575 research outputs found
Adaptive hypermedia system interoperability : a 'real world' evaluation
Adaptive Hypermedia (AH) authoring is widely acknowledged to be complex and time consuming, yet this vital process is rarely evaluated. Recent research has approached the authoring problem by ensuring that previously created materials can be converted from one system to another. This paper evaluates the results of this research, specifically the creation of adaptive materials in MOT and their conversion and subsequent delivery in WHURLE. A group of technically experienced IT users who are novice AH authors were exposed to MOT and WHURLE during an introductory week long course. This paper interprets the results of these authors using a "write once, deliver many" paradigm of adaptive hypermedia creation
How is Learning Fluctuating? FutureLearn MOOCs Fine-Grained Temporal Analysis and Feedback to Teachers
Data-intensive analysis of massive open online courses (MOOCs) is popular. Researchers have been proposing various parameters conducive to analysis and prediction of student behaviour and outcomes in MOOCs, as well as different methods to analyse and use these parameters, ranging from statistics, to NLP, to ML, and even graph analysis. In this paper, we focus on patterns to be extracted, and apply systematic data analysis methods in one of the few genuinely large-scale data collection of 5 MOOCs, spread over 21 runs, on FutureLearn, a UK-based MOOCs provider, that, whilst offering a broad range of courses from many universities, NGOs and other institutions, has been less evaluated, in comparison to, e.g., its American counterparts. We analyse temporal quiz solving patterns; specifically, the less explored issue on how the first number of weeks of data predicts activities in the last weeks; we also address the classical MOOC question on the completion chance. Finally, we discuss the type of feedback a teacher or designer could receive on their MOOCs, in terms of fine-grained analysis of their material, and what personalisation could be provided to a student
Response, relaxation and transport in unconventional superconductors
We investigate the collision-limited electronic Raman response and the
attenuation of ultrasound in spin-singlet d-wave superconductors at low
temperatures. The dominating elastic collisions are treated within a t-matrix
approximation, which combines the description of weak (Born) and strong
(unitary) impurity scattering. In the long wavelength limit a two-fluid
description of both response and transport emerges. Collisions are here seen to
exclusively dominate the relaxational dynamics of the (Bogoliubov)
quasiparticle system and the analysis allows for a clear connection of response
and transport phenomena. When applied to quasi-2-d superconductors like the
cuprates, it turns out that the transport parameter associated with the Raman
scattering intensity for B1g and B2g photon polarization is closely related to
the corresponding components of the shear viscosity tensor, which dominates the
attenuation of ultrasound. At low temperatures we present analytic solutions of
the transport equations, resulting in a non-power-law behavior of the transport
parameters on temperature.Comment: 22 pages, 3 figure
Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions
We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb
superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson
junctions with high critical current densities, large normal resistance times
area products, high quality factors, and very good spatial uniformity. For
these junctions a transition from 0- to \pi-coupling is observed for a
thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The
magnetic field dependence of the \pi-coupled junctions demonstrates good
spatial homogeneity of the tunneling barrier and ferromagnetic interlayer.
Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane
anisotropy and large saturation magnetization, indicating negligible dead
layers at the interfaces. A careful analysis of Fiske modes provides
information on the junction quality factor and the relevant damping mechanisms
up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at
low frequencies, the damping is dominated by the finite surface resistance of
the junction electrodes at high frequencies. High quality factors of up to 30
around 200 GHz have been achieved. Our analysis shows that the fabricated
junctions are promising for applications in superconducting quantum circuits or
quantum tunneling experiments.Comment: 15 pages, 9 figure
Nonlinear Viscous Vortex Motion in Two-Dimensional Josephson-Junction Arrays
When a vortex in a two-dimensional Josephson junction array is driven by a
constant external current it may move as a particle in a viscous medium. Here
we study the nature of this viscous motion. We model the junctions in a square
array as resistively and capacitively shunted Josephson junctions and carry out
numerical calculations of the current-voltage characteristics. We find that the
current-voltage characteristics in the damped regime are well described by a
model with a {\bf nonlinear} viscous force of the form , where is the vortex velocity,
is the velocity dependent viscosity and and are
constants for a fixed value of the Stewart-McCumber parameter. This result is
found to apply also for triangular lattices in the overdamped regime. Further
qualitative understanding of the nature of the nonlinear friction on the vortex
motion is obtained from a graphic analysis of the microscopic vortex dynamics
in the array. The consequences of having this type of nonlinear friction law
are discussed and compared to previous theoretical and experimental studies.Comment: 14 pages RevTex, 9 Postscript figure
Inflation, cold dark matter, and the central density problem
A problem with high central densities in dark halos has arisen in the context
of LCDM cosmologies with scale-invariant initial power spectra. Although n=1 is
often justified by appealing to the inflation scenario, inflationary models
with mild deviations from scale-invariance are not uncommon and models with
significant running of the spectral index are plausible. Even mild deviations
from scale-invariance can be important because halo collapse times and
densities depend on the relative amount of small-scale power. We choose several
popular models of inflation and work out the ramifications for galaxy central
