6,784 research outputs found
The Ground State of the Pseudogap in Cuprate Superconductors
We present studies of the electronic structure of La2-xBaxCuO4, a system
where the superconductivity is strongly suppressed as static spin and charge
orders or "stripes" develop near the doping level of x=1/8. Using
angle-resolved photoemission and scanning tunneling microscopy, we detect an
energy gap at the Fermi surface with magnitude consistent with d-wave symmetry
and with linear density of states, vanishing only at four nodal points, even
when superconductivity disappears at x=1/8. Thus, the non-superconducting,
"striped" state at x=1/8 is consistent with a phase incoherent d-wave
superconductor whose Cooper pairs form spin/charge ordered structures instead
of becoming superconducting.Comment: This is the author's version of the wor
Sexual Experimentation in Heterosexual, Bisexual, Lesbian/Gay, and Questioning Adolescents From Ages 11 to 15.
To examine adolescent sexuality development, we analyzed data from a British cohort study (NÂ =Â 5,070), which assessed the same 12-14 sexual activities at ages 11, 12, 13, and 15, and sexual orientation identity at age 15. The sexual activities ranged from low (e.g., cuddling), moderate (e.g., kissing), to high (e.g., sexual intercourse) intensity. We found that most adolescents having sexual activities of low-to-moderate intensity with same-sex individuals also had them with other-sex individuals, and adolescents having other-sex contacts of low intensity often reported them nonexclusively. Furthermore, other-sex and same-sex sexual activities did not reliably distinguish between sexual orientation identities. Sex differences in these phenomena were absent or small. These findings suggest that many adolescents have low-intensity nonexclusive sexual behaviors.A Gates Cambridge Scholarship and an honorary Cambridge Australia Scholarship awarded to J. T. M. Davis. Research funding awarded to G. Li from the Department of Psychology, University of Cambridg
FactorMatte: Redefining Video Matting for Re-Composition Tasks
We propose "factor matting", an alternative formulation of the video matting
problem in terms of counterfactual video synthesis that is better suited for
re-composition tasks. The goal of factor matting is to separate the contents of
video into independent components, each visualizing a counterfactual version of
the scene where contents of other components have been removed. We show that
factor matting maps well to a more general Bayesian framing of the matting
problem that accounts for complex conditional interactions between layers.
Based on this observation, we present a method for solving the factor matting
problem that produces useful decompositions even for video with complex
cross-layer interactions like splashes, shadows, and reflections. Our method is
trained per-video and requires neither pre-training on external large datasets,
nor knowledge about the 3D structure of the scene. We conduct extensive
experiments, and show that our method not only can disentangle scenes with
complex interactions, but also outperforms top methods on existing tasks such
as classical video matting and background subtraction. In addition, we
demonstrate the benefits of our approach on a range of downstream tasks. Please
refer to our project webpage for more details: https://factormatte.github.ioComment: Project webpage: https://factormatte.github.i
Discrete-step evaporation of an atomic beam
We present a theoretical analysis of the evaporative cooling of a
magnetically guided atomic beam by means of discrete radio-frequency antennas.
First we derive the changes in flux and temperature, as well as in collision
rate and phase-space density, for a single evaporation step. Next we show how
the occurrence of collisions during the propagation between two successive
antennas can be probed. Finally, we discuss the optimization of the evaporation
ramp with several antennas to reach quantum degeneracy. We estimate the number
of antennas required to increase the phase-space density by several orders of
magnitude. We find that at least 30 antennas are needed to gain a factor
in phase-space density.Comment: Submitted to Eur. Phys. J.
Precise Radial Velocities of Polaris: Detection of Amplitude Growth
We present a first results from a long-term program of a radial velocity
study of Cepheid Polaris (F7 Ib) aimed to find amplitude and period of
pulsations and nature of secondary periodicities. 264 new precise radial
velocity measurements were obtained during 2004-2007 with the fiber-fed echelle
spectrograph Bohyunsan Observatory Echelle Spectrograph (BOES) of 1.8m
telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We find a
pulsational radial velocity amplitude and period of Polaris for three seasons
of 2005.183, 2006.360, and 2007.349 as 2K = 2.210 +/- 0.048 km/s, 2K = 2.080
+/- 0.042 km/s, and 2K = 2.406 +/- 0.018 km/s respectively, indicating that the
pulsational amplitudes of Polaris that had decayed during the last century is
now increasing rapidly. The pulsational period was found to be increasing too.
