14,792 research outputs found
Constraints on the three-fluid model of curvaton decay
A three fluid system describing the decay of the curvaton is studied by
numerical and analytical means. We place constraints on the allowed interaction
strengths between the fluids and initial curvaton density by requiring that the
curvaton decays before nucleosynthesis while nucleosynthesis, radiation-matter
equality and decoupling occur at correct temperatures. We find that with a
continuous, time-independent interaction, a small initial curvaton density is
naturally preferred along with a low reheating temperature. Allowing for a
time-dependent interaction, this constraint can be relaxed. In both cases, a
purely adiabatic final state can be generated, but not without fine-tuning.
Unlike in the two fluid system, the time-dependent interactions are found to
have a small effect on the curvature perturbation itself due to the different
nature of the system. The presence of non-gaussianity in the model is
discussed.Comment: 9 pages, 10 figure
Evidence for multiple superconducting gaps in optimally doped BaFeCoAs from infrared spectroscopy
We performed combined infrared reflection and ellipsometry measurements of
the in-plane optical reponse of single crystals of the pnictide high
temperature superconductor BaFeCoAs with = 24.5
K. We observed characteristic superconductivity-induced changes which provide
evidence for at least three different energy gaps. We show that a BCS-model of
isotropic gaps with 2 of 3.1, 4.7, and 9.2 reproduces the
experimental data rather well. We also determine the low-temperature value of
the in-plane magnetic penetration depth of 270 nm
Cohomological aspects of Abelian gauge theory
We discuss some aspects of cohomological properties of a two-dimensional free
Abelian gauge theory in the framework of BRST formalism. We derive the
conserved and nilpotent BRST- and co-BRST charges and express the Hodge
decomposition theorem in terms of these charges and a conserved bosonic charge
corresponding to the Laplacian operator. It is because of the topological
nature of free U(1) gauge theory that the Laplacian operator goes to zero when
equations of motion are exploited. We derive two sets of topological invariants
which are related to each-other by a certain kind of duality transformation and
express the Lagrangian density of this theory as the sum of terms that are
BRST- and co-BRST invariants. Mathematically, this theory captures together
some of the key features of Witten- and Schwarz type of topological field
theories.Comment: 12 pages, LaTeX, no figures, Title and text have been slightly
changed, Journal reference is given and a reference has been adde
Variational Mote Carlo Study of Flat Band Ferromagnetism -- Application to CeRh_3 B_2
A new mechanism for ferromagnetism in CeRh_3B_2 is proposed on the basis of
variational Monte Carlo results. In a one-dimensional Anderson lattice where
each 4f electron hybridizes with a ligand orbital between neighboring Ce sites,
ferromagnetism is stabilized due to a nearly flat band which is a mixture of
conduction and 4f electron states. Because of the strong spin-orbit interaction
in 4f electron states, and of considerable amount of hybridization in the
nearly flat band, the magnetic moments from 4f and conduction electrons tend to
cancel each other. The resultant ferromagnetic moment becomes smaller as
compared with the local 4f moment, and the Fermi surface in the ferromagnetic
ground state is hardly affected by the presence of 4f electrons. These
theoretical results are consistent with experimental observations in CeRh_3B_2
by neutron scattering and dHvA effects.Comment: to be published in J.Phys.Soc.Jp
Abelian 2-form gauge theory: special features
It is shown that the four -dimensional (4D) free Abelian 2-form
gauge theory provides an example of (i) a class of field theoretical models for
the Hodge theory, and (ii) a possible candidate for the quasi-topological field
theory (q-TFT). Despite many striking similarities with some of the key
topological features of the two -dimensional (2D) free Abelian (and
self-interacting non-Abelian) gauge theories, it turns out that the 4D free
Abelian 2-form gauge theory is {\it not} an exact TFT. To corroborate this
conclusion, some of the key issues are discussed. In particular, it is shown
that the (anti-)BRST and (anti-)co-BRST invariant quantities of the 4D 2-form
Abelian gauge theory obey the recursion relations that are reminiscent of the
exact TFTs but the Lagrangian density of this theory is not found to be able to
be expressed as the sum of (anti-)BRST and (anti-)co-BRST exact quantities as
is the case with the {\it topological} 2D free Abelian (and self-interacting
non-Abelian) gauge theories.Comment: LaTeX, 23 pages, journal ref. give
Depth profile of the ferromagnetic order in a YBaCuO / LaCaMnO superlattice on a LSAT substrate: a polarized neutron reflectometry study
Using polarized neutron reflectometry (PNR) we have investigated a
YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed
laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the
high structural quality of the superlattice and the substrate, the specular
reflectivity signal extends with a high signal-to-background ratio beyond the
fourth order superlattice Bragg peak. This allows us to obtain more detailed
and reliable information about the magnetic depth profile than in previous PNR
studies on similar superlattices that were partially impeded by problems
related to the low temperature structural transitions of the SrTiO3 substrates.
