1,037 research outputs found
Light scattering and phase behavior of Lysozyme-PEG mixtures
Measurements of liquid-liquid phase transition temperatures (cloud points) of
mixtures of a protein (lysozyme) and a polymer, poly(ethylene glycol) (PEG)
show that the addition of low molecular weight PEG stabilizes the mixture
whereas high molecular weight PEG was destabilizing. We demonstrate that this
behavior is inconsistent with an entropic depletion interaction between
lysozyme and PEG and suggest that an energetic attraction between lysozyme and
PEG is responsible. In order to independently characterize the lysozyme/PEG
interactions, light scattering experiments on the same mixtures were performed
to measure second and third virial coefficients. These measurements indicate
that PEG induces repulsion between lysozyme molecules, contrary to the
depletion prediction. Furthermore, it is shown that third virial terms must be
included in the mixture's free energy in order to qualitatively capture our
cloud point and light scattering data. The light scattering results were
consistent with the cloud point measurements and indicate that attractions do
exist between lysozyme and PEG.Comment: 5 pages, 2 figures, 1 tabl
Warped compactification on curved manifolds
The characterization of a six- (or seven)-dimensional internal manifold with
metric as having positive, zero or negative curvature is expected to be an
important aspect of warped compactifications in supergravity. In this context,
Douglas and Kallosh recently pointed out that a compact internal space with
negative curvature could help to construct four-dimensional de Sitter solutions
only if the extra dimensions are strongly warped or there are large stringy
corrections. That is, the problem of finding 4-dimensional de Sitter solutions
is well posed, if all extra dimensions are physically compact, which is called
a no-go theorem. Here, we show that the above conclusion does not extend to a
general class of warped compactifications in classical supergravity that allow
a non-compact direction or cosmological solutions for which the internal space
is asymptotic to a cone over a product of compact Einstein spaces or spheres.
For clarity, we present classical solutions that compactify higher-dimensional
spacetime to produce a Robertson--Walker universe with de Sitter-type expansion
plus one extra non-compact direction. Such models are found to admit both an
effective four-dimensional Newton constant that remains finite and a
normalizable zero-mode graviton wavefunction. We also exhibit the possibility
of obtaining 4D de Sitter solutions by including the effect of fluxes (p-form
field strengths).Comment: 24 pages, 1 figure; v5 significant changes in the presentation,
published (journal) versio
Low energy states with different symmetries in the t-J model with two holes on a 32-site lattice
We study the low energy states of the t-J model with two holes on a 32-site
lattice with periodic boundary conditions. In contrary to common belief, we
find that the state with d_{x^2-y^2} symmetry is not always the ground state in
the realistic parameter range 0.2\le J/t\le 0.4. There exist low-lying
finite-momentum p-states whose energies are lower than the d_{x^2-y^2} state
when J/t is small enough. We compare various properties of these low energy
states at J/t=0.3 where they are almost degenerate, and find that those
properties associated with the holes (such as the hole-hole correlation and the
electron momentum distribution function) are very different between the
d_{x^2-y^2} and p states, while their spin properties are very similar.
Finally, we demonstrate that by adding ``realistic'' terms to the t-J model
Hamiltonian, we can easily destroy the d_{x^2-y^2} ground state. This casts
doubt on the robustness of the d_{x^2-y^2} state as the ground state in a
microscopic model for the high temperature superconductors
The late-time behaviour of vortic Bianchi type VIII Universes
We use the dynamical systems approach to investigate the Bianchi type VIII
models with a tilted -law perfect fluid. We introduce
expansion-normalised variables and investigate the late-time asymptotic
behaviour of the models and determine the late-time asymptotic states. For the
Bianchi type VIII models the state space is unbounded and consequently, for all
non-inflationary perfect fluids, one of the curvature variables grows without
bound. Moreover, we show that for fluids stiffer than dust (), the
fluid will in general tend towards a state of extreme tilt. For dust
(), or for fluids less stiff than dust (), we show that
the fluid will in the future be asymptotically non-tilted. Furthermore, we show
that for all the universe evolves towards a vacuum state but
does so rather slowly, .Comment: 19 pages, 3 ps figures, v2:typos fixed, refs and more discussion
adde
From Euclidean Geometry to Knots and Nets
This document is the Accepted Manuscript of an article accepted for publication in Synthese. Under embargo until 19 September 2018. The final publication is available at Springer via https://doi.org/10.1007/s11229-017-1558-x.This paper assumes the success of arguments against the view that informal mathematical proofs secure rational conviction in virtue of their relations with corresponding formal derivations. This assumption entails a need for an alternative account of the logic of informal mathematical proofs. Following examination of case studies by Manders, De Toffoli and Giardino, Leitgeb, Feferman and others, this paper proposes a framework for analysing those informal proofs that appeal to the perception or modification of diagrams or to the inspection or imaginative manipulation of mental models of mathematical phenomena. Proofs relying on diagrams can be rigorous if (a) it is easy to draw a diagram that shares or otherwise indicates the structure of the mathematical object, (b) the information thus displayed is not metrical and (c) it is possible to put the inferences into systematic mathematical relation with other mathematical inferential practices. Proofs that appeal to mental models can be rigorous if the mental models can be externalised as diagrammatic practice that satisfies these three conditions.Peer reviewe
The Extreme Outer Regions of Disk Galaxies: I. Chemical Abundances of HII Regions
We present the first results of an ongoing project to investigate the
present-day chemical abundances of the extreme outer parts of galactic disks,
as probed by the emission line spectra of a new sample of HII regions. The
galaxies studied here, NGC628, NGC1058 and NGC6946, are all late-type spiral
galaxies, characterized by larger than average HI-to-optical sizes. Our deep
Halpha images have revealed the existence of recent massive star formation,
traced by HII regions, out to, and beyond, two optical radii in these galaxies
(defined by the B-band 25th magnitude isophote). Optical spectra of these
newly-discovered HII regions are used to investigate their densities,
ionization parameters, extinctions and in particular their oxygen and nitrogen
abundances. Our measurements reveal gas-phase abundances of O/H~10-15% of the
solar value, and N/O~20-25% of the solar value, at radii of 1.5-2 R25. Clear
evidence also exists for diminished dust extinction (Av~0-0.2) at large radii.
