The causal structure of dynamical charged black holes

We study the causal structure of dynamical charged black holes, with a sufficient number of massless fields, using numerical simulations. Neglecting Hawking radiation, the inner horizon is a null Cauchy horizon and a curvature singularity due to mass inflation. When we include Hawking radiation, the inner horizon becomes space-like and is separated from the Cauchy horizon, which is parallel to the out-going null direction. Since a charged black hole must eventually transit to a neutral black hole, we studied the neutralization of the black hole and observed that the inner horizon evolves into a space-like singularity, generating a Cauchy horizon which is parallel to the in-going null direction. Since the mass function is finite around the inner horizon, the inner horizon is regular and penetrable in a general relativistic sense. However, since the curvature functions become trans-Planckian, we cannot saymore about the region beyond the inner horizon, and it is natural to say that there is a 'physical' space-like singularity. However, if we assume an exponentially large number of massless scalar fields, our results can be extended beyond the inner horizon. In this case, strong cosmic censorship and black hole complementarity can be violated.Comment: 23 pages, 23 figure

Dynamics of false vacuum bubbles: beyond the thin shell approximation

We numerically study the dynamics of false vacuum bubbles which are inside an almost flat background; we assumed spherical symmetry and the size of the bubble is smaller than the size of the background horizon. According to the thin shell approximation and the null energy condition, if the bubble is outside of a Schwarzschild black hole, unless we assume Farhi-Guth-Guven tunneling, expanding and inflating solutions are impossible. In this paper, we extend our method to beyond the thin shell approximation: we include the dynamics of fields and assume that the transition layer between a true vacuum and a false vacuum has non-zero thickness. If a shell has sufficiently low energy, as expected from the thin shell approximation, it collapses (Type 1). However, if the shell has sufficiently large energy, it tends to expand. Here, via the field dynamics, field values of inside of the shell slowly roll down to the true vacuum and hence the shell does not inflate (Type 2). If we add sufficient exotic matters to regularize the curvature near the shell, inflation may be possible without assuming Farhi-Guth-Guven tunneling. In this case, a wormhole is dynamically generated around the shell (Type 3). By tuning our simulation parameters, we could find transitions between Type 1 and Type 2, as well as between Type 2 and Type 3. Between Type 2 and Type 3, we could find another class of solutions (Type 4). Finally, we discuss the generation of a bubble universe and the violation of unitarity. We conclude that the existence of a certain combination of exotic matter fields violates unitarity.Comment: 40 pages, 41 figure

The effect of strain on hot‐electron and hole longitudinal diffusion and noise in Si and Si0.9Ge0.1

Monte Carlo methods are used to model the electron and hole high‐field transport in both unstrained and compressively strained silicon and silicon‐germanium alloy. The data are analyzed to determine in what way the thermal noise properties of the carriers are affected by compressive, in‐plane strain. Results include the longitudinal diffusion coefficient, the longitudinal noise temperature, and the longitudinal noise spectral density, for electric fields in the range of 0–20 kV/cm. The results are qualitatively similar for silicon with 1% compressive biaxial strain and for Si0.9Ge0.1/Si(001). The effects of strain are found to be more pronounced for electrons than for holes and are primarily related to changes in the conductivity effective mass. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70547/2/JAPIAU-78-9-5454-1.pd

Calculation of electron and hole impact ionization coefficients in SiGe alloys

Silicon–germanium alloys offer a system where the ratio of the electron impact ionization coefficient (α) and hole impact ionization coefficient (β) varies from a value larger than unity (in high silicon content alloys), to a value smaller than unity (in high germanium content alloys). We report results for α and β for this alloy system. The electron results are based on a multivalley nonparabolic band structure. The hole results are based on a six‐band k⋅p model for low energies coupled to an eight‐band model for high energies. We find that for the alloy Si0.4Ge0.6, α∼β. Alloy scattering is found to play an important role in determining the impact ionization coefficient. For compositions around Si0.5Ge0.5, the strong alloy scattering is found to suppress the impact ionization coefficient. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70702/2/JAPIAU-80-12-6773-1.pd

