48,148 research outputs found
An Introduction to Conformal Ricci Flow
We introduce a variation of the classical Ricci flow equation that modifies
the unit volume constraint of that equation to a scalar curvature constraint.
The resulting equations are named the Conformal Ricci Flow Equations because of
the role that conformal geometry plays in constraining the scalar curvature.
These equations are analogous to the incompressible Navier-Stokes equations of
fluid mechanics inasmuch as a conformal pressure arises as a Lagrange
multiplier to conformally deform the metric flow so as to maintain the scalar
curvature constraint. The equilibrium points are Einstein metrics with a
negative Einstein constant and the conformal pressue is shown to be zero at an
equilibrium point and strictly positive otherwise. The geometry of the
conformal Ricci flow is discussed as well as the remarkable analytic fact that
the constraint force does not lose derivatives and thus analytically the
conformal Ricci equation is a bounded perturbation of the classical
unnormalized Ricci equation. That the constraint force does not lose
derivatives is exactly analogous to the fact that the real physical pressure
force that occurs in the Navier-Stokes equations is a bounded function of the
velocity. Using a nonlinear Trotter product formula, existence and uniqueness
of solutions to the conformal Ricci flow equations is proven. Lastly, we
discuss potential applications to Perelman's proposed implementation of
Hamilton's program to prove Thurston's 3-manifold geometrization conjectures.Comment: 52 pages, 1 figur
Spin-Orbit Coupling in LaAlO/SrTiO interfaces: Magnetism and Orbital Ordering
The combination of Rashba spin-orbit coupling and electron correlations can
induce unusual phenomena in the metallic interface between SrTiO and
LaAlO. We consider effects of Rashba spin-orbit coupling at this interface
in the context of the recent observation of anisotropic magnetism. Firstly, we
show how Rashba spin-orbit coupling in a system near a band-edge can account
for the observed magnetic anisotropy. Secondly, we investigate the coupling
between in-plane magnetic-moment anisotropy and nematicity in the form of an
orbital imbalance between d / d orbitals. We estimate this
coupling to be substantial in the low electron density regime. Such an orbital
ordering can affect magneto transport
High-Resolution NIR Observations of the Circumstellar Disk System in the Bok Globule CB 26
We report on results of near-infrared and optical observations of the mm disk
embedded in the Bok globule CB 26 (Launhardt & Sargent 2001). The near-infrared
images show a bipolar reflection nebula with a central extinction lane which
coincides with the mm disk. Imaging polarimetry of this object yielded a
polarization pattern which is typical for a young stellar object surrounded by
a large circumstellar disk and an envelope, seen almost edge-on. The strong
linear polarization in the bipolar lobes is caused by single scattering at dust
grains and allowed to locate the illuminating source which coincides with the
center of the mm disk. The spectral energy distribution of the YSO embedded in
CB 26 resembles that of a ClassI source with a luminosity of 0.5 L_sun.Using
the pre-main-sequence evolutionary tracks and the stellar mass inferred from
the rotation curve of the disk, we derive an age of the system of <10^6 yr.
H_alpha and [SII] narrow-band imaging as well as optical spectroscopy revealed
an Herbig-Haro object 6.15 arcmin northwest of CB 26 YSO 1, perfectly aligned
with the symmetry axis of the bipolar nebula. This Herbig-Haro object (HH 494)
indicates ongoing accretion and outflow activity in CB 26 YSO 1. Its excitation
characteristics indicate that the Herbig-Haro flow is propagating into a
low-density environment. We suggest that CB 26 YSO 1 represents the transition
stage between embedded protostellar accretion disks and more evolved
protoplanetary disks around T Tauri stars in an undisturbed environment.Comment: 21 pages, 6 figures (reduced resolution), ApJ accepte
Novel self-assembled morphologies from isotropic interactions
We present results from particle simulations with isotropic medium range
interactions in two dimensions. At low temperature novel types of aggregated
structures appear. We show that these structures can be explained by
spontaneous symmetry breaking in analytic solutions to an adaptation of the
spherical spin model. We predict the critical particle number where the
symmetry breaking occurs and show that the resulting phase diagram agrees well
with results from particle simulations.Comment: 4 pages, 4 figure
Spin-Glass Attractor on Tridimensional Hierarchical Lattices in the Presence of an External Magnetic Field
A nearest-neighbor-interaction Ising spin glass, in the presence of an
external magnetic field, is studied on different hierarchical lattices that
approach the cubic lattice. The magnetic field is considered as uniform, or
random (following either a bimodal or a Gaussian probability distribution). In
