18,364 research outputs found
Crystal growth and magnetic structure of MnBi2Te4
Millimeter-sized MnBiTe single crystals are grown out of Bi-Te flux
and characterized by measuring magnetic and transport properties, scanning
tunneling microscope (STM) and spectroscopy (STS). The magnetic structure of
MnBiTe below T is determined by powder and single crystal neutron
diffraction measurements. Below T=24\,K, Mn moments order
ferromagnetically in the \textit{ab} plane but antiferromagnetically along the
crystallographic \textit{c} axis. The ordered moment is 4.04(13) /Mn
at 10\,K and aligned along the crystallographic \textit{c}-axis. The electrical
resistivity drops upon cooling across T or when going across the
metamagnetic transition in increasing fields below T. A critical scattering
effect was observed in the vicinity of T in the temperature dependence of
thermal conductivity. However, A linear temperature dependence was observed for
thermopower in the temperature range 2K-300K without any anomaly around T.
These indicate that the magnetic order in Mn-Te layer has negligible effect on
the electronic band structure, which makes possible the realization of proposed
topological properties in MnBiTe after fine tuning of the electronic
band structure
A natural-norm Successive Constraint Method for inf-sup lower bounds
We present a new approach for the construction of lower bounds for the inf-sup stability constants required in a posteriori error analysis of reduced basis approximations to affinely parametrized partial differential equations. We combine the “linearized” inf-sup statement of the natural-norm approach with the approximation procedure of the Successive Constraint Method (SCM): the former (natural-norm) provides an economical parameter expansion and local concavity in parameter—a small(er) optimization problem which enjoys intrinsic lower bound properties; the latter (SCM) provides a systematic optimization framework—a Linear Program (LP) relaxation which readily incorporates continuity and stability constraints. The natural-norm SCM requires a parameter domain decomposition: we propose a greedy algorithm for selection of the SCM control points as well as adaptive construction of the optimal subdomains. The efficacy of the natural-norm SCM is illustrated through numerical results for two types of non-coercive problems: the Helmholtz equation (for acoustics, elasticity, and electromagnetics), and the convection–diffusion equation.United States. Air Force Office of Scientific Research (Grant No. FA 9550-07-1-0425
Liquid-like thermal conduction in a crystalline solid
A solid conducts heat through both transverse and longitudinal acoustic
phonons, but a liquid employs only longitudinal vibrations. Here, we report
that the crystalline solid AgCrSe2 has liquid-like thermal conduction. In this
compound, Ag atoms exhibit a dynamic duality that they are exclusively involved
in intense low-lying transverse acoustic phonons while they also undergo local
fluctuations inherent in an order-to-disorder transition occurring at 450 K. As
a consequence of this extreme disorder-phonon coupling, transverse acoustic
phonons become damped as approaching the transition temperature, above which
they are not defined anymore because their lifetime is shorter than the
relaxation time of local fluctuations. Nevertheless, the damped longitudinal
acoustic phonon survives for thermal transport. This microscopic insight might
reshape the fundamental idea on thermal transport properties of matter and
facilitates the optimization of thermoelectrics.Comment: four figures, supplemental informatio
Mercury Contamination in Forest and Freshwater Ecosystems in the Northeastern United States
Eastern North America receives elevated atmospheric mercury deposition from a combination of local, regional, and global sources. Anthropogenic emissions originate largely from electric utilities, incinerators, and industrial processes. The mercury species in these emissions have variable atmospheric residence times, which influence their atmospheric transport and deposition patterns. Forested regions with a prevalence of wetlands and of unproductive surface waters promote high concentrations of mercury in freshwater biota and thus are particularly sensitive to mercury deposition. Through fish consumption, humans and wildlife are exposed to methylmercury, which markedly bioaccumulates up the freshwater food chain. Average mercury concentrations in yellow perch fillets exceed the Environmental Protection Agency’s human health criterion across the region, and mercury concentrations are high enough in piscivorous wildlife to cause adverse behavioral, physiological, and reproductive effects. Initiatives are under way to decrease mercury emissions from electric utilities in the United States by roughly 70%
