Is the electrostatic force between a point charge and a neutral metallic object always attractive?

We give an example of a geometry in which the electrostatic force between a point charge and a neutral metallic object is repulsive. The example consists of a point charge centered above a thin metallic hemisphere, positioned concave up. We show that this geometry has a repulsive regime using both a simple analytical argument and an exact calculation for an analogous two-dimensional geometry. Analogues of this geometry-induced repulsion can appear in many other contexts, including Casimir systems.Comment: 7 pages, 7 figure

Solid 4He and the Supersolid Phase: from Theoretical Speculation to the Discovery of a New State of Matter? A Review of the Past and Present Status of Research

The possibility of a supersolid state of matter, i.e., a crystalline solid exhibiting superfluid properties, first appeared in theoretical studies about forty years ago. After a long period of little interest due to the lack of experimental evidence, it has attracted strong experimental and theoretical attention in the last few years since Kim and Chan (Penn State, USA) reported evidence for nonclassical rotational inertia effects, a typical signature of superfluidity, in samples of solid 4He. Since this "first observation", other experimental groups have observed such effects in the response to the rotation of samples of crystalline helium, and it has become clear that the response of the solid is extremely sensitive to growth conditions, annealing processes, and 3He impurities. A peak in the specific heat in the same range of temperatures has been reported as well as anomalies in the elastic behaviour of solid 4He with a strong resemblance to the phenomena revealed by torsional oscillator experiments. Very recently, the observation of unusual mass transport in hcp solid 4He has also been reported, suggesting superflow. From the theoretical point of view, powerful simulation methods have been used to study solid 4He, but the interpretation of the data is still rather difficult; dealing with the question of supersolidity means that one has to face not only the problem of the coexistence of quantum coherence phenomena and crystalline order, exploring the realm of spontaneous symmetry breaking and quantum field theory, but also the problem of the role of disorder, i.e., how defects, such as vacancies, impurities, dislocations, and grain boundaries, participate in the phase transition mechanism.Comment: Published on J. Phys. Soc. Jpn., Vol.77, No.11, p.11101

Simulations of Pregalactic Structure Formation with Radiative Feedback

We present results from three-dimensional hydrodynamic simulations of the high redshift collapse of pregalactic clouds including feedback effects from a soft H2 photodissociating UV radiation field. The simulations use an Eulerian adaptive mesh refinement technique to follow the nonequilibrium chemistry of nine chemical species with cosmological initial conditions drawn from a popular Lambda-dominated cold dark matter model. The results confirm that the soft UV background can delay the cooling and collapse of small halos (~10^6 Msun). For reasonable values of the photo-dissociating flux, the H2 fraction is in equilibrium throughout most of the objects we simulate. We determine the mass threshold for collapse for a range of soft-UV fluxes and also derive a simple analytic expression. Continuing the simulations beyond the point of initial collapse demonstrates that the fraction of gas which can cool depends mostly on the virial mass of the halo and the amount of soft-UV flux, with remarkably little scatter. We parameterize this relation, for use in semi-analytic models.Comment: 18 pages, 7 figures, submitted to Ap

Bond-disordered spin systems: Theory and application to doped high-Tc compounds

We examine the stability of magnetic order in a classical Heisenberg model with quenched random exchange couplings. This system represents the spin degrees of freedom in high-$T_\textrm{c}$ compounds with immobile dopants. Starting from a replica representation of the nonlinear $\sigma$-model, we perform a renormalization-group analysis. The importance of cumulants of the disorder distribution to arbitrarily high orders necessitates a functional renormalization scheme. From the renormalization flow equations we determine the magnetic correlation length numerically as a function of the impurity concentration and of temperature. From our analysis follows that two-dimensional layers can be magnetically ordered for arbitrarily strong but sufficiently diluted defects. We further consider the dimensional crossover in a stack of weakly coupled layers. The resulting phase diagram is compared with experimental data for La$_{2-x}$Sr$_x$CuO$_4$.Comment: 12 pages, 5 figure

Quantum Films Adsorbed on Graphite: Third and Fourth Helium Layers

Using a path-integral Monte Carlo method for simulating superfluid quantum films, we investigate helium layers adsorbed on a substrate consisting of graphite plus two solid helium layers. Our results for the promotion densities and the dependence of the superfluid density on coverage are in agreement with experiment. We can also explain certain features of the measured heat capacity as a function of temperature and coverage.Comment: 13 pages in the Phys. Rev. two-column format, 16 Figure

