Monte Carlo estimators for the spherically- and
system-averaged electron pair density (i.e. the probability density of finding
two electrons separated by a relative distance u), also known as the
spherically-averaged electron position intracule density I(u), using the
general zero-variance zero-bias principle for observables, introduced by
Assaraf and Caffarel. The calculation of I(u) is made vastly more efficient by
replacing the average of the local delta-function operator by the average of a
smooth non-local operator that has several orders of magnitude smaller
variance. These new estimators also reduce the systematic error (or bias) of
the intracule density due to the approximate trial wave function. Used in
combination with the optimization of an increasing number of parameters in
trial Jastrow-Slater wave functions, they allow one to obtain well converged
correlated intracule densities for atoms and molecules. These ideas can be
applied to calculating any pair-correlation function in classical or quantum
Monte Carlo calculations.Comment: 13 pages, 9 figures, published versio
Pressure-energy correlations and thermodynamic scaling in viscous Lennard-Jones liquids
We use molecular dynamics simulation results on viscous binary Lennard-Jones
mixtures to examine the correlation between the potential energy and the
virial. In accord with a recent proposal [U. R. Pedersen et. al. Phys. Rev.
Lett. 100, 015701 (2008)], the fluctuations in the two quantities are found to
be strongly correlated, exhibiting a proportionality constant, Gamma,
numerically equal to one-third the slope of an inverse power law approximation
to the intermolecular potential function. The correlation is stronger at higher
densities, where interatomic separations are in the range where the inverse
power law approximation is more accurate. These same liquids conform to
thermodynamic scaling of their dynamics, with the scaling exponent equal to
Gamma. Thus, the properties of strong correlation between energy and pressure
and thermodynamic scaling both reflect the ability of an inverse power law
representation of the potential to capture interesting features of the dynamics
of dense, highly viscous liquids.Comment: 5 pages, 4 figures; published version, one figure remove
Jet measurements by the CMS experiment in pp and PbPb collisions
The energy loss of fast partons traversing the strongly interacting matter
produced in high-energy nuclear collisions is one of the most interesting
observables to probe the nature of the produced medium. The multipurpose
Compact Muon Solenoid (CMS) detector is well designed to measure these hard
scattering processes with its high resolution calorimeters and high precision
silicon tracker. Analyzing data from pp and PbPb collisions at a center-of-mass
energy of 2.76 TeV parton energy loss is observed as a significant imbalance of
dijet transverse momentum. To gain further understanding of the parton energy
loss mechanism the redistribution of the quenched jet energy was studied using
the transverse momentum balance of charged tracks projected onto the direction
of the leading jet. In contrast to pp collisions, a large fraction the momentum
balance for asymmetric jets is found to be carried by low momentum particles at
large angular distance to the jet axis. Further, the fragmentation functions
for leading and subleading jets were reconstructed and were found to be
unmodified compared to measurements in pp collisions. The results yield a
detailed picture of parton propagation in the hot QCD medium.Comment: 7 pages, 5 figures, Quark Matter 2011 conference proceeding
Pattern phase diagram for 2D arrays of coupled limit-cycle oscillators
Arrays of coupled limit-cycle oscillators represent a paradigmatic example
for studying synchronization and pattern formation. They are also of direct
relevance in the context of currently emerging experiments on nano- and
optomechanical oscillator arrays. We find that the full dynamical equations for
the phase dynamics of such an array go beyond previously studied Kuramoto-type
equations. We analyze the evolution of the phase field in a two-dimensional
array and obtain a "phase diagram" for the resulting stationary and
non-stationary patterns. The possible observation in optomechanical arrays is
discussed briefly
Lissajous curves and semiclassical theory: The two-dimensional harmonic oscillator
The semiclassical treatment of the two-dimensional harmonic oscillator
provides an instructive example of the relation between classical motion and
the quantum mechanical energy spectrum. We extend previous work on the
anisotropic oscillator with incommensurate frequencies and the isotropic
oscillator to the case with commensurate frequencies for which the Lissajous
curves appear as classical periodic orbits. Because of the three different
scenarios depending on the ratio of its frequencies, the two-dimensional
harmonic oscillator offers a unique way to explicitly analyze the role of
symmetries in classical and quantum mechanics.Comment: 9 pages, 3 figures; to appear in Am. J. Phy
Funding Universal Service Obligations with an Essential Facility: Charges vs. Taxes and Subsidies
