9,912 research outputs found
Turbulent Stresses in Local Simulations of Radiation-Dominated Accretion Disks, and the Possibility of the LIghtman-Eardley Instability
We present the results of a series of radiation-MHD simulations of a local
patch of an accretion disk, with fixed vertical gravity profile but with
different surface mass densities and a broad range of radiation to gas pressure
ratios. Each simulation achieves a thermal equilibrium that lasts for many
cooling times. After averaging over times long compared to a cooling time, we
find that the vertically integrated stress is approximately proportional to the
vertically-averaged total thermal (gas plus radiation) pressure. We map
out--for the first time on the basis of explicit physics--the thermal
equilibrium relation between stress and surface density: the stress decreases
(increases) with increasing surface mass density when the simulation is
radiation (gas) pressure dominated. The dependence of stress on surface mass
density in the radiation pressure dominated regime suggests the possibility of
a Lightman-Eardley inflow instability, but global simulations or shearing box
simulations with much wider radial boxes will be necessary to confirm this and
determine its nonlinear behavior.Comment: accepted for publication in The Astrophysical Journa
First-principles study on scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
Scanning tunneling microscopy images of hydrogen-terminated Si(110) surfaces
are studied using first-principles calculations. Our results show that the
calculated filled-state images and local density of states are consistent with
recent experimental results, and the empty-state images appear significantly
different from the filled-state ones. To elucidate the origin of this
difference, we examined in detail the local density of states, which affects
the images, and found that the bonding and antibonding states of surface
silicon atoms largely affect the difference between the filled- and empty-state
images.Comment: 4 pages, and 4 figure
Si/Ge hole-tunneling double-barrier resonant tunneling diodes formed on sputtered flat Ge layers
We have demonstrated Si/Ge hole-tunneling double-barrier resonant tunneling diodes (RTDs) formed on flat Ge layers with a relaxation rate of 89% by our proposed method; in this method, the flat Ge layers can be directly formed on highly B-doped Si(001) substrates using our proposed sputter epitaxy method. The RTDs exhibit clear negative differential resistance effects in the static current–voltage (I–V) curves at room temperature. The quantized energy level estimation suggests that resonance peaks that appeared in the I–V curves are attributed to hole tunneling through the first heavy- and light-hole energy levels
Transport properties in network models with perfectly conducting channels
We study the transport properties of disordered electron systems that contain
perfectly conducting channels. Two quantum network models that belong to
different universality classes, unitary and symplectic, are simulated
numerically. The perfectly conducting channel in the unitary class can be
realized in zigzag graphene nano-ribbons and that in the symplectic class is
known to appear in metallic carbon nanotubes. The existence of a perfectly
conducting channel leads to novel conductance distribution functions and a
shortening of the conductance decay length.Comment: 4 pages, 6 figures, proceedings of LT2
Estimating the Prompt Electromagnetic Luminosity of a Black Hole Merger
Although recent work in numerical relativity has made tremendous strides in
quantifying the gravitational wave luminosity of black hole mergers, very
little is known about the electromagnetic luminosity that might occur in
immediate conjunction with these events. We show that whenever the heat
deposited in the gas near a pair of merging black holes is proportional to its
total mass, and the surface density of the gas in the immediate vicinity is
greater than the (quite small) amount necessary to make it optically thick, the
characteristic scale of the luminosity emitted in direct association with the
merger is the Eddington luminosity independent of the gas mass. The duration of
the photon signal is proportional to the gas mass, and is generally rather
longer than the merger event. At somewhat larger distances, dissipation
associated with realigning the gas orbits to the new spin orientation of the
black hole can supplement dissipation of the energy gained from orbital
adjustment to the mass lost in gravitational radiation; these two heat sources
can combine to augment the electromagnetic radiation over longer timescales.Comment: 14 pages. Ap J, in pres
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