Recommendations for a national agenda to substantially reduce cervical cancer

PURPOSE: Prophylactic human papillomavirus (HPV) vaccines and new HPV screening tests, combined with traditional Pap test screening, provide an unprecedented opportunity to greatly reduce cervical cancer in the USA. Despite these advances, thousands of women continue to be diagnosed with and die of this highly preventable disease each year. This paper describes the initiatives and recommendations of national cervical cancer experts toward preventing and possibly eliminating this disease. METHODS: In May 2011, Cervical Cancer-Free America, a national initiative, convened a cervical cancer summit in Washington, DC. Over 120 experts from the public and private sector met to develop a national agenda for reducing cervical cancer morbidity and mortality in the USA. RESULTS: Summit participants evaluated four broad challenges to reducing cervical cancer: (1) low use of HPV vaccines, (2) low use of cervical cancer screening, (3) screening errors, and (4) lack of continuity of care for women diagnosed with cervical cancer. The summit offered 12 concrete recommendations to guide future national and local efforts toward this goal. CONCLUSIONS: Cervical cancer incidence and mortality can be greatly reduced by better deploying existing methods and systems. The challenge lies in ensuring that the array of available prevention options are accessible and utilized by all age-appropriate women-particularly minority and underserved women who are disproportionately affected by this disease. The consensus was that cervical cancer can be greatly reduced and that prevention efforts can lead the way towards a dramatic reduction in this preventable disease in our country

Atypical disengagement from faces and its modulation by the control of eye fixation in children with Autism Spectrum Disorder

By using the gap overlap task, we investigated disengagement from faces and objects in children (9–17 years old) with and without autism spectrum disorder (ASD) and its neurophysiological correlates. In typically developing (TD) children, faces elicited larger gap effect, an index of attentional engagement, and larger saccade-related event-related potentials (ERPs), compared to objects. In children with ASD, by contrast, neither gap effect nor ERPs differ between faces and objects. Follow-up experiments demonstrated that instructed fixation on the eyes induces larger gap effect for faces in children with ASD, whereas instructed fixation on the mouth can disrupt larger gap effect in TD children. These results suggest a critical role of eye fixation on attentional engagement to faces in both groups

The Curse of Online Friends: The Detrimental Effects of Online Social Network Usage on Well-Being

In the pursuit of happiness, it has been conventionally accepted that more friends would bring us a better quality of life. However, with the advent of social networking sites, unprecedented social influence has pervaded our daily lives. Across two studies we show that even though people feel more satisfied with their lives when they view the friends added on Facebook, reading friends’ posts reduces their well-being. This is because the more friends people have on Facebook, the more ostentatious information they see. The resultant drop in life satisfaction occurs because people fail to draw a connection between the number of friends and the amount of ostentatious information. Moreover, this decrease in life satisfaction is mediated by envy. We contribute to the literature on consumer well-being by identifying a novel and ubiquitous phenomenon of making social comparisons with hundreds of people, a phenomenon that arose with the advent of social networking and was previously outside the scope of social comparison literature

Path integral Monte Carlo simulation of the second layer of helium-4 adsorbed on graphite

We have developed a path integral Monte Carlo method for simulating helium films and apply it to the second layer of helium adsorbed on graphite. We use helium-helium and helium-graphite interactions that are found from potentials which realistically describe the interatomic interactions. The Monte Carlo sampling is over both particle positions and permutations of particle labels. From the particle configurations and static structure factor calculations, we find that this layer possesses, in order of increasing density, a superfluid liquid phase, a sqrt(7) x sqrt(7) commensurate solid phase that is registered with respect to the first layer, and an incommensurate solid phases. By applying the Maxwell construction to the dependence of the low-temperature total energy on the coverage, we are able to identify coexistence regions between the phases. From these, we deduce an effectively zero-temperature phase diagram. Our phase boundaries are in agreement with heat capacity and torsional oscillator measurements, and demonstrate that the experimentally observed disruption of the superfluid phase is caused by the growth of the commensurate phase. We further observe that the superfluid phase has a transition temperature consistent with the two-dimensional value. Promotion to the third layer occurs for densities above 0.212 atom/A^2, in good agreement with experiment. Finally, we calculate the specific heat for each phase and obtain peaks at temperatures in general agreement with experiment.Comment: 14 double-column pages, 10 figures, revtex. Accepted for publication in Phys. Rev. B. 3 figures added, some text revisions, 6 figures remove