27 research outputs found
Protostellar Disk Evolution Over Million-Year Timescales with a Prescription for Magnetized Turbulence
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
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
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
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
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
Electrically and Mechanically Tunable Electron Spins in Silicon Carbide Color Centers
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