471 research outputs found
Gauge Theories in Noncommutative Homogeneous K\"ahler Manifolds
We construct a gauge theory on a noncommutative homogeneous K\"ahler
manifold, where we employ the deformation quantization with separation of
variables for K\"ahler manifolds formulated by Karabegov. A key point in this
construction is to obtaining vector fields which act as inner derivations for
the deformation quantization. We show that these vector fields are the only
Killing vector fields. We give an explicit construction of this gauge theory on
noncommutative and noncommutative .Comment: 27 pages, typos correcte
TIM23-mediated insertion of transmembrane alpha-helices into the mitochondrial inner membrane
While overall hydrophobicity is generally recognized as the main characteristic of transmembrane (TM) alpha-helices, the only membrane system for which there are detailed quantitative data on how different amino acids contribute to the overall efficiency of membrane insertion is the endoplasmic reticulum (ER) of eukaryotic cells. Here, we provide comparable data for TIM23-mediated membrane protein insertion into the inner mitochondrial membrane of yeast cells. We find that hydrophobicity and the location of polar and aromatic residues are strong determinants of membrane insertion. These results parallel what has been found previously for the ER. However, we see striking differences between the effects elicited by charged residues flanking the TM segments when comparing the mitochondrial inner membrane and the ER, pointing to an unanticipated difference between the two insertion systems. Keywords: CoxVa , membrane protein , Mgm1p , mitochondria , TIM2
Probing early structure and model-independent neutrino mass with high-redshift CMB lensing mass maps
CMB lensing maps probe the mass distribution in projection out to high
redshifts, but significant sensitivity to low-redshift structure remains. In
this paper we discuss a method to remove the low-redshift contributions from
CMB lensing mass maps by subtracting suitably scaled galaxy density maps,
nulling the low redshift structure with a model-insensitive procedure that is
similar to delensing. This results in a high--only mass map that can provide
a probe of structure growth at uniquely high redshifts: if systematics can be
controlled, we forecast that CMB-S4 lensing combined with a Rubin-LSST-like
galaxy survey can probe the amplitude of structure at redshifts
() to within (). We then discuss other example applications
of such high- CMB lensing maps. In standard analyses of CMB lensing,
assuming the wrong dark energy model (or wrong model parametrization) can lead
to biases in neutrino mass constraints. In contrast, we show with forecasts
that a high- mass map constructed from CMB-S4 lensing and LSST galaxies can
provide a nearly model-independent neutrino mass constraint, with only
negligible sensitivity to the presence of non-standard dark energy models,
irrespective of their parametrization.Comment: 19 pages, 11 figure
First-order phase transition in the tethered surface model on a sphere
We show that the tethered surface model of Helfrich and Polyakov-Kleinert
undergoes a first-order phase transition separating the smooth phase from the
crumpled one. The model is investigated by the canonical Monte Carlo
simulations on spherical and fixed connectivity surfaces of size up to N=15212.
The first-order transition is observed when N>7000, which is larger than those
in previous numerical studies, and a continuous transition can also be observed
on small-sized surfaces. Our results are, therefore, consistent with those
obtained in previous studies on the phase structure of the model.Comment: 6 pages with 7 figure
Orbital and spin interplay in spin-gap formation in pyroxene titanium oxides ATiSi2O6 (A=Na, Li)
Interplay between orbital and spin degrees of freedom is theoretically
studied for the phase transition to the spin-singlet state with lattice
dimerization in pyroxene titanium oxides ATiSi2O6 (A=Na, Li). For the quasi
one-dimensional spin-1/2 systems, we derive an effective spin-orbital-lattice
coupled model in the strong correlation limit with explicitly taking account of
the t_2g orbital degeneracy, and investigate the model by numerical simulation
as well as the mean-field analysis. We find a nontrivial feedback effect
between orbital and spin degrees of freedom; as temperature decreases,
development of antiferromagnetic spin correlations changes the sign of orbital
correlations from antiferro to ferro type, and finally the ferro-type orbital
correlations induce the dimerization and the spin-singlet formation. As a
result of this interplay, the system undergoes a finite-temperature transition
to the spin-dimer and orbital-ferro ordered phase concomitant with the
Jahn-Teller lattice distortion. The numerical results for the magnetic
susceptibility show a deviation from the Curie-Weiss behavior, and well
reproduce the experimental data. The results reveal that the Jahn-Teller energy
scale is considerably small and the orbital and spin exchange interactions play
a decisive role in the pyroxene titanium oxides.Comment: 13 pages, 9 figures; final version. Text, Fig.1, and references are
revised. To appear in Phys. Rev.
NMR relaxation rate in the field-induced octupolar liquid phase of spin-1/2 J1-J2 frustrated chains
In the spin-1/2 frustrated chain with nearest-neighbor ferromagnetic exchange
J1 and next-nearest-neighbor antiferromagnetic exchange J2 under magnetic
field, magnetic multipolar-liquid (quadrupolar, octupolar, and hexadecapolar)
phases are widely expanded from the saturation down to a low-field regime.
