11,001 research outputs found
Connected Hopf algebras and iterated Ore extensions
We investigate when a skew polynomial extension T = R[x; {\sigma}, {\delta}]
of a Hopf algebra R admits a Hopf algebra structure, substantially generalising
a theorem of Panov. When this construction is applied iteratively in
characteristic 0 one obtains a large family of connected noetherian Hopf
algebras of finite Gelfand-Kirillov dimension, including for example all
enveloping algebras of finite dimensional solvable Lie algebras and all
coordinate rings of unipotent groups. The properties of these Hopf algebras are
investigated
Neutrino Emission From Direct Urca Processes in Pion Condensed Quark Matter
We study neutrino emission from direct Urca processes in pion condensed quark
matter. In compact stars with high baryon density, the emission is dominated by
the gapless modes of the pion condensation which leads to an enhanced
emissivity. While for massless quarks the enhancement is not remarkable, the
emissivity is significantly larger and the cooling of the condensed matter is
considerably faster than that in normal quark matter when the mass difference
between - and -quarks is sizable.Comment: 12 pages,6 figures, published versio
Liquid Crystal-Solid Interface Structure at the Antiferroelectric-Ferroelectric Phase Transition
Total Internal Reflection (TIR) is used to probe the molecular organization
at the surface of a tilted chiral smectic liquid crystal at temperatures in the
vicinity of the bulk antiferroelectric-ferroelectric phase transition. Data are
interpreted using an exact analytical solution of a real model for
ferroelectric order at the surface. In the mixture T3, ferroelectric surface
order is expelled with the bulk ferroelectric-antiferroelectric transition. The
conditions for ferroelectric order at the surface of an antiferroelectric bulk
are presented
Mgb2 Nonlinear Properties Investigated under Localized High RF Magnetic Field Excitation
In order to increase the accelerating gradient of Superconducting Radio
Frequency (SRF) cavities, Magnesium Diboride (MgB2) opens up hope because of
its high transition temperature and potential for low surface resistance in the
high RF field regime. However, due to the presence of the small superconducting
gap in the {\pi} band, the nonlinear response of MgB2 is potentially quite
large compared to a single gap s-wave superconductor (SC) such as Nb.
Understanding the mechanisms of nonlinearity coming from the two-band structure
of MgB2, as well as extrinsic sources, is an urgent requirement. A localized
and strong RF magnetic field, created by a magnetic write head, is integrated
into our nonlinear-Meissner-effect scanning microwave microscope [1]. MgB2
films with thickness 50 nm, fabricated by a hybrid physical-chemical vapor
deposition technique on dielectric substrates, are measured at a fixed location
and show a strongly temperature-dependent third harmonic response. We propose
that at least two mechanisms are responsible for this nonlinear response, one
of which involves vortex nucleation and penetration into the film. [1] T. M.
Tai, X. X. Xi, C. G. Zhuang, D. I. Mircea, S. M. Anlage, "Nonlinear Near-Field
Microwave Microscope for RF Defect Localization in Superconductors", IEEE
Trans. Appl. Supercond. 21, 2615 (2011).Comment: 6 pages, 6 figure
Household visitation during the COVID-19 pandemic
The COVID-19 pandemic has posed novel risks related to the indoor mixing of individuals from different households and challenged policymakers to adequately regulate this behaviour. While in many cases household visits are necessary for the purpose of social care, they have been linked to broadening community transmission of the virus. In this study we propose a novel, privacy-preserving framework for the measurement of household visitation at national and regional scales, making use of passively collected mobility data. We implement this approach in England from January 2020 to May 2021. The measures expose significant spatial and temporal variation in household visitation patterns, impacted by both national and regional lockdown policies, and the rollout of the vaccination programme. The findings point to complex social processes unfolding differently over space and time, likely informed by variations in policy adherence, vaccine relaxation, and regional interventions
Attribution of aerosol light absorption to black carbon, brown carbon, and dust in China ? interpretations of atmospheric measurements during EAST-AIRE
International audienceBlack carbon, brown carbon, and mineral dust are three of the most important light absorbing aerosols. Their optical properties differ greatly and are distinctive functions of the wavelength of light. Most optical instruments that quantify light absorption, however, are unable to distinguish one type of absorbing aerosol from another. It is thus instructive to separate total absorption from these different light absorbers to gain a better understanding of the optical characteristics of each aerosol type. During the EAST-AIRE (East Asian Study of Tropospheric Aerosols: an International Regional Experiment) campaign near Beijing, we measured light scattering using a nephelometer, and light absorption using an aethalometer and a particulate soot absorption photometer. We also measured the total mass concentrations of carbonaceous (elemental and organic carbon) and inorganic particulates, as well as aerosol number and mass distributions. We were able to identify periods during the campaign that were dominated by dust, biomass burning, fresh (industrial) chimney plumes, other coal burning pollution, and relatively clean (background) air for Northern China. Each of these air masses possessed distinct intensive optical properties, including the single scatter albedo and Ã…ngstrom exponents. Based on the wavelength-dependence and particle size distribution, we apportioned total light absorption to black carbon, brown carbon, and dust; their mass absorption efficiencies at 550 nm were estimated to be 9.5, 0.5, and 0.03 m2/g, respectively. While agreeing with the common consensus that BC is the most important light absorber in the mid-visible, we demonstrated that brown carbon and dust could also cause significant absorption, especially at shorter wavelengths
1/N_c- expansion of the quark condensate at finite temperature
Previously the quark and meson properties in a many quark system at finite
temperature have been studied within effective QCD approaches in the Hartree
approximation. In the present paper we consider the influence of the mesonic
correlations on the quark self-energy and on the quark propagator within a
systematic - expansion. Using a general separable ansatz for the
nonlocal interaction, we derive a selfconsistent equation for the
correction to the quark propagator. For a separable model with cut-off
formfactor, we obtain a decrease of the condensate of the order of 20\% at zero
temperature. A lowering the critical temperature for the onset of the chiral
restoration transition due to the inclusion of mesonic correlations is obtained
what seems to be closer to the results from lattice calculations.Comment: 19 pages, REVTeX, 5 figure
Electronic and magnetic states in doped LaCoO_3
The electronic and magnetic states in doped perovskite cobaltites, (La,
Sr)CoO_3, are studied in the numerically exact diagonalization method on
Co_2O_{11} clusters. For realistic parameter values, it is shown that a high
spin state and an intermediate spin state coexist in one-hole doped clusters
due to strong p-d mixing. The magnetic states in the doped cobaltites obtained
in the calculation explain various experimental results.Comment: 4 pages, 2 figures, epsfj.st
Phase field modeling and computer implementation: A review
This paper presents an overview of the theories and computer implementation
aspects of phase field models (PFM) of fracture. The advantage of PFM over
discontinuous approaches to fracture is that PFM can elegantly simulate
complicated fracture processes including fracture initiation, propagation,
coalescence, and branching by using only a scalar field, the phase field. In
addition, fracture is a natural outcome of the simulation and obtained through
the solution of an additional differential equation related to the phase field.
No extra fracture criteria are needed and an explicit representation of a crack
surface as well as complex track crack procedures are avoided in PFM for
fracture, which in turn dramatically facilitates the implementation. The PFM is
thermodynamically consistent and can be easily extended to multi-physics
problem by 'changing' the energy functional accordingly. Besides an overview of
different PFMs, we also present comparative numerical benchmark examples to
show the capability of PFMs
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