Electrical Control of Plasmon Resonance with Graphene
Surface plasmon, with its unique capability to concentrate light into
sub-wavelength volume, has enabled great advances in photon science, ranging
from nano-antenna and single-molecule Raman scattering to plasmonic waveguide
and metamaterials. In many applications it is desirable to control the surface
plasmon resonance in situ with electric field. Graphene, with its unique
tunable optical properties, provides an ideal material to integrate with
nanometallic structures for realizing such control. Here we demonstrate
effective modulation of the plasmon resonance in a model system composed of
hybrid graphene-gold nanorod structure. Upon electrical gating the strong
optical transitions in graphene can be switched on and off, which leads to
significant modulation of both the resonance frequency and quality factor of
plasmon resonance in gold nanorods. Hybrid graphene-nanometallic structures, as
exemplified by this combination of graphene and gold nanorod, provide a general
and powerful way for electrical control of plasmon resonances. It holds promise
for novel active optical devices and plasmonic circuits at the deep
subwavelength scale
Recent breakthroughs in carrier depletion based silicon optical modulators
The majority of the most successful optical modulators in silicon demonstrated in recent years operate via the plasma dispersion effect and are more specifically based upon free carrier depletion in a silicon rib waveguide. In this work we overview the different types of free carrier depletion type optical modulators in silicon. A summary of some recent example devices for each configuration is then presented together with the performance that they have achieved. Finally an insight into some current research trends involving silicon based optical modulators is provided including integration, operation in the mid-infrared wavelength range and application in short and long haul data transmission link
Magnetic Coherence in Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO observed a {\it magnetic coherence effect}, i.e.,
strong frequency and momentum dependent changes of the spin susceptibility,
, in the superconducting phase. We show that this effect is a direct
consequence of changes in the damping of incommensurate antiferromagnetic spin
fluctuations due to the appearance of a d-wave gap in the fermionic spectrum.
Our theoretical results provide a quantitative explanation for the weak
momentum dependence of the observed spin-gap. Moreover, we predict {\bf (a)} a
Fermi surface in LaSrCuO which is closed around up
to optimal doping, and {\bf (b)} similar changes in for all cuprates
with an incommensurate magnetic response.Comment: 5 pages, 4 figures, Fig.3 is in colo
Nkx2-5 and Sarcospan genetically interact in the development of the muscular ventricular septum of the heart
The muscular ventricular septum separates the flow of oxygenated and de-oxygenated blood in air-breathing vertebrates. Defects within it, termed muscular ventricular septal defects (VSDs), are common, yet less is known about how they arise than rarer heart defects. Mutations of the cardiac transcription factor NKX2-5 cause cardiac malformations, including muscular VSDs. We describe here a genetic interaction between Nkx2-5 and Sarcospan (Sspn) that affects the risk of muscular VSD in mice. Sspn encodes a protein in the dystrophin-glycoprotein complex. Sspn knockout (Sspn(KO)) mice do not have heart defects, but Nkx2-5(+/−)/Sspn(KO) mutants have a higher incidence of muscular VSD than Nkx2-5(+/−) mice. Myofibers in the ventricular septum follow a stereotypical pattern that is disrupted around a muscular VSD. Subendocardial myofibers normally run in parallel along the left ventricular outflow tract, but in the Nkx2-5(+/−)/Sspn(KO) mutant they commonly deviate into the septum even in the absence of a muscular VSD. Thus, Nkx2-5 and Sspn act in a pathway that affects the alignment of myofibers during the development of the ventricular septum. The malalignment may be a consequence of a defect in the coalescence of trabeculae into the developing ventricular septum, which has been hypothesized to be the mechanistic basis of muscular VSDs
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