This paper compares three schemes for funding Universal Service Obligations in network industries with an essential facility: an uplift to the network access charge, the establishment of a Universal Service (US) fund financed through a lump-sum tax and a US fund financed through a unit tax. The comparison is made under a duopoly structure with a potential entrant and an incumbent, which owns the essential facility and is responsible for universal service. The incumbent is also constrained to offer the same price on all markets. Using a social welfare criteria, we show that the US fund financed with a lump sum tax dominates the other two schemes, while the US fund with unit tax is equivalent to the access charge uplift.UNIVERSAL SERVICE OBLIGATIONS; ACCESS CHARGES; REGULATION
Freezing point depression and freeze-thaw damage by nano-fuidic salt trapping
A remarkable variety of organisms and wet materials are able to endure
temperatures far below the freezing point of bulk water. Cryo-tolerance in
biology is usually attributed to "anti-freeze" proteins, and yet massive
supercooling (C) is also possible in porous media containing only
simple aqueous electrolytes. For concrete pavements, the common wisdom is that
freeze-thaw damage results from the expansion of water upon freezing, but this
cannot explain the large pressures (~MPa) required to damage concrete,
the observed correlation between pavement damage and de-icing salts, or the
damage of cement paste loaded with benzene (which contracts upon freezing). In
this Letter, we propose a different mechanism -- nanofluidic salt trapping --
which can explain the observations, using simple mathematical models of
dissolved ions confined to thin liquid films between growing ice and charged
surfaces. Although trapped salt lowers the freezing point, ice nucleation in
charged pores causes enormous disjoining pressures via the rejected ions, until
their removal by precipitation or surface adsorption at a lower temperatures
releases the pressure and allows complete freezing. The theory is able to
predict the non-monotonic salt-concentration dependence of freeze-thaw damage
in concreter and provides a general framework to understand the origins of
cryo-tolerance.Comment: 5 figure
Design and fabrication of a centrifugally driven microfluidic disk for fully integrated metabolic assays on whole blood
For the first time, we present a novel and fully integrated centrifugal microfluidic â lab-on-a-diskâ for rapid metabolic assays in human whole blood. All essential steps comprising blood sampling, metering, plasma extraction and the final optical detection are conducted within t = 150 s in passive structures integrated on one disposable disk. Our technology features a novel plasma extraction structure (V = 500 nL, CV < 5%) without using any hydrophobic microfluidics where the purified plasma (cRBC< 0.11%) is centrifugally separated and subsequently extracted through a capillarily primed extraction channel into the detection chamber. While this capillary extraction requires precisely defined, narrow micro-structures, the reactive mixing and detection is most efficient within larger cavities. The corresponding manufacturing technique of these macro- and micro structures in the range of 30 ” m to 1000 ” m is also presented for the first time: A novel, cost-efficient hybrid prototyping technique of a multiscale epoxy master for subsequent hot embossing of polymer disks
Are polar liquids less simple?
Strong correlation between equilibrium fluctuations of the potential energy,
U, and the virial, W, is a characteristic of a liquid that implies the presence
of certain dynamic properties, such as density scaling of the relaxation times
and isochronal superpositioning of the relaxation function. In this work we
employ molecular dynamics simulations (mds) on methanol and two variations,
lacking hydrogen bonds and a dipole moment, to assess the connection between
the correlation of U and W and these dynamic properties. We show, in accord
with prior results of others [T.S. Ingebrigtsen, T.B. Schroder, J.C. Dyre,
Phys. Rev. X 2, 011011 (2012).], that simple van der Waals liquids exhibit both
strong correlations and the expected dynamic behavior. However, for polar
liquids this correspondence breaks down - weaker correlation between U and W is
not associated with worse conformance to density scaling or isochronal
superpositioning. The reason for this is that strong correlation between U and
W only requires their proportionality, whereas the expected dynamic behavior
depends primarily on constancy of the proportionality constant for all state
points. For hydrogen-bonded liquids, neither strong correlation nor adherence
to the dynamic properties is observed; however, this nonconformance is not
directly related to the concentration of hydrogen bonds, but rather to the
greater deviation of the intermolecular potential from an inverse power law
(IPL). Only (hypothetical) liquids having interactions governed strictly by an
IPL are perfectly correlating and exhibit the consequent dynamic properties
over all thermodynamic conditions.Comment: 14 pages, 8 figure
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