Model of multiphoton transitions in a current-biased Josephson junction

We present a simple model for multiphoton transitions between the quasi-bound states of a current-driven Josephson junction. The transitions are induced by applying an ac voltage with controllable frequency and amplitude across the junction. The voltage induces transitions when the ac frequency equals n times the splitting between the ground and first excited quasi-bound state of the junction. We calculate the transition matrix elements as functions of the dc bias current I, and the frequency and amplitude of the ac voltage, for representative junction parameters. We also calculate the frequency-dependent absorption coefficient by solving the relevant Bloch equations when the ac amplitude is sufficiently small. In this regime, the absorption coefficient is a sum of Lorentzian lines centered at the n-photon absorption frequency, of strength proportional to the squared matrix elements. For fixed ac voltage amplitude, the n-photon transition rate usually decreases with increasing n. We also find a characteristic even-odd effect: The absorption coefficient typically increases with I for n even but decreases for n odd. Our results agree qualitatively with recent experiments.Comment: 15 pages, 13 figures, accepted for publication in Physical Review

Methodology for a security audit of ERTMS

In this paper we discuss the methodology we used for a security audit of the European Railway Traffic Management System (ERTMS) specifications. ERTMS is a major industrial project that aims at replacing the many different national train control and command systems in Europe. We discuss the stages of the audit, threat model used, and the output of each stage of the audit

Vortex Fractionalization in a Josephson Ladder

We show numerically that, in a Josephson ladder with periodic boundary conditions and subject to a suitable transverse magnetic field, a vortex excitation can spontaneously break up into two or more fractional excitations. If the ladder has N plaquettes, and N is divisible by an integer q, then in an applied transverse field of 1/q flux quanta per plaquette the ground state is a regular pattern of one fluxon every q plaquettes. When one additional fluxon is added to the ladder, it breaks up into q fractional fluxons, each carrying 1/q units of vorticity. The fractional fluxons are basically walls between different domains of the ground state of the underlying 1/q lattice. The fractional fluxons are all depinned at the same applied current and move as a unit. For certain applied fields and ladder lengths, we show that there are isolated fractional fluxons. It is shown that the fractional fluxons would produce a time-averaged voltage related in a characteristic way to the ac voltage frequency.Comment: 13 Figures. 10 page

How secure is ERTMS?

This paper reports on the results of a security analysis of the European Railway Traffic Management System (ERTMS) specifications. ERTMS is designed to be fail-safe and the general philosophy of ‘if in doubt, stop the train’ makes it difficult to engineer a train accident. However, it is possible to exploit the fail-safe behaviour of ERTMS and create a situation that causes a train to halt. Thus, denial of service attacks are possible, and could be launched at a time and place of the attacker’s choosing, perhaps designed to cause maximum disruption or passenger discomfort. Causing an accident is more difficult but not impossible

Longitudinal changes in the mental health of UK young male and female adults during the COVID-19 pandemic

An increasing body of research indicates that, whilst young adults are at the lowest risk of becoming severely physically ill as a result of COVID-19, they are at the greatest risk of adverse mental health outcomes. Using data from the Understanding Society COVID-19 survey, the current study examined the mental health of 18-25-year-olds during the pandemic. Current mental health was measured at six time points using the 12-item General Health Questionnaire (GHQ-12), a validated measure for mental distress. The analytic sample included 880 young adults (292 = males; 588 = females). The trajectory of mental health was modeled from April to November 2020, using demographic information and health behaviors (physical activity, alcohol consumption, and smoking cigarettes) as covariates. Growth curve modeling indicated that alcohol consumption, smoking, being female, having a lower income, and having a pre-existing mental health condition were risk factors for worse mental health during the pandemic. For females, their mental health was lowest in April but gradually improved until September, when it began to decline again. Males, in contrast, had a relatively stable trajectory of mental health across the pandemic. These findings can help inform targeted interventions for at risk groups to minimize the adverse impact of the pandemic on young adults’ mental health

Quantum Monte Carlo study of a magnetic-field-driven 2D superconductor-insulator transition

