11,537 research outputs found
Drastic annealing effects in transport properties of single crystals of the YbNi2B2C heavy fermion system
We report temperature dependent resistivity, specific heat, magnetic
susceptibility and thermoelectric power measurements made on the heavy fermion
system YbNi2B2C, for both as grown and annealed single crystals. Our results
demonstrate a significant variation in the temperature dependent electrical
resistivity and thermoelectric power between as grown crystals and crystals
that have undergone optimal (150 hour, 950 C) annealing, whereas the
thermodynamic properties: (c_p(T) and chi(T)) remain almost unchanged. We
interpret these results in terms of redistributions of local Kondo temperatures
associated with ligandal disorder for a small (~ 1%) fraction of the Yb sites.Comment: 5 pages, 4 figures, submitted to PR
Direct observation of Fe spin reorientation in single crystalline YbFe6Ge6
We have grown single crystals of YbFe6Ge6 and LuFe6Ge6 and characterized
their anisotropic behaviour through low field magnetic susceptibility,
field-dependent magnetization, resistivity and heat capacity measurements. The
Yb+3 valency is confirmed by LIII XANES measurements. YbFe6Ge6 crystals exhibit
a field-dependent, sudden reorientation of the Fe spins at about 63 K, a unique
effect in the RFe6Ge6 family (R = rare earths) where the Fe ions order
anti-ferromagnetically with Neel temperatures above 450 K and the R ions'
magnetism appears to behave independently. The possible origins of this unusual
behaviour of the ordered Fe moments in this compound are discussed.Comment: 12 pages, 8 figures, accepted in J. Phys.: Cond. Matte
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Numerical investigation of high-speed droplet impact using a multiscale two-fluid approach
A single droplet impact onto solid surfaces remains a fundamental and challenging topic in both experimental and numerical studies with significant importance in a plethora of industrial applications, ranging from printing technologies to fuel injection in internal combustion engines. Under high-speed impact conditions, additional complexities arise as a result of the prompt droplet splashing and the subsequent violent fragmentation; thus, different flow regimes and a vast spectrum of sizes for the produced secondary flow structures coexist in the flow field. The present work introduces a numerical methodology to capture the multiscale processes involved with respect to local topological characteristics. The proposed methodology concerns a compressible Σ-Υ two-fluid model with dynamic interface sharpening based on an advanced flow topology detection algorithm. The model has been developed in OpenFOAM® and provides the flexibility of dealing with the multiscale character of droplet splashing, by switching between a sharp and a diffuse interface within the Eulerian-Eulerian framework in segregated and dispersed flow regions, respectively. An additional transport equation for the interface surface area density (Σ) introduces important information for the sub-grid scale phenomena, which is exploited in the dispersed flow regions to provide an insight into the extended cloud of secondary droplets after impact on the target. A high-speed water droplet impact case has been examined and evaluated against new experimental data; these refer to a millimetre size droplet impacting a solid dry smooth surface at velocity as high as 150m/s, which corresponds to a Weber number of ~7.6×10^5. At the investigated impact conditions compressibility effects dominate the early stages of droplet splashing. A strong shock wave forms and propagates inside the droplet, where transonic Mach numbers occur; local Mach numbers up to 2.5 are observed for the expelled surrounding gas outside the droplet. The proposed numerical approach is found to capture relatively accurately the phenomena and provide significant information regarding the produced flow structure dimensions, which is not available from the experiment
A New Shear Estimator for Weak Lensing Observations
We present a new shear estimator for weak lensing observations which properly
accounts for the effects of a realistic point spread function (PSF). Images of
faint galaxies are subject to gravitational shearing followed by smearing with
the instrumental and/or atmospheric PSF. We construct a `finite resolution
shear operator' which when applied to an observed image has the same effect as
a gravitational shear applied prior to smearing. This operator allows one to
calibrate essentially any shear estimator. We then specialize to the case of
weighted second moment shear estimators. We compute the shear polarizability
which gives the response of an individual galaxy's polarization to a
gravitational shear. We then compute the response of the population of
galaxies, and thereby construct an optimal weighting scheme for combining shear
estimates from galaxies of various shapes, luminosities and sizes. We define a
figure of merit --- an inverse shear variance per unit solid angle --- which
characterizes the quality of image data for shear measurement. The new method
is tested with simulated image data. We discuss the correction for anisotropy
of the PSF and propose a new technique involving measuring shapes from images
which have been convolved with a re-circularizing PSF. We draw attention to a
hitherto ignored noise related bias and show how this can be analyzed and
corrected for. The analysis here draws heavily on the properties of real PSF's
and we include as an appendix a brief review, highlighting those aspects which
are relevant for weak lensing.Comment: 39 pages, 9 figure
Collapse of the ESR fine structure throughout the coherent temperature of the Gd-doped Kondo Semiconductor
Experiments on the Electron Spin Resonance (ESR) in the filled
skutterudite (), at temperatures
where the host resistivity manifests a smooth insulator-metal crossover,
provides evidence of the underlying Kondo physics associated with this system.
At low temperatures (below ), behaves
as a Kondo-insulator with a relatively large hybridization gap, and the
ESR spectra displays a fine structure with lorentzian line shape,
typical of insulating media. The electronic gap is attributed to the large
hybridization present in the coherent regime of a Kondo lattice, when Ce
4f-electrons cooperate with band properties at half-filling. Mean-field
calculations suggest that the electron-phonon interaction is fundamental at
explaining the strong 4f-electron hybridization in this filled skutterudite.
