494 research outputs found
Free Energy of a Dilute Bose Gas: Lower Bound
A lower bound is derived on the free energy (per unit volume) of a
homogeneous Bose gas at density and temperature . In the dilute
regime, i.e., when , where denotes the scattering length of
the pair-interaction potential, our bound differs to leading order from the
expression for non-interacting particles by the term . Here, denotes the critical density for
Bose-Einstein condensation (for the non-interacting gas), and denotes
the positive part. Our bound is uniform in the temperature up to temperatures
of the order of the critical temperature, i.e., or smaller.
One of the key ingredients in the proof is the use of coherent states to extend
the method introduced in [arXiv:math-ph/0601051] for estimating correlations to
temperatures below the critical one.Comment: LaTeX2e, 53 page
Free Energies of Dilute Bose gases: upper bound
We derive a upper bound on the free energy of a Bose gas system at density
and temperature . In combination with the lower bound derived
previously by Seiringer \cite{RS1}, our result proves that in the low density
limit, i.e., when , where denotes the scattering length of
the pair-interaction potential, the leading term of the free energy
difference per volume between interacting and ideal Bose gases is equal to
4\pi a (2\rho^2-[\rho-\rhoc]^2_+). Here, \rhoc(T) denotes the critical
density for Bose-Einstein condensation (for the ideal gas), and
denotes the positive part.Comment: 56 pages, no figure
Phylogenetic relationships of African Caecilians (Amphibia: Gymnophiona): insights from mitochondrial rRNA gene sequences
Africa (excluding the Seychelles) has a diverse caecilian fauna, including the endemic family Scolecomorphidae and six endemic genera of the more cosmopolitan Caeciliidae. Previous molecular phylogenetic studies have not included any caecilians from the African mainland. Partial 12S and 16S mitochondrial gene sequences were obtained for two species of the endemic African Scolecomorphidae and five species and four genera of African Caeciliids, aligned against previously reported sequences for 16 caecilian species, and analysed using parsimony, maximum likelihood, Bayesian and distance methods. Results are in agreement with traditional taxonomy in providing support for the monophyly of the African Caeciliid genera Boulengerula and Schistometopum and for the Scolecomorphidae. They disagree in indicating that the Caeciliidae is paraphyletic with respect to the Scolecomorphidae. Although more data from morphology and/or molecules will be required to resolve details of the interrelationships of the African caecilian genera, the data provide strong support for at least two origins of caecilians in which the eye is reduced and covered with bone, and do not support the hypotheses that the caecilian assemblages of Africa, and of East and of West Africa are monophyletic
Developing specialist leaders of education: a research engagement approach
There has been little research to date on the continuing professional
development needs of the several thousand Specialist Leaders of
Education (SLE) now designated by the National College for Teaching
and Leadership in England to work across schools as consultants on
school-to-school support. This case study reports on the second and
third stages of a four-stage research process designed to address these
needs. The
fi
rst stage reported on the creation of a professional devel-
opment framework for SLE
’
s using consultancy research. These middle
stages test out this framework with a stakeholder group of SLEs, head-
teachers and broker in a Teaching Schools Alliance. The fourth stage will
track the implementation of professional development activities arising
from these
fi
ndings. Apart from the speci
fi
c needs of SLE, this study will
have wider relevance for all practitioners and researchers working in and
with schools on leadership development using Research Engagement
strategies and Joint Practice Development approaches in a so-called
‘
self-
improving
’
school system
Investigation into the mechanisms by which microwave heating enhances separation of water-in-oil emulsions
The separation of water-in-oil emulsions made with Azeri crude was investigated using natural gravity settling and microwave heating techniques. Separation times could be reduced by an order of magnitude compared with untreated emulsions. Increasing the salinity of the water phase leads to a 15% average decrease in the settling time for untreated emulsions compared with over 90% for microwave-heated emulsions. An image analysis technique showed that the observed increases in settling time could not be attributed to changes in viscosity alone. Significant coalescence of water droplets occurs during microwave heating, however the effects of coalescence and viscosity reduction cannot be completely decoupled. Despite this, it is clear that it is the thermal effect of microwave heating that leads to improvements in settling times, and that any advantages in microwave heating over conventional heating can be explained by selective heating of the aqueous phase rather than so-called non-thermal effects
Drying Kinetics of Salt Solution Droplets:Water Evaporation Rates and Crystallization
Drying and crystallization
of solution droplets is a problem of
broad relevance, determining the microstructures of particles formed
in spray-drying, the phase of particles delivered by, for example,
aerosol formulations for inhalation therapies, and the impact of aerosols
on radiative forcing and climate. The ephemeral nature of free droplets,
particularly when considering the drying kinetics of droplets with
highly volatile constituents, has often precluded the accurate measurement
of transient properties such as droplet size and composition, preventing
the robust assessment of predictive models of droplet-drying rates,
nucleation, and crystallization. Here, we report novel measurements
of the drying kinetics of individual aqueous sodium chloride solution
droplets using an electrodynamic balance to isolate and trap single
aerosol droplets (radius ≈ 25 μm). The initial solution
droplet size and composition are shown to be highly reproducible in
terms of drying rate and crystallization time when examined over hundreds
of identical evaporating droplets. We introduce a numerical model
that determines the concentration gradient across the radial profile
of the droplet as it dries, considering both the surface recession
because of evaporation and the diffusion of components within the
droplet. Drying-induced crystallization is shown to be fully determined
for this system, with nucleation and instantaneous crystallization
occurring once a critical supersaturation level of 2.04 ± 0.02
is achieved at the surface of the evaporating droplet. This phenomenological
model provides a consistent account of the timescale and surface concentration
of free-droplet crystallization on drying for the different drying
conditions studied, a necessary step in progress toward achieving
control over rates of crystallization and the competitive formation
of amorphous particles
Finite Temperature Properties of Quantum Antiferromagnets in a Uniform Magnetic Field in One and Two Dimensions
Consider a -dimensional antiferromagnet with a quantum disordered ground
state and a gap to bosonic excitations with non-zero spin. In a finite external
magnetic field, this antiferromagnet will undergo a phase transition to a
ground state with non-zero magnetization, describable as the condensation of a
dilute gas of bosons. The finite temperature properties of the Bose gas in the
vicinity of this transition are argued to obey a hypothesis of ZERO
SCALE-FACTOR UNIVERSALITY for , with logarithmic violations in .
Scaling properties of various experimental observables are computed in an
expansion in , and exactly in .Comment: 27 pages, REVTEX 3.0, 8 Postscript figures appended, YCTP-xyz
Quantum spin systems at positive temperature
We develop a novel approach to phase transitions in quantum spin models based
on a relation to their classical counterparts. Explicitly, we show that
whenever chessboard estimates can be used to prove a phase transition in the
classical model, the corresponding quantum model will have a similar phase
transition, provided the inverse temperature and the magnitude of the
quantum spins \CalS satisfy \beta\ll\sqrt\CalS. From the quantum system we
require that it is reflection positive and that it has a meaningful classical
limit; the core technical estimate may be described as an extension of the
Berezin-Lieb inequalities down to the level of matrix elements. The general
theory is applied to prove phase transitions in various quantum spin systems
with \CalS\gg1. The most notable examples are the quantum orbital-compass
model on and the quantum 120-degree model on which are shown to
exhibit symmetry breaking at low-temperatures despite the infinite degeneracy
of their (classical) ground state.Comment: 47 pages, version to appear in CMP (style files included
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