Recently, we have clarified characteristic temperature and field dependence of
the NMR relaxation rate 1/T_1 in the quadrupolar phase. In this paper, we
examine those of 1/T_1 in the octupolar phase combining field theoretical
method with numerical data. The relevance of the results to quasi
one-dimensional J1-J2 magnets such as PbCuSO4(OH)2, Rb2Cu2Mo3O12 and Li2ZrCuO4
is shortly discussed.Comment: 6 pages (1 column), 3 figure
HST/NICMOS Imaging Polarimetry of Proto-Planetary Nebulae: Probing of the Dust Shell Structure via Polarized Light
Using NICMOS on HST, we have performed imaging polarimetry of proto-planetary
nebulae. Our objective is to study the structure of optically thin
circumstellar shells of post-asymptotic giant branch stars by separating
dust-scattered, linearly polarized star light from unpolarized direct star
light. This unique technique allows us to probe faint reflection nebulae around
the bright central star, which can be buried under the point-spread-function of
the central star in conventional imaging. Our observations and archival search
have yielded polarimetric images for five sources: IRAS 07134+1005 (HD 56126),
IRAS 06530-0213, IRAS 04296+3429, IRAS (Z)02229+6208, and IRAS 16594-4656.
These images have revealed the circumstellar dust distribution in an
unprecedented detail via polarized intensity maps, providing a basis to
understand the 3-D structure of these dust shells. We have observationally
confirmed the presence of the inner cavity caused by the cessation of AGB mass
loss and the internal shell structures which is strongly tied to the progenitor
star's mass loss history on the AGB. We have also found that equatorial
enhancement in these circumstellar shells comes with various degrees of
contrast, suggesting a range of optical depths in these optically thin shells.
Our data support the interpretation that the dichotomy of PPN morphologies is
due primarily to differences in optical depth and secondary to the inclination
effect. The polarization maps reveal a range of inclination angles for these
optically thin reflection nebulae, dispelling the notion that elliptical
nebulae are pole-on bipolar nebulae.Comment: 17 pages in emulateapj format, 12 figures. To be published in the
March 2005 issue of The Astronomical Journa
Near-Infrared Photometric Survey of Proto-Planetary Nebula Candidates
We present JHK' photometric measurements of 78 objects mostly consisting of
proto-planetary nebula candidates. Photometric magnitudes are determined by
means of imaging and aperture photometry. Unlike the observations with a
photometer with a fixed-sized beam, the method of imaging photometry permits
accurate derivation of photometric values because the target sources can be
correctly identified and confusion with neighboring sources can be easily
avoided. Of the 78 sources observed, we report 10 cases in which the source
seems to have been misidentified or confused by nearby bright sources. We also
present nearly two dozen cases in which the source seems to have indicated a
variability which prompts a follow-up monitoring. There are also a few sources
that show previously unreported extendedness. In addition, we present H band
finding charts of the target sources.Comment: 3 tables, 1 figur
Scalar chiral ground states of spin ladders with four-spin exchanges
We show that scalar chiral order can be induced by four-spin exchanges in the
two-leg spin ladder, using the spin-chirality duality transformation and
matrix-product ansatz. Scalar-chiral-ordered states are found to be exact
ground states in a family of spin ladder models. In this scalar chiral phase,
there is a finite energy gap above the doubly degenerate ground states and a
symmetry is fully broken. It is also shown that the
SU(4)-symmetric model, which is self-dual under the duality transformation, is
on a multicritical point surrounded by the staggered dimer phase, the staggered
scalar chiral phase, and the gapless phase.Comment: 8 pages, 2 figures, to appear in Phys. Rev.
HST/NICMOS Imaging Polarimetry of Proto-Planetary Nebulae II: Macro-morphology of the Dust Shell Structure via Polarized Light
The structure of the dusty circumstellar envelopes (CSEs) of proto-planetary
nebulae (PPNs) reveals the mass-loss history of these sources and how such
histories may differ for elliptical (SOLE) and bipolar (DUPLEX) PPNs. To study
the PPN structures via dust-scattered linearly polarized starlight, we have
compiled the imaging-polarimetric data for all 18 evolved stars that have been
obtained to date with NICMOS on-board the Hubble Space Telescope (HST). This
alternative imaging technique provides a unique way to probe the distribution
of dust grains that scatter light around evolved stars. The new perspective
gained from the imaging-polarimetric data has revealed several new aspects to
the structures of PPNs. Point-symmetry is a prevalent imaging-polarimetric
characteristic resulting from the azimuthal density gradient in the CSEs. Among
these point-symmetric nebulae, three detailed morphological types can be
differentiated by their polarized intensity, I_pol, and polarization strength,
P. While the azimuthal density gradient is reversed above and below the
equatorial plane in optically thicker bipolar nebulae, there is no gradient
reversal in optically thinner elliptical nebulae. The equatorial plane of the
system defined by the integrated angle of polarization is not necessarily
orthogonal to the axis of the apparent bipolar structure in the total intensity
data.Comment: 27 pages, 14 figures, 3 tables; to appear in the Astronomical Journal
March 2007 issu
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