The resulting electronic structure is strongly temperature dependent, and at
about the system undergoes an insulator-to-metal
transition induced by the withdrawal of 4f-electrons from the Fermi volume, the
system becoming metallic and non-magnetic. The ESR fine structure
coalesces into a single dysonian resonance, as in metals. Still, our
simulations suggest that exchange-narrowing via the usual Korringa mechanism,
alone, is not capable of describing the thermal behavior of the ESR spectra in
the entire temperature region ( - K). We propose that temperature
activated fluctuating-valence of the Ce ions is the missing ingredient that,
added to the usual exchange-narrowing mechanism, fully describes this unique
temperature dependence of the ESR fine structure observed in
.Comment: 19 pages, 6 figure
Turbulent dissipation in the ISM: the coexistence of forced and decaying regimes and implications for galaxy formation and evolution
We discuss the dissipation of turbulent kinetic energy Ek in the global ISM
by means of 2-D, MHD, non-isothermal simulations in the presence of model
radiative heating and cooling. We argue that dissipation in 2D is
representative of that in three dimensions as long as it is dominated by shocks
rather than by a turbulent cascade. Energy is injected at a few isolated sites
in space, over relatively small scales, and over short time periods. This leads
to the coexistence of forced and decaying regimes in the same flow. We find
that the ISM-like flow dissipates its turbulent energy rapidly. In simulations
with forcing, the input parameters are the radius l_f of the forcing region,
the total kinetic energy e_k each source deposits into the flow, and the rate
of formation of those regions, sfr_OB. The global dissipation time t_d depends
mainly on l_f. In terms of measurable properties of the ISM, t_d >= Sigma_g
u_rms^2/(e_k sfr_OB), where Sigma_g is the average gas surface density and
u_rms is the rms velocity dispersion. For the solar neighborhood, t_d >=
1.5x10^7 yr. The global dissipation time is consistently smaller than the
crossing time of the largest energy-containing scales. In decaying simulations,
Ek decreases with time as t^-n, where n~0.8-0.9. This suggests a decay with
distance d as Ek\propto d^{-2n/(2-n)} in the mixed forced+decaying case. If
applicable to the vertical direction, our results support models of galaxy
evolution in which stellar energy injection provides significant support for
the gas disk thickness, but not models of galaxy formation in which this energy
injection is supposed to reheat an intra-halo medium at distances of up to
10-20 times the optical galaxy size, as the dissipation occurs on distances
comparable to the disk height.Comment: 23 pages, including figures. To appear in ApJ. Abstract abridge
Avaliação da resistência à penetração em diferentes sistemas em lote de reforma agrária no município de Itapeva, SP.
O sistema vivo, complexo e dinâmico do solo tem sido, no ultimo século, considerado apenas um suporte inerte para a obtenção de lucro pela agricultura convencional e os passivos de degradação de sua estrutura física, química e biológica já são entraves à melhoria de qualidade de vida dos agricultores, dentro da realidade de escassez de recursos e assistência técnica dos assentamentos rurais do estado de São Paulo (PRIMAVESI, 2002; GLIESSMAN, 2009). Um dos legados do manejo inadequado com maquinaria pesada e cultivo intensivo, deixado nos solos da maioria das áreas destinadas à assentamentos rurais, é a compactação ou o chamado "pé-de-grande" (RAMOS FILHO e PELLEGRINI, 2006). A destruição da bioestrutura do solo compromete a estabilidade, resistência às intempéries, fornecimento de água e oxigênio para os cultivos e, portanto, constitui-se na principal causa dos problemas de produtividade na agricultura tropical (PRIMAVESI, 2002). Dentro deste contexto têm surgido nos últimos anos diversas iniciativas de alternativas de manejo agroecológico do solo na forma de sistemas agroflorestais (SAFs). Pelo fato de aliar no mesmo espaço e/ou tempo o cultivo de grande diversidade de espécies agrícolas anuais, florestais perenes, herbáceas, arbustivas e arbóreas, os SAFs imitam e se fundamentam na estrutura multiestratificada complexa e na dinâmica dos processos ecológicos de ecossistemas florestais naturais (PENEIREIRO et al., 2002). A camada de estratos acima do solo e a "capa" de folhas o protege dos impactos do sol, chuva e vento, evitando a destruição de sua estrutura, a sua erosão e compactação. A incorporação frequente de matéria orgânica reconstrói o solo e a camada ampla e complexa de raízes abaixo da superfície promove a descompactação, aeração e aumento da infiltração de água. Experiências de SAF com objetivo de aliar conservação do solo com produção agrícola vêm sendo construídas no assentamento Pirituba II, localizado no município de Itapeva, SP, cujo histórico anterior é de produção convencional de grãos com uso intensivo de maquinaria pesada em extensas áreas de monocultivo. O presente estudo foi realizado no lote dos agricultores João Pereira da Silva e Eva Conceição da Silva que tem trabalhado com agrofloresta desde 2006. Dentro deste contexto, o monitoramento do sistema por indicadores de sustentabilidade se constitui em uma ferramenta essencial de qualificação da experiência e de base para melhorias de práticas de manejo, indicando modificações em atributos-chave na dinâmica do agroecossistema (DEPONTI et al., 2002). Neste sentido o objetivo principal deste trabalho foi analisar os efeitos do uso e manejo agroecológico do solo na forma de SAF, através de medições com equipamento de medição de compactação, o penetrômetro
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