Phases of lattice hard core bosons in a periodic superlattice

We study by Quantum Monte Carlo simulations the phase diagram of lattice hard core bosons with nearest-neighbour repulsive interactions, in the presence of a super-lattice of adsorption sites. For a moderate adsorption strength, the system forms crystal phases registered with the adsorption lattice; a "supersolid" phase exists, on both the vacancy and interstitial sides, whereas at commensuration the superfluid density vanishes. The possible relevance of these results to experiments on $^4$He films adsorbed on graphite is discussed.Comment: 5 pages, 5 figure

The magnetic structure of Gd_2Ti_2O_7

We attempt to solve the magnetic structure of the gadolinium analogue of `spin-ice', using a mixture of experimental and theoretical assumptions. The eventual predictions are essentially consistent with both the Mossbauer and neutron measurements but are unrelated to previous proposals. We find two possible distinct states, one of which is coplanar and the other is fully three-dimensional. We predict that close to the initial transition the preferred state is coplanar but that at the lowest temperature the ground-state becomes fully three-dimensional. Unfortunately the energetics are consequently complicated. There is a dominant nearest-neighbour Heisenberg interaction but then a compromise solution for lifting the final degeneracy resulting from a competition between longer-range Heisenberg interactions and direct dipolar interactions on similar energy scales.Comment: 12 pages, 15 figure

Discovery of 15-second oscillations in Hubble Space Telescope observations of WZ Sagittae following the 2001 outburst

We report the discovery of 15-s oscillations in ultraviolet observations of WZ Sge obtained with the Hubble Space Telescope approximately one month after the peak of the 2001 outburst. This is the earliest detection of oscillations in WZ Sge following an outburst and the first time that a signal near 15 s has been seen to be dominant. The oscillations are quite strong (amplitude about 5%), but not particularly coherent. In one instance, the oscillation period changed by 0.7 s between successive observations separated by less than 1 hour. We have also found evidence for weaker signals with periods near 6.5 s in some of our data. We discuss the implications of our results for the models that have been proposed to account for the 28-s oscillations seen in quiescence. If the periods of the 15-s oscillations can be identified with the periods of revolution of material rotating about the white dwarf, the mass of the white dwarf must satisfy M_WD > 0.71 M_sun. The corresponding limit for the 6.5-s signals is M_WD > 1.03 M_sun.Comment: accepted for publication in ApJ Letters; 13 pages, 4 postscript figures; new version corrects a few typos and matches version that will appear in ApJ

Cosmological Constant, Dark Matter, and Electroweak Phase Transition

Accepting the fine tuned cosmological constant hypothesis, we have recently proposed that this hypothesis can be tested if the dark matter freeze out occurs at the electroweak scale and if one were to measure an anomalous shift in the dark matter relic abundance. In this paper, we numerically compute this relic abundance shift in the context of explicit singlet extensions of the Standard Model and explore the properties of the phase transition which would lead to the observationally most favorable scenario. Through the numerical exploration, we explicitly identify a parameter space in a singlet extension of the standard model which gives order unity observable effects. We also clarify the notion of a temperature dependence in the vacuum energy.Comment: 58 pages, 10 figure

New effective interactions in RMF theory with non-linear terms and density-dependent meson-nucleon coupling

New parameter sets for the Lagrangian density in the relativistic mean field (RMF) theory, PK1 with nonlinear sigma- and omega-meson self-coupling, PK1R with nonlinear sigma-, omega- and rho-meson self-coupling and PKDD with the density-dependent meson-nucleon coupling, are proposed. They are able to provide an excellent description not only for the properties of nuclear matter but also for the nuclei in and far from the valley of beta-stability. For the first time in the parametrization of the RMF Lagrangian density, the center-of-mass correction is treated by a microscopic way, which is essential to unify the description of nuclei from light to heavy regions with one effective interaction.Comment: 22 pages, 16 EPS figures, RevTeX

