50,033 research outputs found
Broken Time Translation Symmetry as a model for Quantum State Reduction
The symmetries that govern the laws of nature can be spontaneously broken,
enabling the occurrence of ordered states. Crystals arise from the breaking of
translation symmetry, magnets from broken spin rotation symmetry and massive
particles break a phase rotation symmetry. Time translation symmetry can be
spontaneously broken in exactly the same way. The order associated with this
form of spontaneous symmetry breaking is characterised by the emergence of
quantum state reduction: systems which spontaneously break time translation
symmetry act as ideal measurement machines. In this review the breaking of time
translation symmetry is first compared to that of other symmetries such as
spatial translations and rotations. It is then discussed how broken time
translation symmetry gives rise to the process of quantum state reduction and
how it generates a pointer basis, Born's rule, etc. After a comparison between
this model and alternative approaches to the problem of quantum state
reduction, the experimental implications and possible tests of broken time
translation symmetry in realistic experimental settings are discussed.Comment: 15 pages, 5 figure
Anaerobic degradation of dimethylsulfoniopropionate to 3-S-methylmercaptopropionate by a marine Desulfobacterium strain
Dimethylsulfoniopropionate, an osmolyte of marine algae, is thought to be the major precursor of dimethyl sulfide, which plays a dominant role in biogenic sulfur emission. The marine sulfate-reducing bacterium Desulfobacterium strain PM4 was found to degrade dimethylsulfoniopropionate to 3-S-methylmercaptopropionate. The oxidation of one of the methyl groups of dimethylsulfoniopropionate was coupled to the reduction of sulfate; this process is similar to the degradation of betaine to dimethylglycine which was described earlier for the same strain. Desulfobacterium PM4 is the first example of an anaerobic marine bacterium that is able to demethylate dimethylsulfoniopropionate.
Bernstein-Szego Polynomials Associated with Root Systems
We introduce multivariate generalizations of the Bernstein-Szego polynomials,
which are associated to the root systems of the complex simple Lie algebras.
The multivariate polynomials in question generalize Macdonald's Hall-Littlewood
polynomials associated with root systems. For the root system of type A1
(corresponding to the Lie algebra SL (2;C)) the classic Bernstein-Szego
polynomials are recovered.Comment: LaTeX, 12 page
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
Short-time fluctuations of displacements and work
A recent theorem giving the initial behavior of very short-time fluctuations
of particle displacements in classical many-body systems is discussed. It has
applications to equilibrium and non-equilibrium systems, one of which is a
series expansion of the distribution of work fluctuations around a Gaussian
function. To determine the time-scale at which this series expansion is valid,
we present preliminary numerical results for a Lennard-Jones fluid. These
results suggest that the series expansion converges up to time scales on the
order of a picosecond, below which a simple Gaussian function for the
distribution of the displacements can be used.Comment: 10 pages, 3 figure
Method for locating low-energy solutions within DFT+U
The widely employed DFT+U formalism is known to give rise to many self-consistent yet energetically distinct solutions in correlated systems, which can be highly problematic for reliably predicting the thermodynamic and physical properties of such materials. Here we study this phenomenon in the bulk materials UO_2, CoO, and NiO, and in a CeO_2 surface. We show that the following factors affect which self-consistent solution a DFT+U calculation reaches: (i) the magnitude of U; (ii) initial correlated orbital occupations; (iii) lattice geometry; (iv) whether lattice symmetry is enforced on the charge density; and (v) even electronic mixing parameters. These various solutions may differ in total energy by hundreds of meV per atom, so identifying or approximating the ground state is critical in the DFT+U scheme. We propose an efficient U-ramping method for locating low-energy solutions, which we validate in a range of test cases. We also suggest that this method may be applicable to hybrid functional calculations
To swim or not to swim: an interpretation of farmed mink's motivation for a water bath
How an animal’s behavioural (ethological) needs can be met is a pivotal issue in the assessment of welfare for captive animals. The value of swimming water for farmed mink is an example how scientific and societal questions relating to animal welfare can be answered. A number of studies have addressed the issue of the indispensability of swimming water for mink; however, so far with inconclusive evidence. In this paper, the results of these studies and related literature are reviewed.
First, the biological definition of need is discussed. Subsequently, attention is paid to the effects of the presence, absence and the removal of swimming water on behavioural and physiological correlates of well-being including stereotypic and anticipatory behaviour and urinary cortisol. Thereafter we discuss individual differences in the use of swimming water, the price animals pay for access to a water bath, and the effect of access to swimming water on juvenile play.