all cases, a spin-glass attractor is found, in the plane magnetic field versus
temperature, associated with a low-temperature phase. The physical consequences
of this attractor are discussed, in view of the present scenario of the
spin-glass problem.Comment: Accepted for publication in Physical Review
Solute channels of the outer membrane: from bacteria to chloroplasts
Chloroplasts, unique organelles of plants, originated from endosymbiosis of an ancestor of today's cyanobacteria with a mitochondria-containing host cell. It is assumed that the outer envelope membrane, which delimits the chloroplast from the surrounding cytosol, was thus inherited from its Gram-negative bacterial ancestor. This plastid-specific membrane is thus equipped with elements of prokaryotic and eukaryotic origin. In particular, the membrane-intrinsic outer envelope proteins (OEPs) form solute channels with properties reminiscent of porins and channels in the bacterial outer membrane. OEP channels are characterised by distinct specificities for metabolites and a quite peculiar expression pattern in specialised plant organs and plastids, thus disproving the assumption that the outer envelope is a non-specific molecular sieve. The same is true for the outer membrane of Gram-negative bacteria, which functions as a permeability barrier in addition to the cytoplasmic membrane, and embeds different classes of channel pores. The channels of these prokaryotic prototype proteins, ranging from unspecific porins to specific channels to ligand-gated receptors, are exclusively built of P-barrels. Although most of the OEP channels are formed by P-strands as well, phylogeny based on sequence homology alone is not feasible. Thus, the comparison of structural and functional properties of chloroplast outer envelope and bacterial outer membrane channels is required to pinpoint the ancestral OEP `portrait gallery'
Correlated projection operator approach to non-Markovian dynamics in spin baths
The dynamics of an open quantum system is usually studied by performing a
weak-coupling and weak-correlation expansion in the system-bath interaction.
For systems exhibiting strong couplings and highly non-Markovian behavior this
approach is not justified. We apply a recently proposed correlated projection
superoperator technique to the model of a central spin coupled to a spin bath
via full Heisenberg interaction. Analytical solutions to both the
Nakajima-Zwanzig and the time-convolutionless master equation are determined
and compared with the results of the exact solution. The correlated projection
operator technique significantly improves the standard methods and can be
applied to many physical problems such as the hyperfine interaction in a
quantum dot
Disentangling the near infrared continuum spectral components of the inner 500 pc of Mrk 573: two-dimensional maps
We present a near infrared study of the spectral components of the continuum
in the inner 500500 pc of the nearby Seyfert galaxy Mrk573 using
adaptive optics near-infrared integral field spectroscopy with the instrument
NIFS of the Gemini North Telescope at a spatial resolution of 50 pc. We
performed spectral synthesis using the {\sc starlight} code and constructed
maps for the contributions of different age components of the stellar
population: young ( Myr), young-intermediate (
Myr), intermediate-old ( Myr Gyr) to the
near-IR K-band continuum, as well as their contribution to the total stellar
mass. We found that the old stellar population is dominant within the inner 250
pc, while the intermediate age components dominate the continuum at larger
distances. A young stellar component contributes up to 20% within the
inner 70 pc, while hot dust emission and featureless continuum components
are also necessary to fit the nuclear spectrum, contributing up to 20% of the
K-band flux there. The radial distribution of the different age components in
the inner kiloparsec of Mrk573 is similar to those obtained by our group for
the Seyfert galaxies Mrk1066, Mrk1157 and NGC1068 in previous works using a
similar methodology. Young stellar populations (100 Myr) are seen in the
inner 200-300 pc for all galaxies contributing with 20% of the K-band
flux, while the near-IR continuum is dominated by the contribution of
intermediate-age stars (100 Myr-2 Gyr) at larger distances. Older stellar
populations dominate in the inner 250 pc
A Comparison of Phycocyanins from Three Different Species of Cyanobacteria Employing Resonance-Enhanced Coherent Anti-Stokes Raman Spectroscopy
Resonance-enhanced coherent anti-Stokes Raman spectra are recorded for monomers and trimers of phycocyanin from three different cyanobacteria: Westiellopsis prolifica, Mastigocladus laminosus and Spirulina platensis. It is shown that upon aggregation from monomer to trimer the electronic structures of both the α84 and β84 chromophores are changed. The spectra of the trimers originating from S. platensis and M. laminosus are very similar to each other, but distinctly different from the spectrum of W. prolifica
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