A New Parameter In Accretion Disk Model

Taking optically thin accretion flows as an example, we investigate the dynamics and the emergent spectra of accretion flows with different outer boundary conditions (OBCs) and find that OBC plays an important role in accretion disk model. This is because the accretion equations describing the behavior of accretion flows are a set of {\em differential} equations, therefore, accretion is intrinsically an initial-value problem. We argue that optically thick accretion flow should also show OBC-dependent behavior. The result means that we should seriously consider the initial physical state of the accretion flow such as its angular momentum and its temperature. An application example to Sgr A$^*$ is presented.Comment: 6 pages, 4 figures, to appear in the Proceeding of "Pacific Rim Conference on Stellar Astrophysics", Aug. 1999, HongKong, Chin

A constraint-based methodology for product design with virtual reality

This paper presents a constraint-based methodology for product design with advanced virtual reality technologies. A hierarchically structured and constraint-based data model is developed to support product design from features to parts and further to assemblies in a VR environment. Product design in the VR environment is performed in an intuitive manner through precise constraint-based manipulations. Constraint-based manipulations are accompanied with automatic constraint recognition and precise constraint satisfaction to establish constraints between objects, and are further realized by allowable motions for precise 3D interactions in the VR environment. The allowable motions are represented as a mathematical matrix and derived from constraints between objects by constraint solving. A procedure-based degrees-of-freedom combination approach is presented for 3D constraint solving. A rule-based constraint recognition engine is developed for both constraint-based manipulations and implicitly incorporating constraints into the VR environment. An intuitive method is presented for recognizing pairs of mating features between assembly components. Examples are presented to demonstrate the efficacy of the proposed methodology

Influence of an external magnetic field on the decoherence of a central spin coupled to an antiferromagnetic environment

Using the spin wave approximation, we study the decoherence dynamics of a central spin coupled to an antiferromagnetic environment under the application of an external global magnetic field. The external magnetic field affects the decoherence process through its effect on the antiferromagnetic environment. It is shown explicitly that the decoherence factor which displays a Gaussian decay with time depends on the strength of the external magnetic field and the crystal anisotropy field in the antiferromagnetic environment. When the values of the external magnetic field is increased to the critical field point at which the spin-flop transition (a first-order quantum phase transition) happens in the antiferromagnetic environment, the decoherence of the central spin reaches its highest point. This result is consistent with several recent quantum phase transition witness studies. The influences of the environmental temperature on the decoherence behavior of the central spin are also investigated.Comment: 29 preprint pages, 4 figures, to appear in New Journal of Physic

Soil moisture and land use are major determinants of soil microbial community composition and biomass at a regional scale in northeastern China

Global environmental factors impact soil microbial communities and further affect organic matter decomposition, nutrient cycling and vegetation dynamic. However, little is known about the relative contributions of climate factors, soil properties, vegetation types, land management practices and spatial structure (which serves as a proxy for underlying effects of temperature and precipitation for spatial variation) on soil microbial community composition and biomass at large spatial scales. Here, we compared soil microbial communities using phospholipid fatty acid method across 7 land use types from 23 locations at a regional scale in northeastern China (850 × 50 km). The results showed that soil moisture and land use changes were most closely related to microbial community composition and biomass at the regional scale, while soil total C content and climate effects were weaker but still significant. Factors such as spatial structure, soil texture, nutrient availability and vegetation types were not important. Higher contributions of gram-positive bacteria were found in wetter soils, whereas higher contributions of gram-negative bacteria and fungi were observed in drier soils. The contributions of gram-negative bacteria and fungi were lower in heavily disturbed soils than historically disturbed and undisturbed soils. The lowest microbial biomass appeared in the wettest and driest soils. In conclusion, dominant climate and soil properties were not the most important drivers governing microbial community composition and biomass because of inclusion of irrigated and managed practices, and thus soil moisture and land use appear to be primary determinants of microbial community composition and biomass at the regional scale in northeastern China

Effects of rare earth elements on callus growth, soluble protein content, peroxidase activity and shoot differentiation of Echinacea angustifolia cultures in vitro

The effects of lanthanum nitrate (La3+) and cerium nitrate (Ce4+) on Echinacea angustifolia callus growth and subculture were studied by the measurement of callus fresh weight, dry weight and time-course curve. The effects of La3+ on soluble protein content, peroxidase activity and shoot differentiation of callus were also investigated. Low concentrations of La3+ and Ce4+ (0.01, 0.1 and 1 mg/l) showed enhancing action on callus growth while the suppress effect was found at high concentrationtreatments (10 and 100 mg/l). The result from time-course experiment indicated that La3+ or Ce4+ showed promotion on callus growth after 15 days cultured and Ce4+ displayed a better effect than La3+. Differentproportion mixture of La3+ and Ce4+ at 1 mg/l in total showed stimulating action on callus growth and the optimum proportion was the mixture of 40% La3+ and 60% Ce4+. Inhibitory effects were observed when the calli derived from the treatments with La3+ and Ce4+ were subcultured for the first generation. The dose-dependent effects of La3+ on soluble protein content, peroxidase activity and shoot differentiation were also noticed. The optimum concentration for callus growth and shoot differentiation was 0.1 mg/l and the green, loose calli with low percentage of brown callus were obtained at this concentration

Effect of process parameters on the force parameters in warm skew rolling of copper ball

In order to better control the forming quality of copper ball by warm skew rolling process, a Finite Element Model (FEM) of copper ball warm skew rolling for the coupling of thermal and mechanical was established. The influence of process parameters on force and rolling torque was analyzed by using single factor research method. The results show that the smaller the cross angle, the lower the rolling temperature, the slower the rolling rotation speed, the greater the forming force and rolling torque, the more difficult for forming. The optimum rolling temperature is 600 °C; the optimum cross angle is 2,5°; the optimum rolling rotation speed is 60 rpm

Probe of Spin Dynamics in Superconducting NbN Thin Films via Spin Pumping

The emerging field of superconductor (SC) spintronics has attracted intensive attentions recently. Many fantastic spin dependent properties in SC have been discovered, including the observation of large magnetoresistance, long spin lifetimes and the giant spin Hall effect in SC, as well as spin supercurrent in Josephson junctions, etc. Regarding the spin dynamic in SC films, few studies has been reported yet. Here, we report the investigation of the spin dynamics in an s-wave superconducting NbN film via spin pumping from an adjacent insulating ferromagnet GdN layer. A profound coherence peak of the Gilbert damping is observed slightly below the superconducting critical temperature of the NbN layer, which is consistent with recent theoretical studies. Our results further indicate that spin pumping could be a powerful tool for investigating the spin dynamics in 2D crystalline superconductors.Comment: 11 pages, 4 figures, and S