The main conclusions of the literature review are that 1) the use of a water bath for mink is most likely related to foraging behaviour (foraging areas: land and water); 2) absence of swimming water, without prior experience, does not lead to consistent changes in level of stereotypic behaviour, or anticipatory responses; 3) removal of a previously experienced water bath may induce short-term stress as indicated by behavioural parameters and elevated cortisol responses; 4) mink work hard for access to a swimming bath and running wheel in consumer demand studies. Other cage modifications such as tunnels and biting objects, may also provide environmental enrichment, if they are added to otherwise impoverished conditions; 5) There are individual differences in the use of swimming water: these are related in part to variation in prior experience of aquatic resources.; 6) As prior experience is important both with respect to individual use of swimming water and the response to deprivation, swimming water can not be described as biological need in the sense of a fixed requirement for survival. As swimming water appears to act as an incentive that induces its own motivation a more accurate term may be an “incentive induced or environmentally facilitated need”. Given the available evidence, it is not possible to conclude whether mink that have never experienced swimming water, suffer as a consequence of its absence. However, it is possible to predict that mink with access to water have improved quality of life, due to increased behavioural opportunities, in comparison to farmed mink without access to swimming water. In practical terms, it is still open to debate whether mink should be provided with swimming water, or if alternative, less valued, but easier to install and maintain forms of environmental enrichment, should be provided in mink housing.
To clarify these issues a number of future studies would be valuable. These include; 1) whether specific environmental cues affect motivation to swim, such as the form of drinking water delivery systems ; 2) whether prior experience of swimming water affects its incentive value; in other words “can you miss what you never experienced?”; 3) do behavioural parameters such as stereotypic behaviour; rebound effects and vacuum activity have any general utility in assessing the value of absent resources; 4) what are preferences for and the value of alternative resources which may act as substitutes for swimming water. In addition we would recommend further work investigating: relationship between access to swimming water and positive indicators of welfare such as play and/or anticipatory behaviour; the effects of preventing the performance of rewarding behaviours and deprivation of a previous experienced resource; and health and hygeine issues related to provision of a water bath. In future work, it would be desirable to present be the actual percentages of animals using a water bath during the experiment and the use of power analyses, to aid their interpretation
A source of high-velocity white dwarfs
We investigate whether the recently-observed population of high-velocity
white dwarfs can be derived from a population of binaries residing initially
within the thin disk of the Galaxy. In particular we consider binaries where
the primary is sufficiently massive to explode as a type II supernova. A large
fraction of such binaries are broken up when the primary then explodes as a
supernova owing to the combined effects of the mass loss from the primary and
the kick received by the neutron star on its formation. For binaries where the
primary evolves to fill its Roche lobe, mass transfer from the primary leads to
the onset of a common envelope phase during which the secondary and the core of
the primary spiral together as the envelope is ejected. Such binaries are the
progenitors of X-ray binaries if they are not broken up when the primary
explodes. For those systems which are broken up, a large number of the
secondaries receive kick velocities ~100 - 200 km/s and subsequently evolve
into white dwarfs. We compute trajectories within the Galactic potential for
this population of stars and relate the birthrate of these stars over the
entire Galaxy to those seen locally with high velocities relative to the LSR.
We show that for a reasonable set of assumptions concerning the Galactic
supernova rate and the binary population, our model produces a local number
density of high-velocity white dwarfs compatible with that inferred from
observations. We therefore propose that a population of white dwarfs
originating in the thin disk may make a significant contribution to the
observed population of high-velocity white dwarfs.Comment: 6 pages, 5 figures; revised version, MNRAS in pres
Spin glass like transition in a highly concentrated Fe-C nanoparticle system
A highly concentrated (17 vol.%) Fe-C nano-particle system, with a narrow
size distribution nm, has been investigated using magnetic ac
susceptibility measurements covering a wide range of frequencies (17 mHz - 170
Hz). A dynamic scaling analysis gives evidence for a phase transition to a low
temperature spin-glass-like phase. The critical exponents associated with the
transition are and . The reason why
the scaling analysis works for this sample, while it may not work for other
samples exhibiting collective behavior as evidenced by aging phenomena, is that
the single particle contribution to is vanishingly small for
and hence all slow dynamics is due to collective behavior. This criterion can
only be fulfilled for a highly concentrated nano-particle sample with a narrow
size distribution.Comment: 2 pages, 3 figures, Proceeding for ICM200
Self diffusion of particles in complex fluids: temporary cages and permanent barriers
We study the self diffusion of individual particles in dense (non-)uniform
complex fluids within dynamic density functional theory and explicitly account
for their coupling to the temporally fluctuating background particles. Applying
the formalism to rod-like particles in uniaxial nematic and smectic liquid
crystals, we find correlated diffusion in different directions: The temporary
cage formed by the neighboring particles competes with permanent barriers in
periodic inhomogeneous systems such as the lamellar smectic state and delays
self diffusion of particles even in uniform systems. We compare our theory with
recent experimental data on the self diffusion of fluorescently labelled
filamentous virus particles in aqueous dispersions in the smectic phase and
find qualitative agreement. This demonstrates the importance of explicitly
dealing with the time-dependent self-consistent molecular field that every
particle experiences.Comment: submitte
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