989 research outputs found
Panel Data Models with Multiple Time-Varying Individual Effects
This paper considers a panel data model with time-varying individual effects. The data are assumed to contain a large number of cross-sectional units repeatedly observed over a fixed number of time periods. The model has a feature of the fixed-effects model in that the effects are assumed to be correlated with the regressors. The unobservable individual effects are assumed to have a factor structure. For consistent estimation of the model, it is important to estimate the true number of factors. We propose a generalized methods of moments procedure by which both the number of factors and the regression coefficients can be consistently estimated. Some important identification issues are also discussed. Our simulation results indicate that the proposed methods produce reliable estimates.panel data, time-varying individual effects, factor models
cDNA Cloning of Biologically Active Chicken Interleukin-18
By searching a chicken EST database, we identified a cDNA clone that appeared to contain the entire open
reading frame (ORF) of chicken interleukin-18 (ChIL-18). The encoded protein consists of 198 amino acids
and exhibits approximately 30% sequence identity to IL-18 of humans and various others mammals. Sequence
comparisons reveals a putative caspase-1 cleavage site at aspartic acid 29 of the primary translation product,
indicating that mature ChIL-18 might consist of 169 amino acids. Bacterially expressed ChIL-18 in which the
N-terminal 29 amino acids of the putative precursor molecule were replaced by a histidine tag induced the
synthesis of interferon-γ (IFN-γ) in cultured primary chicken spleen cells, indicating that the recombinant
protein is biologically active
Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights
The temperature dependence and anisotropy of optical spectral weights
associated with different multiplet transitions is determined by the spin and
orbital correlations. To provide a systematic basis to exploit this close
relationship between magnetism and optical spectra, we present and analyze the
spin-orbital superexchange models for a series of representative
orbital-degenerate transition metal oxides with different multiplet structure.
For each case we derive the magnetic exchange constants, which determine the
spin wave dispersions, as well as the partial optical sum rules. The magnetic
and optical properties of early transition metal oxides with degenerate
orbitals (titanates and vanadates with perovskite structure) are shown
to depend only on two parameters, viz. the superexchange energy and the
ratio of Hund's exchange to the intraorbital Coulomb interaction, and on
the actual orbital state. In systems important corrections follow from
charge transfer excitations, and we show that KCuF can be classified as a
charge transfer insulator, while LaMnO is a Mott insulator with moderate
charge transfer contributions. In some cases orbital fluctuations are quenched
and decoupling of spin and orbital degrees of freedom with static orbital order
gives satisfactory results for the optical weights. On the example of cubic
vanadates we describe a case where the full quantum spin-orbital physics must
be considered. Thus information on optical excitations, their energies,
temperature dependence and anisotropy, combined with the results of magnetic
neutron scattering experiments, provides an important consistency test of the
spin-orbital models, and indicates whether orbital and/or spin fluctuations are
important in a given compound.Comment: 34 pages, 16 figure
Higher spin AdS_3 holography with extended supersymmetry
We propose a holographic duality between a higher spin AdS_3 gravity with
so(p) extended supersymmetry and a large N limit of a 2-dimensional
Grassmannian-like model with a specific critical level k=N and a non-diagonal
modular invariant. As evidence, we show the match of one-loop partition
functions. Moreover, we construct symmetry generators of the coset model for
low spins which are dual to gauge fields in the supergravity. Further, we
discuss a possible relation to superstring theory by noticing an N=3
supersymmetry of critical level model at finite k,N. In particular, we examine
BPS states and marginal deformations. Inspired by the supergravity side, we
also propose and test another large N CFT dual obtained as a Z_2 automorphism
truncation of a similar coset model, but at a non-critical level.Comment: 44 pages, published versio
Cosmological Simulations with Self-Interacting Dark Matter I: Constant Density Cores and Substructure
We use cosmological simulations to study the effects of self-interacting dark
matter (SIDM) on the density profiles and substructure counts of dark matter
halos from the scales of spiral galaxies to galaxy clusters, focusing
explicitly on models with cross sections over dark matter particle mass
\sigma/m = 1 and 0.1 cm^2/g. Our simulations rely on a new SIDM N-body
algorithm that is derived self-consistently from the Boltzmann equation and
that reproduces analytic expectations in controlled numerical experiments. We
find that well-resolved SIDM halos have constant-density cores, with
significantly lower central densities than their CDM counterparts. In contrast,
the subhalo content of SIDM halos is only modestly reduced compared to CDM,
with the suppression greatest for large hosts and small halo-centric distances.
Moreover, the large-scale clustering and halo circular velocity functions in
SIDM are effectively identical to CDM, meaning that all of the large-scale
successes of CDM are equally well matched by SIDM. From our largest cross
section runs we are able to extract scaling relations for core sizes and
central densities over a range of halo sizes and find a strong correlation
between the core radius of an SIDM halo and the NFW scale radius of its CDM
counterpart. We construct a simple analytic model, based on CDM scaling
relations, that captures all aspects of the scaling relations for SIDM halos.
