Positivity of Entropy in the Semi-Classical Theory of Black Holes and Radiation

Quantum stress-energy tensors of fields renormalized on a Schwarzschild background violate the classical energy conditions near the black hole. Nevertheless, the associated equilibrium thermodynamical entropy $\Delta S$ by which such fields augment the usual black hole entropy is found to be positive. More precisely, the derivative of $\Delta S$ with respect to radius, at fixed black hole mass, is found to vanish at the horizon for {\it all} regular renormalized stress-energy quantum tensors. For the cases of conformal scalar fields and U(1) gauge fields, the corresponding second derivative is positive, indicating that $\Delta S$ has a local minimum there. Explicit calculation shows that indeed $\Delta S$ increases monotonically for increasing radius and is positive. (The same conclusions hold for a massless spin 1/2 field, but the accuracy of the stress-energy tensor we employ has not been confirmed, in contrast to the scalar and vector cases). None of these results would hold if the back-reaction of the radiation on the spacetime geometry were ignored; consequently, one must regard $\Delta S$ as arising from both the radiation fields and their effects on the gravitational field. The back-reaction, no matter how "small",Comment: 19 pages, RevTe

Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite

Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3+ cations and their impact on relevant processes such as charge recombination and exciton dissociation are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K) and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation is present. Around room temperature, the characteristic relaxation times for the C4 rotation is found to be ps while for the C3 rotation ps. The -dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C4 rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH3NH3+ and the associated dipole have important implications on understanding the low exciton binding energy and slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance

Preparing and probing atomic number states with an atom interferometer

We describe the controlled loading and measurement of number-squeezed states and Poisson states of atoms in individual sites of a double well optical lattice. These states are input to an atom interferometer that is realized by symmetrically splitting individual lattice sites into double wells, allowing atoms in individual sites to evolve independently. The two paths then interfere, creating a matter-wave double-slit diffraction pattern. The time evolution of the double-slit diffraction pattern is used to measure the number statistics of the input state. The flexibility of our double well lattice provides a means to detect the presence of empty lattice sites, an important and so far unmeasured factor in determining the purity of a Mott state

Meson Condensation in Dense Matter Revisited

The results for meson condensation in the literature vary markedly depending on whether one uses chiral perturbation theory or the current-algebra-plus-PCAC approach. To elucidate the origin of this discrepancy, we re-examine the role of the sigma-term in meson condensation. We find that the resolution of the existing discrepancy requires a knowledge of terms in the Lagrangian that are higher order in density than hitherto considered.Comment: 10pages, USC(NT)-94-

Hypercritical Advection Dominated Accretion Flow

In this note we study the accretion disc that arises in hypercritical accretion of $\dot M\sim 10^8 M_{\rm Edd}$ onto a neutron star while it is in common envelope evolution with a massive companion. In order to raise the temperature high enough that the disc might cool by neutrino emission, Chevalier found a small value of the $\alpha$-parameter, where the kinematic coefficient of shear viscosity is $\nu=\alpha c_s H$, with $c_s$ the velocity of sound and $H$ the disc height; namely, $\alpha\sim 10^{-6}$ was necessary for gas pressure to dominate. He also considered results with higher values of $\alpha$, pointing out that radiation pressure would then predominate. With these larger $\alpha$'s, the temperatures of the accreting material are much lower, \lsim 0.35 MeV. The result is that neutrino cooling during the flow is negligible, satisfying very well the advection dominating conditions. The low temperature of the accreting material means that it cannot get rid of its energy rapidly by neutrino emission, so it piles up, pushing its way through the accretion disc. An accretion shock is formed, far beyond the neutron star, at a radius \gsim 10^8 cm, much as in the earlier spherically symmetric calculation, but in rotation. Two-dimensional numerical simulation shows that an accretion disc is reformed inside of the accretion shock, allowing matter to accrete onto the neutron star with pressure high enough so that neutrinos can carry off the energy.Comment: 6 pages, ApJ, submitte

Extreme alpha-clustering in the 18O nucleus

The structure of the 18O nucleus at excitation energies above the alpha decay threshold was studied using 14C+alpha resonance elastic scattering. A number of states with large alpha reduced widths have been observed, indicating that the alpha-cluster degree of freedom plays an important role in this N not equal Z nucleus. However, the alpha-cluster structure of this nucleus is very different from the relatively simple pattern of strong alpha-cluster quasi-rotational bands in the neighboring 16O and 20Ne nuclei. A 0+ state with an alpha reduced width exceeding the single particle limit was identified at an excitation energy of 9.9+/-0.3 MeV. We discuss evidence that states of this kind are common in light nuclei and give possible explanations of this feature.Comment: 4 pages, 2 figures, 1 table. Resubmission with minor changes for clarity, including removal of one figur

\u3ci\u3eMedicine Meets Virtual Reality 16\u3c/i\u3e

Chapter, Validating Advanced Robot-Assisted Laparoscopic Training Task in Virtual Reality, co-authored by Nicholas Stergiou, UNO faculty member. We humans are tribal, grouping ourselves by a multitude of criteria: physical, intellectual, political, emotional, etc. The Internet and its auxiliary technologies have enabled a novel dimension in tribal behavior during our recent past. This growing connectivity begs the question: will individuals and their communities come together to solve some very urgent global problems? At MMVR, we explore ways to harness information technology to solve healthcare problems – and in the industrialized nations we are making progress. In the developing world however, things are more challenging. Massive urban poverty fuels violence and misery. Will global networking bring a convergence of individual and tribal problem-solving? Recently, a barrel-shaped water carrier that rolls along the ground was presented, improving daily life for many people. Also the One Laptop per Child project is a good example of how the industrialized nations can help the developing countries. They produce durable and simple laptops which are inexpensive to produce. At MMVR, we focus on cutting-edge medical technology, which is generally pretty expensive. While the benefits of innovation trickle downward, from the privileged few to the broader masses, we should expand this trickle into a flood. Can breakthrough applications in stimulation, visualization, robotics, and informatics engender tools as ingeniously as the water carrier or laptop? With some extra creativity, we can design better healthcare for the developing world too.https://digitalcommons.unomaha.edu/facultybooks/1234/thumbnail.jp

Kaon Condensation in the Bound-State Approach to the Skyrme Model

We explore kaon condensation using the bound-state approach to the Skyrme model on a 3-sphere. The condensation occurs when the energy required to produce a $K^-$ falls below the electron fermi level. This happens at the baryon number density on the order of 3--4 times nuclear density.Comment: LaTeX format, 15 pages. 3 Postscript figures, compressed and uuencode