27 research outputs found

    Protostellar Disk Evolution Over Million-Year Timescales with a Prescription for Magnetized Turbulence

    Get PDF
    Magnetorotational instability (MRI) is the most promising mechanism behind accretion in low-mass protostellar disks. Here we present the first analysis of the global structure and evolution of non-ideal MRI-driven T-Tauri disks on million-year timescales. We accomplish this in a 1+1D simulation by calculating magnetic diffusivities and utilizing turbulence activity criteria to determine thermal structure and accretion rate without resorting to a 3-D magnetohydrodynamical (MHD) simulation. Our major findings are as follows. First, even for modest surface densities of just a few times the minimum-mass solar nebula, the dead zone encompasses the giant planet-forming region, preserving any compositional gradients. Second, the surface density of the active layer is nearly constant in time at roughly 10 g/cm2, which we use to derive a simple prescription for viscous heating in MRI-active disks for those who wish to avoid detailed MHD computations. Furthermore, unlike a standard disk with constant-alpha viscosity, the disk midplane does not cool off over time, though the surface cools as the star evolves along the Hayashi track. The ice line is firmly in the terrestrial planet-forming region throughout disk evolution and can move either inward or outward with time, depending on whether pileups form near the star. Finally, steady-state mass transport is a poor description of flow through an MRI-active disk. We caution that MRI activity is sensitive to many parameters, including stellar X-ray flux, grain size, gas/small grain mass ratio and magnetic field strength, and we have not performed an exhaustive parameter study here.Comment: Accepted for publication in Astrophysical Journal. 19 pages, including 8 figure

    The Principles of Social Order. Selected Essays of Lon L. Fuller, edited With an introduction by Kenneth I. Winston

    Get PDF
    The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields

    Polytype control of spin qubits in silicon carbide

    Get PDF
    Crystal defects can confine isolated electronic spins and are promising candidates for solid-state quantum information. Alongside research focusing on nitrogen vacancy centers in diamond, an alternative strategy seeks to identify new spin systems with an expanded set of technological capabilities, a materials driven approach that could ultimately lead to "designer" spins with tailored properties. Here, we show that the 4H, 6H and 3C polytypes of SiC all host coherent and optically addressable defect spin states, including spins in all three with room-temperature quantum coherence. The prevalence of this spin coherence shows that crystal polymorphism can be a degree of freedom for engineering spin qubits. Long spin coherence times allow us to use double electron-electron resonance to measure magnetic dipole interactions between spin ensembles in inequivalent lattice sites of the same crystal. Together with the distinct optical and spin transition energies of such inequivalent spins, these interactions provide a route to dipole-coupled networks of separately addressable spins.Comment: 28 pages, 5 figures, and supplementary information and figure

    Electrically and mechanically tunable electron spins in silicon carbide color centers

    Full text link
    The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab-initio simulations, we show that spin-spin interactions within SiC neutral divacancies give rise to spin states with an enhanced Stark effect, sub-10**-6 strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15-36%. These results establish SiC color centers as compelling systems for sensing nanoscale fields.Comment: 10 pages, 4 figures, 1 tabl

    Scientists' warning on climate change and insects

    Get PDF
    Climate warming is considered to be among the most serious of anthropogenic stresses to the environment, because it not only has direct effects on biodiversity, but it also exacerbates the harmful effects of other human-mediated threats. The associated consequences are potentially severe, particularly in terms of threats to species preservation, as well as in the preservation of an array of ecosystem services provided by biodiversity. Among the most affected groups of animals are insects—central components of many ecosystems—for which climate change has pervasive effects from individuals to communities. In this contribution to the scientists' warning series, we summarize the effect of the gradual global surface temperature increase on insects, in terms of physiology, behavior, phenology, distribution, and species interactions, as well as the effect of increased frequency and duration of extreme events such as hot and cold spells, fires, droughts, and floods on these parameters. We warn that, if no action is taken to better understand and reduce the action of climate change on insects, we will drastically reduce our ability to build a sustainable future based on healthy, functional ecosystems. We discuss perspectives on relevant ways to conserve insects in the face of climate change, and we offer several key recommendations on management approaches that can be adopted, on policies that should be pursued, and on the involvement of the general public in the protection effort

    An extended data-flow architecture for data analysis and visualization

    No full text

    Efficient alias-free rendering using bit-masks and look-up tables

    No full text

    Electrically and Mechanically Tunable Electron Spins in Silicon Carbide Color Centers

    No full text
    The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields
    corecore