We numerically study the superconductor-insulator phase transition in a model disordered 2D superconductor as a function of applied magnetic field. The calculation involves quantum Monte Carlo calculations of the (2+1)D XY model in the presence of both disorder and magnetic field. The XY coupling is assumed to have the form -J\cos(\theta_i-\theta_j-A_{ij}), where A_{ij} has a mean of zero and a standard deviation \Delta A_{ij}. In a real system, such a model would be approximately realized by a 2D array of small Josephson-coupled grains with slight spatial disorder and a uniform applied magnetic field. The different values \Delta A_{ij} then corresponds to an applied field such that the average number of flux quanta per plaquette has various integer values N: larger N corresponds to larger \Delta A_{ij}. For any value of \Delta A_{ij}, there appears to be a critical coupling constant K_c(\Delta A_{ij})=\sqrt{[J/(2U)]_c}, where U is the charging energy, above which the system is a Mott insulator; there is also a corresponding critical conductivity \sigma^*(\Delta A_{ij}) at the transition. For \Delta A_{ij}=\infty, the order parameter of the transition is a renormalized coupling constant g. Using a numerical technique appropriate for disordered systems, we show that the transition at this value of \Delta A_{ij} takes place from an insulating (I) phase to a Bose glass (BG) phase, and that the dynamical critical exponent characterizing this transition is z \sim 1.3. By contrast, z=1 for this model at \Delta A_{ij}=0. We suggest that the superconductor to insulator transition is actually of this I to BG class at all nonzero \Delta A_{ij}'s, and we support this interpretation by both numerical evidence and an analytical argument based on the Harris criterion.Comment: 17 pages, 23 figures, accepted for publication in Phys. Rev.

A Quantitative Exploration of the Educational Paths to Completion Taken by First Generation College Students and Students Who Have a Parent with A Four-Year College Degree

Title from PDF of title page, viewed June 20, 2017Dissertation advisor: Jennifer FriendVitaIncludes bibliographical references (pages 139 -162)Thesis (Ed.D.)-- School of Education. University of Missouri--Kansas City, 2017This quantitative study examines students’ survey responses as they begin the transition from high school into and through their initial year of college then to completion of a four-year college degree, to explore differences for both first generation college students and students whose parents have a four-year college degree. The research design uses data from four points in time to analyze and report the characteristics of a sample population of more than 16,000 students spread across 750 public and private secondary institutions in the United States (Gall, Gall & Borg, 2007). The data are derived from the Education Longitudinal Study of 2002, specifically the Student Questionnaires and phases that include the Base Year (2002), the First Follow Up (2004),the Second Follow Up (2006), and the Third Follow Up (2012), which offers the opportunity to see the data through different lenses. Students who responded to the survey were separated into two groups for the purposes of analysis: first generation college students (FGCS) and students who have a parent with a 4-year college degree (SPCD). This data disaggregation and the use of Binary Logistic Regression allowed the researcher to analyze and discuss the factors involved in both groups’ progression to completion of a four-year college degree. Results of the study showed that FGCS were 1.5 times less likely to persist to a four-year college degree than SPCD. Further, in conducting the regression models when all of the variables selected for this study are considered together, only school motivation, familial involvement and a student’s confidence significantly predict FGCS’ persistence to completion of a four-year college degree.Introduction -- Literature review -- Methodology -- Results -- Discussion -- Appendix A. IRB approval letter -- Appendix B. A personal account -- Appendix C. Overview of the instrumen

Beyond Eliashberg superconductivity in MgB2: anharmonicity, two-phonon scattering, and multiple gaps

Density-functional calculations of the phonon spectrum and electron-phonon coupling in MgB$_2$ are presented. The $E_{2g}$ phonons, which involve in-plane B displacements, couple strongly to the $p_{x,y}$ electronic bands. The isotropic electron-phonon coupling constant is calculated to be about 0.8. Allowing for different order parameters in different bands, the superconducting $\lambda$ in the clean limit is calculated to be significantly larger. The $E_{2g}$ phonons are strongly anharmonic, and the non-linear contribution to the coupling between the $E_{2g}$ modes and the p$_{x,y}$ bands is significant.Comment: 4 pages, 3 figure

How Reasoning Aims at Truth

Many hold that theoretical reasoning aims at truth. In this paper, I ask what it is for reasoning to be thus aim-directed. Standard answers to this question explain reasoning’s aim-directedness in terms of intentions, dispositions, or rule-following. I argue that, while these views contain important insights, they are not satisfactory. As an alternative, I introduce and defend a novel account: reasoning aims at truth in virtue of being the exercise of a distinctive kind of cognitive power, one that, unlike ordinary dispositions, is capable of fully explaining its own exercises. I argue that this account is able to avoid the difficulties plaguing standard accounts of the relevant sort of mental teleology