densities. For each model, we calculate its COBE-normalized power spectrum and
deduce the implied halo densities using a semi-analytic method calibrated
against N-body simulations. We compare our predictions to a sample of dark
matter-dominated galaxies using a non-parametric measure of the density. While
standard n=1, LCDM halos are overdense by a factor of 6, several of our example
inflation+CDM models predict halo densities well within the range preferred by
observations. We also show how the presence of massive (0.5 eV) neutrinos may
help to alleviate the central density problem even with n=1. We conclude that
galaxy central densities may not be as problematic for the CDM paradigm as is
sometimes assumed: rather than telling us something about the nature of the
dark matter, galaxy rotation curves may be telling us something about inflation
and/or neutrinos. An important test of this idea will be an eventual consensus
on the value of sigma_8, the rms overdensity on the scale 8 h^-1 Mpc. Our
successful models have values of sigma_8 approximately 0.75, which is within
the range of recent determinations. Finally, models with n>1 (or sigma_8 > 1)
are highly disfavored.Comment: 13 pages, 6 figures. Minor changes made to reflect referee's
Comments, error in Eq. (18) corrected, references updated and corrected,
conclusions unchanged. Version accepted for publication in Phys. Rev. D,
scheduled for 15 August 200
Conformal Invariance, Dark Energy, and CMB Non-Gaussianity
In addition to simple scale invariance, a universe dominated by dark energy
naturally gives rise to correlation functions possessing full conformal
invariance. This is due to the mathematical isomorphism between the conformal
group of certain 3 dimensional slices of de Sitter space and the de Sitter
isometry group SO(4,1). In the standard homogeneous isotropic cosmological
model in which primordial density perturbations are generated during a long
vacuum energy dominated de Sitter phase, the embedding of flat spatial sections
in de Sitter space induces a conformal invariant perturbation spectrum and
definite prediction for the shape of the non-Gaussian CMB bispectrum. In the
case in which the density fluctuations are generated instead on the de Sitter
horizon, conformal invariance of the horizon embedding implies a different but
also quite definite prediction for the angular correlations of CMB
non-Gaussianity on the sky. Each of these forms for the bispectrum is intrinsic
to the symmetries of de Sitter space and in that sense, independent of specific
model assumptions. Each is different from the predictions of single field slow
roll inflation models which rely on the breaking of de Sitter invariance. We
propose a quantum origin for the CMB fluctuations in the scalar gravitational
sector from the conformal anomaly that could give rise to these
non-Gaussianities without a slow roll inflaton field, and argue that conformal
invariance also leads to the expectation for the relation n_S-1=n_T between the
spectral indices of the scalar and tensor power spectrum. Confirmation of this
prediction or detection of non-Gaussian correlations in the CMB of one of the
bispectral shape functions predicted by conformal invariance can be used both
to establish the physical origins of primordial density fluctuations and
distinguish between different dynamical models of cosmological vacuum dark
energy.Comment: 73 pages, 9 figures. Final Version published in JCAP. New Section 4
added on linearized scalar gravitational potentials; New Section 8 added on
gravitational wave tensor perturbations and relation of spectral indices n_T
= n_S -1; Table of Contents added; Eqs. (3.14) and (3.15) added to clarify
relationship of bispectrum plotted to CMB measurements; Some other minor
modification
Theory and simulation of quantum photovoltaic devices based on the non-equilibrium Green's function formalism
This article reviews the application of the non-equilibrium Green's function
formalism to the simulation of novel photovoltaic devices utilizing quantum
confinement effects in low dimensional absorber structures. It covers
well-known aspects of the fundamental NEGF theory for a system of interacting
electrons, photons and phonons with relevance for the simulation of
optoelectronic devices and introduces at the same time new approaches to the
theoretical description of the elementary processes of photovoltaic device
operation, such as photogeneration via coherent excitonic absorption,
phonon-mediated indirect optical transitions or non-radiative recombination via
defect states. While the description of the theoretical framework is kept as
general as possible, two specific prototypical quantum photovoltaic devices, a
single quantum well photodiode and a silicon-oxide based superlattice absorber,
are used to illustrated the kind of unique insight that numerical simulations
based on the theory are able to provide.Comment: 20 pages, 10 figures; invited review pape
Transport properties of strongly correlated metals:a dynamical mean-field approach
The temperature dependence of the transport properties of the metallic phase
of a frustrated Hubbard model on the hypercubic lattice at half-filling are
calculated. Dynamical mean-field theory, which maps the Hubbard model onto a
single impurity Anderson model that is solved self-consistently, and becomes
exact in the limit of large dimensionality, is used. As the temperature
increases there is a smooth crossover from coherent Fermi liquid excitations at
low temperatures to incoherent excitations at high temperatures. This crossover
leads to a non-monotonic temperature dependence for the resistance,
thermopower, and Hall coefficient, unlike in conventional metals. The
resistance smoothly increases from a quadratic temperature dependence at low
temperatures to large values which can exceed the Mott-Ioffe-Regel value, hbar
a/e^2 (where "a" is a lattice constant) associated with mean-free paths less
than a lattice constant. Further signatures of the thermal destruction of
quasiparticle excitations are a peak in the thermopower and the absence of a
Drude peak in the optical conductivity. The results presented here are relevant
to a wide range of strongly correlated metals, including transition metal
oxides, strontium ruthenates, and organic metals.Comment: 19 pages, 9 eps figure
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