Our results show that halo core densities in \sigma/m = 1 cm^2/g models are too
low to match observations of galaxy clusters, low surface brightness spirals
(LSBs), and dwarf spheroidal galaxies. However, SIDM with \sigma/m ~ 0.1 cm^2/g
appears capable of reproducing reported core sizes and central densities of
dwarfs, LSBs, and galaxy clusters without the need for velocity dependence.
(abridged)Comment: 26 pages, 16 figures, all figures include colors, submitted for
publication in MNRA
Measuring the Physiologic Properties of Oral Lesions Receiving Fractionated Photodynamic Therapy
Photodynamic therapy (PDT) can treat superficial, early‐stage disease with minimal damage to underlying tissues and without cumulative dose‐limiting toxicity. Treatment efficacy is affected by disease physiologic properties, but these properties are not routinely measured. We assessed diffuse reflectance spectroscopy (DRS) for the noninvasive, contact measurement of tissue hemoglobin oxygen saturation (StO2) and total hemoglobin concentration ([tHb]) in the premalignant or superficial microinvasive oral lesions of patients treated with 5‐aminolevulinic acid (ALA)‐PDT. Patients were enrolled on a Phase 1 study of ALA‐PDT that evaluated fluences of 50, 100, 150 or 200 J cm−2 delivered at 100 mW cm−2. To test the feasibility of incorporating DRS measurements within the illumination period, studies were performed in patients who received fractionated (two‐part) illumination that included a dark interval of 90–180 s. Using DRS, tissue oxygenation at different depths within the lesion could also be assessed. DRS could be performed concurrently with contact measurements of photosensitizer levels by fluorescence spectroscopy, but a separate noncontact fluorescence spectroscopy system provided continuous assessment of photobleaching during illumination to greater tissue depths. Results establish that the integration of DRS into PDT of early‐stage oral disease is feasible, and motivates further studies to evaluate its predictive and dosimetric value.Diffuse reflectance spectroscopy with a contact probe was employed as part of a fluorescence and reflectance spectroscopy system to measure the tissue hemoglobin oxygen saturation and hemoglobin content of lesions of premalignant or early microinvasive cancer of the oral cavity. Studies demonstrate the feasibility of incorporating these measurements into treatment with fractionated (two‐part) photodynamic therapy (PDT) using 5‐aminolevulinic acid. Patient‐specific differences in physiologic parameters were detectable at baseline and at times during and after PDT. Photobleaching of photosensitizer was measured by its fluorescence. Results establish the utility of rationally designed spectroscopy probes toward personalized dosimetry in PDT of oral disease.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113767/1/php12475.pd
Generation of solar spicules and subsequent atmospheric heating
Spicules are rapidly evolving fine-scale jets of magnetized plasma in the solar chromosphere. It remains unclear how these prevalent jets originate from the solar surface and what role they play in heating the solar atmosphere. Using the Goode Solar Telescope at the Big Bear Solar Observatory, we observed spicules emerging within minutes of the appearance of opposite-polarity magnetic flux around dominant-polarity magnetic field concentrations. Data from the Solar Dynamics Observatory showed subsequent heating of the adjacent corona. The dynamic interaction of magnetic fields (likely due to magnetic reconnection) in the partially ionized lower solar atmosphere appears to generate these spicules and heat the upper solar atmosphere
Manganites at Quarter Filling: Role of Jahn-Teller Interactions
We have analyzed different correlation functions in a realistic spin-orbital
model for half-doped manganites. Using a finite-temperature diagonalization
technique the CE phase was found in the charge-ordered phase in the case of
small antiferromagnetic interactions between electrons. It is shown
that a key ingredient responsible for stabilization of the CE-type spin and
orbital-ordered state is the cooperative Jahn-Teller (JT) interaction between
next-nearest Mn neighbors mediated by the breathing mode distortion of
Mn octahedra and displacements of Mn ions. The topological phase
factor in the Mn-Mn hopping leading to gap formation in one-dimensional models
for the CE phase as well as the nearest neighbor JT coupling are not able to
produce the zigzag chains typical for the CE phase in our model.Comment: 16 pages with 16 figures, contains a more detailed parameter estimate
based on the structural data by Radaelli et al. (accepted for publication in
Phys. Rev. B
The Energy Landscape, Folding Pathways and the Kinetics of a Knotted Protein
The folding pathway and rate coefficients of the folding of a knotted protein
are calculated for a potential energy function with minimal energetic
frustration. A kinetic transition network is constructed using the discrete
path sampling approach, and the resulting potential energy surface is
visualized by constructing disconnectivity graphs. Owing to topological
constraints, the low-lying portion of the landscape consists of three distinct
regions, corresponding to the native knotted state and to configurations where
either the N- or C-terminus is not yet folded into the knot. The fastest
folding pathways from denatured states exhibit early formation of the
N-terminus portion of the knot and a rate-determining step where the C-terminus
is incorporated. The low-lying minima with the N-terminus knotted and the
C-terminus free therefore constitute an off-pathway intermediate for this
model. The insertion of both the N- and C-termini into the knot occur late in
the folding process, creating large energy barriers that are the rate limiting
steps in the folding process. When compared to other protein folding proteins
of a similar length, this system folds over six orders of magnitude more
slowly.Comment: 19 page
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