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