2,228 research outputs found
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
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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
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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 are presented. The phonons, which involve in-plane
B displacements, couple strongly to the 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
in the clean limit is calculated to be significantly larger. The
phonons are strongly anharmonic, and the non-linear contribution to
the coupling between the modes and the p 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
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