The combination of our measurements of outer disk HII region abundances with
those for inner disk HII regions published in the literature is a powerful
probe of the shape of abundance gradients over unprecedented radial baselines.
Within the limits of the current dataset, the radial abundance variations are
consistent with single log-linear relationships, although the derived slopes
can often differ considerably from those found if only inner disk HII regions
are used to define the fit. Interestingly, both the mean level of enrichment
and the ratio of N/O measured in extreme outer galactic disks are similar to
those values measured in some high redshift damped Lyman-alpha absorbers,
suggesting that outer disks at the present epoch are relatively unevolved.
(abridged)Comment: 36 pages, 10 embedded postscript files, 3 jpeg files, 7 postscript
tables; accepted for publication in the Astronomical Journal (August issue
Are Small Hyperbolic Universes Observationally Detectable?
Using recent observational constraints on cosmological density parameters,
together with recent mathematical results concerning small volume hyperbolic
manifolds, we argue that, by employing pattern repetitions, the topology of
nearly flat small hyperbolic universes can be observationally undetectable.
This is important in view of the facts that quantum cosmology may favour
hyperbolic universes with small volumes, and from the expectation coming from
inflationary scenarios, that is likely to be very close to one.Comment: 5 pages, 1 figure, LaTeX2e. A reference and two footnotes added. To
appear in Class. Quantum Grav. 18 (2001) in the present for
"Pair" Fermi contour and repulsion-induced superconductivity in cuprates
The pairing of charge carriers with large pair momentum is considered in
connection with high-temperature superconductivity of cuprate compounds. The
possibility of pairing arises due to some essential features of
quasi-two-dimensional electronic structure of cuprates: (i) The Fermi contour
with strong nesting features; (ii) The presence of extended saddle point near
the Fermi level; (iii) The existence of some ordered state (for example,
antiferromagnetic) close to the superconducting one as a reason for an
appearing of "pair" Fermi contour resulting from carrier redistribution in
momentum space. In an extended vicinity of the saddle point, momentum space has
hyperbolic (pseudoeuclidean) metrics, therefore, the principal values of
two-dimensional reciprocal reduced effective mass tensor have unlike signs.
Rearrangement of holes in momentum space results in a rise of "pair" Fermi
contour which may be defined as zero-energy line for relative motion of the
pair. The superconducting gap arises just on this line. Pair Fermi contour
formation inside the region of momentum space with hyperbolic metrics results
in not only superconducting pairing but in a rise of quasi-stationary state in
the relative motion of the pair. Such a state has rather small decay and may be
related to the pseudogap regime of underdoped cuprates. It is concluded that
the pairing in cuprates may be due to screened Coulomb repulsion. In this case,
the superconducting energy gap in hole-doped cuprates exists in the region of
hole concentration which is bounded both above and below. The superconducting
state with positive condensation energy exists in more narrow range of doping
level inside this region. Such hole concentration dependence correlates with
typical phase diagram of cuprates.Comment: 23 pages, 11 figures. Submitted to Phys. Rev.
Radiation Damping in FRW Space-times with Different Topologies
We study the role played by the compactness and the degree of connectedness
in the time evolution of the energy of a radiating system in the
Friedmann-Robertson-Walker (FRW) space-times whose spacelike
sections are the Euclidean 3-manifold and six topologically
non-equivalent flat orientable compact multiply connected Riemannian
3-manifolds. An exponential damping of the energy is present in the
case, whereas for the six compact flat 3-spaces it is found
basically the same pattern for the evolution of the energy, namely relative
minima and maxima occurring at different times (depending on the degree of
connectedness) followed by a growth of . Likely reasons for this
divergent behavior of in these compact flat 3-manifolds are discussed
and further developments are indicated. A misinterpretation of Wolf's results
regarding one of the six orientable compact flat 3-manifolds is also indicated
and rectified.Comment: 13 pages, RevTeX, 5 figures, To appear in Phys. Rev. D 15, vol. 57
(1998
Commensurate dynamic magnetic correlations in La2(Cu,Li)O4
When sufficient numbers of holes are introduced into the two-dimensional CuO2
square lattice, dynamic magnetic correlations become incommensurate with
underlying lattice in all previously investigated La_{2-x}A_xCu_{1-z}B_zO_{4+y}
(A=Sr or Nd, B=Zn) including high T_C superconductors and insulators, and in
bilayered superconducting YBa_2Cu_3O_{6.6} and Bi_2Sr_2CaCu_2O_8. Magnetic
correlations also become incommensurate in structurally related La_2NiO_4 when
doped with Sr or O. We report an exception to this so-far well established
experimental "rule" in La_2Cu_{1-z}Li_{z}O_4 in which magnetic correlations
remain commensurate.Comment: 4 pages, 3 figures, revised version as for publicatio
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