In agreement with the previous reports, our PNR data reveal a strong magnetic
proximity effect showing that the depth profile of the magnetic potential
differs significantly from the one of the nuclear potential that is given by
the YBCO and LCMO layer thickness. We present fits of the PNR data using
different simple block-like models for which either a ferromagnetic moment is
induced on the YBCO side of the interfaces or the ferromagnetic order is
suppressed on the LCMO side. We show that a good agreement with the PNR data
and with the average magnetization as obtained from dc magnetization data can
only be obtained with the latter model where a so-called depleted layer with a
strongly suppressed ferromagnetic moment develops on the LCMO side of the
interfaces. The models with an induced ferromagnetic moment on the YBCO side
fail to reproduce the details of the higher order superlattice Bragg peaks and
yield a wrong magnitude of the average magnetization. We also show that the PNR
data are still consistent with the small, ferromagnetic Cu moment of 0.25muB
that was previously identified with x-ray magnetic circular dichroism and x-ray
resonant magnetic reflectometry measurements on the same superlattice.Comment: 11 pages, 7 figure
Coexistence and competition of magnetism and superconductivity on the nanometer scale in underdoped BaFe1.89Co0.11As2
We report muon spin rotation (muSR) and infrared (IR) spectroscopy
experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and
superconductivity (SC) coexist and compete on the nanometer length scale. Our
combined data reveal a bulk magnetic order, likely due to an incommensurate
spin density wave (SDW), which develops below Tmag \approx 32 K and becomes
reduced in magnitude (but not in volume) below Tc = 21.7 K. A slowly
fluctuating precursor of the SDW seems to develop alrady below the structural
transition at Ts \approx 50 K. The bulk nature of SC is established by the muSR
data which show a bulk SC vortex lattice and the IR data which reveal that the
majority of low-energy states is gapped and participates in the condensate at T
<< Tc
Absolutely anticommuting (anti-)BRST symmetry transformations for topologically massive Abelian gauge theory
We demonstrate the existence of the nilpotent and absolutely anticommuting
Becchi-Rouet-Stora-Tyutin (BRST) and anti-BRST symmetry transformations for the
four (3 + 1)-dimensional (4D) topologically massive Abelian U(1) gauge theory
that is described by the coupled Lagrangian densities (which incorporate the
celebrated (B \wedge F) term). The absolute anticommutativity of the (anti-)
BRST symmetry transformations is ensured by the existence of a Curci-Ferrari
type restriction that emerges from the superfield formalism as well as from the
equations of motion that are derived from the above coupled Lagrangian
densities. We show the invariance of the action from the point of view of the
symmetry considerations as well as superfield formulation. We discuss,
furthermore, the topological term within the framework of superfield formalism
and provide the geometrical meaning of its invariance under the (anti-) BRST
symmetry transformations.Comment: LaTeX file, 22 pages, journal versio
Hematoma Enlargement Among Patients with Traumatic Brain Injury: Analysis of a Prospective Multicenter Clinical Trial
Observational studies suggest that hematomas continue to enlarge during hospitalization in patients with traumatic brain injury (TBI). There is limited data regarding factors associated with hematoma enlargement and on whether hematoma enlargement contributes directly to death and disability in patients with TBI.
We analyzed data collected as part of the Resuscitation Outcomes Consortium Hypertonic Saline and TBI Study. Hematoma enlargement was ascertained and collected as a predefined safety endpoint. We evaluated the effect of hematoma enlargement on the risk of death and disability at 6 months based on the Extended Glasgow Outcome Scale (GOSE) (dichotomized as \u3e4 or ≤4) using stepwise logistic regression analysis. We adjusted for age (continuous variable), admission GCS score (dichotomized at \u3e5 and ≤5), and computed tomography (CT) scan classification (Marshall grades entered as a categorical variable).
Of the 1200 patients with severe TBI analyzed, 238 (19.8%) patients were reported to have hematoma enlargement as an adverse event. The proportion of patients who reached favorable outcome at 6 months was significantly lower (defined by GOSE of \u3e4) among patients with hematoma enlargement (29.0% vs. 40.1%, p\u3c.0001). The proportion of patients who died within 6 months was significantly higher among patients with hematoma enlargement (31.9% vs. 20.7%, p\u3c.0001). After adjusting for age, admission GCS score, and initial injury score, the odds of favorable outcome was lower in patients with hematoma enlargement (odds ratio 0.7, 95% confidence interval [CI]; 0.5–0.97).
Our results suggest that hematoma enlargement may be a direct contributor to death and disability in patients with TBI at 6 months. Future clinical trials must continue to evaluate new therapeutic interventions aimed at reducing hematoma enlargement with a favorable risk benefit ratio in patients with TBI
A field-theoretic model for Hodge theory
We demonstrate that the four (3 + 1)-dimensional free Abelian 2-form gauge
theory presents a tractable field theoretical model for the Hodge theory where
the well-defined symmetry transformations correspond to the de Rham
cohomological operators of differential geometry. The conserved charges,
corresponding to the above continuous symmetry transformations, obey an algebra
that is reminiscent of the algebra obeyed by the cohomological operators. The
discrete symmetry transformation of the theory represents the realization of
the Hodge duality operation that exists in the relationship between the
exterior and co-exterior derivatives of differential geometry. Thus, we provide
the realizations of all the mathematical quantities, associated with the de
Rham cohomological operators, in the language of the symmetries of the present
4D free Abelian 2-form gauge theory.Comment: LaTeX file, 24 pages, journal reference is give
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