Dielectric Breakdown of a Mott Insulator

We study the nonequilibrium steady state of a Mott insulator coupled to a thermostat and driven by a constant electric field, starting from weak fields, until the dielectric breakdown, and beyond. We find that the conventional Zener picture does not describe the steady-state physics. In particular, the current at weak field is found to be controlled by the dissipation. Moreover, in connection with the electric-field-driven dimensional crossover, we find that the dielectric breakdown occurs when the field strength is on the order of the Mott gap of the corresponding lower-dimensional system. We also report a resonance and the meltdown of the quasiparticle peak when the field strength is half of this Mott gap.Comment: 5 pages, 5 figures. v2: references adde

Permanent spin currents in cavity-qubit systems

In a recent remarkable experiment [P. Roushan et al., Nature Physics 13, 146 (2017)], a spin current in an architecture of three superconducting qubits was produced during a few microseconds by creating synthetic magnetic fields. The life-time of the current was set by the typical dissipative mechanisms that occur in those systems. We propose a scheme for the generation of permanent currents, even in the presence of such imperfections, and scalable to larger system sizes. It relies on striking a subtle balance between multiple nonequilibrium drives and the dissipation mechanisms, in order to engineer and stimulate chiral excited states which can carry current.Comment: 4 pages, 3 figure

Core-crust transition in neutron stars: predictivity of density developments

The possibility to draw links between the isospin properties of nuclei and the structure of compact stars is a stimulating perspective. In order to pursue this objective on a sound basis, the correlations from which such links can be deduced have to be carefully checked against model dependence. Using a variety of nuclear effective models and a microscopic approach, we study the relation between the predictions of a given model and those of a Taylor density development of the corresponding equation of state: this establishes to what extent a limited set of phenomenological constraints can determine the core-crust transition properties. From a correlation analysis we show that a) the transition density $\rho_t$ is mainly correlated with the symmetry energy slope $L$, b) the proton fraction $Y_{p,t}$ with the symmetry energy and symmetry energy slope $(J,L)$ defined at saturation density, or, even better, with the same quantities defined at $\rho=0.1$ fm$^{-3}$, and c) the transition pressure $P_t$ with the symmetry energy slope and curvature $(J,K_{\rm sym})$ defined at $\rho=0.1$ fm$^{-3}$

Hydrogen content in doped and undoped BaPrO3 and BaCeO3 by cold neutron prompt-gamma activation analysis

Proton uptake in undoped and Y-doped BaPrO3 has been measured by cold neutron prompt-gamma activation analysis, and compared to the proton uptake in Gd-doped BaCeO3, as determined by the same technique. The conventional proton incorporation model of perovskites in which oxygen ion vacancies, generated by the introduction of the trivalent dopant onto the tetravalent perovskite site, are filled with hydroxyl groups upon exposure of the sample to H2O containing atmospheres, predicts that the proton concentration in such materials should be no greater than the dopant concentration. In contradiction to this model, the proton concentration in BaPr1–xYxO3 after humidification is as much as three times greater than the dopant concentration, and even undoped samples contain a high concentration of protons. Moreover, x-ray photoemission spectra suggest that the Pr oxidation state is lowered upon hydration. In contrast, BaCe0.9Y0.1O3 shows a typical hydrogen concentration, of close to 90% of the yttrium concentration. The results are interpreted in terms of the variable valence of Pr, which can become reduced from the 4+ to the 3+ oxidation state upon exposure to water, and effectively behaves as a self-dopant within the structure

Accumulation of tissue factor in endothelial cells induces cell apoptosis, mediated through p38 and p53 activation

We previously reported that high levels of tissue factor (TF) can induce cellular apoptosis in endothelial. In this study, TF-mediated mechanisms of induction of apoptosis were explored. Endothelial cells were transfected to express wild-type TF. Additionally, cells were transfected to express Asp253-substituted, or Ala253-substitued TF to enhance or prevent TF release respectively. Alternatively, cells were pre-incubated with TF-rich and TF-poor microvesicles. Cell proliferation, apoptosis and the expression of cyclin D1, p53, bax and p21 were measured following activation of cells with PAR2-agonist peptide. Greatest levels of cell proliferation and cyclin D1 expression were observed in cells expressing wild-type or Asp253-substituted TF. In contrast, increased cellular apoptosis was observed in cells expressing Ala253-substituted TF, or cells pre-incubated with TF-rich microvesicles. The level of p53 protein, p53-phosphorylation at ser33, p53 nuclear localisation and transcriptional activity, but not p53 mRNA, were increased in cells expressing wild-type and Ala253-substituted TF, or in cells pre-incubated with TF-rich microvesicles. However, the expression of bax and p21 mRNA, and Bax protein were only increased in cells pre-incubated with TF-rich microvesicle and in cells expressing Ala253-substituted TF. Inhibition of the transcriptional activity of p53 using pifithrin-α suppressed the expression of Bax. Finally, siRNA–mediated suppression of p38α, or inhibition using SB202190 significantly reduced the p53 protein levels, p53 nuclear localisation and transcriptional activity, suppressed Bax expression and prevented cellular apoptosis. In conclusion, accumulation of TF within endothelial cell, or sequestered from the surrounding can induce cellular apoptosis through mechanisms mediated by p38, and involves the stabilisation of p53

Octomom and Multi-Fetal Pregnancies: Why Federal Legislation Should Require Insurers to Cover In Vitro Fertilization