This is the first detection of a historical turnaround of pulsational amplitude
change in Cepheids. We clearly find the presence of additional radial velocity
variations on a time scale of about 119 days and an amplitude of about +/- 138
m/s, that is quasi-periodic rather than strictly periodic. We do not confirm
the presence in our data the variation on a time scale 34-45 days found in
earlier radial velocity data obtained in 80's and 90's. We assume that both the
119 day quasi-periodic, noncoherent variations found in our data as well as
34-45 day variations found before can be caused by the 119 day rotation periods
of Polaris and by surface inhomogeneities such as single or multiple spot
configuration varying with the time.Comment: 15 pages, 7 figures, Accepted for publication in The Astronomical
Journa
Thermodynamics of trapped interacting bosons in one dimension
On the basis of Bethe ansatz solution of bosons with delta-function
interaction in a one-dimensional potential well, the thermodynamics equilibrium
of the system in finite temperature is studied by using the strategy of Yang
and Yang. The thermodynamics quantities, such as specific heat etc. are
obtained for the cases of strong coupling limit and weak coupling limit
respectively.Comment: RevTEX, 7 pages, 0 figur
Dispersion of imbibition fronts
We have studied the dispersive behaviour of imbibition fronts in a porous
medium by X-ray tomography. Injection velocities were varied and the porous
medium was initially prewetted or not. At low velocity in the prewetted medium,
the imbibition profiles are found to be distinctly hyperdispersive. The
profiles are anomalously extended when compared to tracer fronts exhibiting
conventional (Gaussian) dispersion. We observe a strong velocity dependence of
the exponent characterizing the divergence of the dispersion coefficient for
low wetting-fluid saturation. Hyperdispersion is absent at high imbibition
velocities or when the medium is not prewetted.Comment: 8 pages, 5 figures; submitted to Europhysics Letter
Influence of boundaries on pattern selection in through-flow
The problem of pattern selection in absolutely unstable open flow systems is
investigated by considering the example of Rayleigh-B\'{e}nard convection. The
spatiotemporal structure of convection rolls propagating downstream in an
externally imposed flow is determined for six different inlet/outlet boundary
conditions. Results are obtained by numerical simulations of the Navier-Stokes
equations and by comparison with the corresponding Ginzburg-Landau amplitude
equation. A unique selection process is observed being a function of the
control parameters and the boundary conditions but independent of the history
and the system length. The problem can be formulated in terms of a nonlinear
eigen/boundary value problem where the frequency of the propagating pattern is
the eigenvalue. PACS: 47.54.+r, 47.20.Bp, 47.27.Te, 47.20.KyComment: 8 pages, 5 Postscript figures, Physica D 97, 253-263 (1996
Using mixed methods to investigate school improvement and the role of leadership: an example of a longitudinal study in England
Purpose-The purpose of this paper is to discuss the use of mixed methods research in a major three year project and focuses on the contribution of quantitative and qualitative approaches to study school improvement. It discusses the procedures and multiple data sources used in studying improvement using the example of a recent study of the role of leadership in promoting improvement in primary and secondary schools’ academic results in England. Although the definition of improvement used was based on robust analyses of data on students’ academic outcomes, the mixed methods design enabled a broader perspective to be achieved.
Design/methodology/approach-The study illustrates how the multilevel analysis of students’ national assessment and examination results based on national data sets for primary and secondary schools in England were used to investigate the concept of academic effectiveness based on value-added methodology. Using three successive years of national results a purposive sample of schools were identified that could be classified as both effective and improving over the period 2003-2005. In addition, surveys and interviews were used to gather evidence of the role of stakeholder perceptions in investigating school improvement strategies and processes.
Findings-National student attainment data sets were used for the identification of improving and effective schools and revealed the importance of considering their different starting points in their classification of three distinctive improvement groups. The combination of quantitative survey data from headteachers and key staff with qualitative case study data enabled a range of analysis strategies and the development of statistical models and deeper understanding of the role of leadership.
Research limitations/implications-The limitations of a focus on only academic outcomes and “value-added” measures of student progress are discussed. The challenges and opportunities faced in analysis and integration of the different sources of evidence are briefly explored.
Practical implications-The study contributes to the knowledge base on the identification of school improvement and use of performance data. The findings on strategies and processes that support improvement are of relevance to policy makers and practitioners, especially school leaders.
Originality/value-The mixed methods design adopted in the study enabled the research to combine rigorous quantitative and in-depth qualitative data in new ways to extend and make new claims to knowledge about the role of school leadership in promoting school improvement based on the study of effective and improved schools’ experiences
- …