Theoretical study on threshold energy and impact ionization coefficient for electrons in Si1−xGex

Threshold energy and electron impact ionization coefficients (α) are calculated for unstrained and strained Si1−xGex on {100} silicon substrate using nonparabolic and ellipsoidal band structure for conduction band and k⋅p method for valence band. The threshold energy in the unstrained Si1−xGex is smaller than that in pure silicon due to the reduced band‐gap energy. The strain causes band degeneracy lifting for both the conduction band and valence band. It gives an additional band‐gap narrowing which leads to a much smaller threshold energy. On the basis of these results, the electron impact ionization coefficient is estimated up to 30% germanium using a Monte Carlo simulation. The reduced threshold energy is found to be the most dominant factor in determining α in the strained Si1−xGex. As a result, the strained Si1−xGex has much larger α than pure silicon while the unstrained Si1−xGex does not due to the effect of alloy scattering and the relatively small change of the threshold energy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70101/2/APPLAB-64-22-2985-1.pd

TCP throughput guarantee in the DiffServ Assured Forwarding service: what about the results?

Since the proposition of Quality of Service architectures by the IETF, the interaction between TCP and the QoS services has been intensively studied. This paper proposes to look forward to the results obtained in terms of TCP throughput guarantee in the DiffServ Assured Forwarding (DiffServ/AF) service and to present an overview of the different proposals to solve the problem. It has been demonstrated that the standardized IETF DiffServ conditioners such as the token bucket color marker and the time sliding window color maker were not good TCP traffic descriptors. Starting with this point, several propositions have been made and most of them presents new marking schemes in order to replace or improve the traditional token bucket color marker. The main problem is that TCP congestion control is not designed to work with the AF service. Indeed, both mechanisms are antagonists. TCP has the property to share in a fair manner the bottleneck bandwidth between flows while DiffServ network provides a level of service controllable and predictable. In this paper, we build a classification of all the propositions made during these last years and compare them. As a result, we will see that these conditioning schemes can be separated in three sets of action level and that the conditioning at the network edge level is the most accepted one. We conclude that the problem is still unsolved and that TCP, conditioned or not conditioned, remains inappropriate to the DiffServ/AF service

Dynamical formation and evolution of (2+1)-dimensional charged black holes

In this paper, we investigate the dynamical formation and evolution of 2 + 1-dimensional charged black holes. We numerically study dynamical collapses of charged matter fields in an anti de Sitter background and note the formation of black holes using the double-null formalism. Moreover, we include re-normalized energy-momentum tensors assuming the S-wave approximation to determine thermodynamical back-reactions to the internal structures. If there is no semi-classical effects, the amount of charge determines the causal structures. If the charge is sufficiently small, the causal structure has a space-like singularity. However, as the charge increases, an inner Cauchy horizon appears. If we have sufficient charge, we see a space-like outer horizon and a time-like inner horizon, and if we give excessive charge, black hole horizons disappear. We have some circumstantial evidences that weak cosmic censorship is still satisfied, even for such excessive charge cases. Also, we confirm that there is mass inflation along the inner horizon, although the properties are quite different from those of four-dimensional cases. Semi-classical back-reactions will not affect the outer horizon, but they will affect the inner horizon. Near the center, there is a place where negative energy is concentrated. Thus, charged black holes in three dimensions have two types of curvature singularities in general: via mass inflation and via a concentration of negative energy. Finally, we classify possible causal structures.Comment: 40 pages, 15 figure

Relationship between temporary emotion of students and performance in learning through comparing facial expression analytics

This paper presents a study on temporary emotion of students and their performance related to learning activities. This paper elucidates different kinds of facial expressions elicited during the activities: quiz and a movie trailer with the help of existing facial expression analyzing applications. The user’s expressions are recorded as video while watching the movie trailer and doing the quiz. The video is processed by different applications which gives the score for different emotions. The results obtained are studied to find which emotion is mostly prevalent among the user in different situations. From this study, it is shown that students experience seemingly different emotions during the activity. The emotions they portrayed were confusion, sadness, anger and neutral. This study explores the use of affective computing for further comprehension of students’ emotion in learning environment