On January 26, 2009, Nadya Suleman, dubbed Octomom by the media, delivered octuplets after using in vitro fertilization. The same day, Congressman Anthony Weiner of New York introduced the Family Building Act of 2009 in the United States House of Representatives—a federal mandate requiring insurers to provide coverage for in vitro fertilization. The octuplets are no longer headline news, but issues associated with in vitro fertilization are still newsworthy. In this paper I propose that Congress should take a serious look at the Family Building Act of 2009. After addressing some additional issues, Congress should pass legislation mandating that insurers provide coverage for in vitro fertilization. Such legislation will have the effect of reducing the number of multi-fetal pregnancies and preterm births, as well as the costs and risks associated with such pregnancies and births. Although in vitro fertilization is used to treat infertility, it has replaced one problem (the inability to procreate) with a more serious problem (multi-fetal pregnancies). This problem exists largely because the fertility industry is not regulated. While the American Society of Reproductive Medicine (ASRM)and the Society for Reproductive Technology (SART) have issued permissive industry guidelines, there is no serious consequence to the physician or clinic that ignores those guidelines. As in vitro fertilization is expensive and largely funded with out-of-pocket monies, patients and their physicians are inclined to transfer more than one embryo for implantation during each cycle of in vitro fertilization. They do so in order to increase the chances of a “successful” pregnancy and maximize a patient’s use of funds. As a result, women who undergo in vitro fertilization often have more than one baby at a time. Moreover, the only federal law that regulates the industry—the Fertility Clinic Success Rate and Certification Act—also encourages multiple embryo transfers by requiring physicians and clinics to report their pregnancy success rates each year. A successful pregnancy is one that results in a live birth—regardless of how many children. This encourages physicians to transfer multiple embryos to increase their chances of a successful pregnancy so that they can attract additional consumers. Because the human uterus is designed to carry only one baby at a time, multi-fetal pregnancies are risky and usually result in preterm babies. The children often have long-term health and other needs. This is a public health concern. The Family Building Act would help regulate the largely unregulated fertility industry. If passed, women and their physicians would be inclined to transfer fewer embryos for implantation during a cycle of in vitro fertilization because an unsuccessful pregnancy would not mean the loss of out-of-pocket dollars. A patient would be more willing to try again if insurance covered the procedure. Furthermore, physician reimbursement rates could be tied to the industry guidelines. A federal mandate requiring insurers to cover in vitro fertilization would strengthen the industry guidelines without a need for an additional regulatory industry. An insurance mandate will reduce the incidence of multi-fetal pregnancies, the largest problem associated with in vitro fertilization, and ultimately increase the incidence of healthy single-baby pregnancies

Dynamics of elastocapillary rise

We present the results of a combined experimental and theoretical investigation of the surface-tension-driven coalescence of flexible structures. Specifically, we consider the dynamics of the rise of a wetting liquid between flexible sheets that are clamped at their upper ends. As the elasticity of the sheets is progressively increased, we observe a systematic deviation from the classical diffusive-like behaviour: the time to reach equilibrium increases dramatically and the departure from classical rise occurs sooner, trends that we elucidate via scaling analyses. Three distinct temporal regimes are identified and subsequently explored by developing a theoretical model based on lubrication theory and the linear theory of plates. The resulting free-boundary problem is solved numerically and good agreement is obtained with experiments

Heart of darkness

Thesis (S.M. in Science Writing)--Massachusetts Institute of Technology, Dept. of Humanities, Graduate Program in Science Writing, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 22-31).A few decades ago, black holes were a theoretical quirk. Highly probable on paper, they were doubted more than touted; many scientists didn't believe they even existed. Today, however, black holes appear to be everywhere, from behemoths in the cores of almost every galaxy to more modest, stellar-mass objects spattering the Milky Way's arms. Astronomers suspect that super massive black holes like Sagittarius A* (the compact dark object at the center of our galaxy) may be a cosmic mafia manipulating the galaxies that house them, possibly even controlling galaxy growth. If this suspicion turns out to be true, black holes may have had more influence on cosmic structure than any other object. This thesis explores how black holes became science from pseudoscience, focusing on three shifts in astronomy: detailed proper motion measurements of stars zooming around the galactic center, the discovery of the apparent relationship between galaxies and their central super massive black holes, and the development of working numerical simulations of black hole mergers. These three steps have led up to the Event Horizon Telescope, a project which will combine radio telescopes around the world to peer into the innermost spacetime warps surrounding Sagittarius A*. If all goes well, astronomers may finally glimpse the "silhouette" of the Milky Way's central super massive black hole within the next decade, directly testing whether Einstein's theory of general relativity is right.by Camille M. Carlisle.S.M.in Science Writin

ILR Impact Brief - Community College Websites and Barriers to Access

[Excerpt] Community colleges, on average, serve 335 students with disabilities, although that number climbs to 5,000 at the largest college surveyed for this project. Nearly all community colleges that participated in the survey rely on the web for a variety of student services, but only half have instituted requirements regarding web accessibility for individuals with disabilities. Actual evaluations of accessibility and ease of use revealed that none of the websites analyzed complied with all federal standards on accessibility, and many web pages encompassed usability obstacles (e.g., unfamiliar terminology, unintuitive navigation schemes, and hard-to-read design elements) that affected disabled and non-disabled individuals alike