Assessing Readability of Online Patient Education Materials for Spine Surgery Procedures

Increased patient reliance on Internet-based health information has amplified the need for comprehensible online patient education articles. As suggested by the AMA and NIH, spine fusion articles should be between a 4th and 6th grade readability level to increase patient comprehension, which may contribute to improved postoperative outcomes. Objective: To determine the average readability level of online healthcare education information relating to anterior cervical discectomy and fusion (ACDF) and lumbar fusion procedures. Design: Online Health-Education Resource Qualitative Analysis. Setting: Rush University Medical Center - Department of Orthopaedic Surgery. Methods: Three popular search engines were utilized to access patient education articles for common cervical and lumbar spine procedures. Relevant articles were analyzed for readability using Readability Studio Professional Edition software (Oleander Software, Ltd). Articles were stratified by organization type as follows: General Medical Websites (GMW), Healthcare Network/Academic Institutions (HNAI), and Private Practices (PP). Thirteen common readability tests were performed with the mean grade level for each readability test compared between subgroups using ANOVA analysis. Results: 79 ACDF and 231 lumbar fusion articles were determined to have a mean readability level of 10.7 ± 1.5 and 11.3 ± 1.6, respectively. GMW, HNAI, and PP subgroups had mean readability levels of 10.9 ± 2.9, 10.7 ± 2.8, and 10.7 ± 2.5 for ACDF and 10.9 ± 3.0, 10.8 ± 2.9, and 11.6 ± 2.7 for lumbar fusion articles. Of 310 total articles, only 6 (3 ACDF and 3 lumbar fusion) were written below the 7th grade reading level. Conclusions: Current online literature from medical websites containing information regarding ACDF and lumbar fusion procedures are written at a grade level higher than the suggested guidelines. Therefore, current patient education articles should be revised to accommodate the average readability level in the United States and may result in improved patient comprehension and postoperative outcomes

The type numbers of closed geodesics

A short survey on the type numbers of closed geodesics, on applications of the Morse theory to proving the existence of closed geodesics and on the recent progress in applying variational methods to the periodic problem for Finsler and magnetic geodesicsComment: 29 pages, an appendix to the Russian translation of "The calculus of variations in the large" by M. Mors

Targeted Automatic Integer Overflow Discovery Using Goal-Directed Conditional Branch Enforcement

We present a new technique and system, DIODE, for auto- matically generating inputs that trigger overflows at memory allocation sites. DIODE is designed to identify relevant sanity checks that inputs must satisfy to trigger overflows at target memory allocation sites, then generate inputs that satisfy these sanity checks to successfully trigger the overflow. DIODE works with off-the-shelf, production x86 binaries. Our results show that, for our benchmark set of applications, and for every target memory allocation site exercised by our seed inputs (which the applications process correctly with no overflows), either 1) DIODE is able to generate an input that triggers an overflow at that site or 2) there is no input that would trigger an overflow for the observed target expression at that site.United States. Defense Advanced Research Projects Agency (Grant FA8650-11-C-7192

Behavior of a frustrated quantum spin chain with bond dimerization

We clarified behavior of the excitation gap in a frustrated S=1/2 quantum spin chain with bond dimerization by using the numerical diagonalization of finite systems and a variational approach. The model interpolates between the independent dimer model and the S=1 spin chain by changing a strength of the dimerization. The energy gap is minimum at the fully-frustrated point, where a localized kink and a freely mobile anti-kink govern the low-lying excitations. Away from the point, a kink and an antikink form a bound state by an effective triangular potential between them. The consequential gap enhancement and the localization length of the bound state is obtained exactly in the continuous limit. The gap enhancement obeys a power law with exponent 2/3. The method and the obtained results are common to other frustrated double spin-chain systems, such as the one-dimensional J_1 - J_2 model, or the frustrated ladder model.Comment: 11 pages, REVTeX, 8 figures in eps-fil