2,274 research outputs found
Theory of fluctuation conductivity from interband pairing in pnictide superconductors
We derive the effective action for superconducting fluctuations in a
four-band model for pnictides, discussing the emergence of a single critical
mode out of a dominant interband pairing mechanism. We then apply our model to
calculate the paraconductivity in two-dimensional and layered three-dimensional
systems, and compare our results with recent resistivity measurements in
SmFeAsOFComment: 4 pages, 1 figure; final versio
Entrepreneurship and new product development: exploring the âadvantage of youthâ and âbusiness acumenâ views
Spinal modelling to investigate postural loading and stability
Numerous mathematical models have been developed to investigate the high
incidence of low back pain associated with lifting activities. These mainly
consider the muscle forces required to support the spine, and few have
considered the additional role of curvature.
One previous model which represented the spine as an arch (Aspden 1987)
indicated the curvature to have a significant effect on both loading and
stability of the spine. However this model included collective loading
patterns for body weight and muscle forces, and only partial representation
of the spine. On the basis that the level of anatomic detail of a model affects
the accuracy of its predictions (McGill and Norman, 1987), this thesis
describes the development of a model which provides greater detail for
investigating spinal stability in the sagittal plane.
The curvature of the whole spine, a distributed loading pattern for body
weight, and the activity of individual spinal muscle groups have been
considered. Comparison with the previous arch model has shown these to be
necessary features for determining the loading and stability associated with a
given posture. In particular, application of individual muscle forces provide
greater control of stability at each vertebral level. By considering the force
requirements of the individual muscle groups and the consequent loads at
each intervertebral joint, possible areas of tissue over load can be identified
Statics and dynamics of charge fluctuations in the t-J model
The equation for the charge vertex of the model is derived and
solved in leading order of an 1/N expansion, working directly in terms of
Hubbard operators. Various quantities which depend crucially on are
then calculated, such as the life time and the transport life time of electrons
due to a charge coupling to other degrees of freedom and the charge-charge
correlation function. Our results show that the static screening of charges and
the dynamics of charge fluctuations depend only weakly on and are mainly
determined by the constraint of having no double occupancies of sites.Comment: 10 latex pages, 4 figures as post-script file
School attendance of children and the work of mothers: a joint multilevel model for India.
This paper investigates the determinants of school attendance of children and their motherâs working status when the mother decides how to allocate her time and that of her children. A multilevel random effects model is applied to study the motherâs participation and the schooling status of her children in a joint framework. Using the second National Family Health Survey (NFHS-2) for India, we find that, controlling for many covariates among which wealth is the most powerful predictor, children of working mothers have a lower probability of attending school. This, together with the result that only illiterate and poor mothers with unskilled or unemployed partners have a high probability of working, points to the need for decent labour market opportunities for females. An implication of our findings is that any policy aiming both at enhancing womenâs empowerment through labour and increasing childrenâs welfare should also target improvements in womenâs conditions in the labour market
Spin and energy relaxation in germanium studied by spin-polarized direct-gap photoluminescence
Spin orientation of photoexcited carriers and their energy relaxation is
investigated in bulk Ge by studying spin-polarized recombination across the
direct band gap. The control over parameters such as doping and lattice
temperature is shown to yield high polarization degree, namely larger than 40%,
as well as a fine-tuning of the angular momentum of the emitted light with a
complete reversal between right- and left-handed circular polarization. By
combining the measurement of the optical polarization state of band-edge
luminescence and Monte Carlo simulations of carrier dynamics, we show that
these very rich and complex phenomena are the result of the electron
thermalization and cooling in the multi-valley conduction band of Ge. The
circular polarization of the direct-gap radiative recombination is indeed
affected by energy relaxation of hot electrons via the X valleys and the
Coulomb interaction with extrinsic carriers. Finally, thermal activation of
unpolarized L valley electrons accounts for the luminescence depolarization in
the high temperature regime
Effect of mesoscopic inhomogeneities on local tunnelling density of states
We carry out a theoretical analysis of the momentum dependence of the
Fourier-transformed local density of states (LDOS) in the superconducting
cuprates within a model considering the interference of quasiparticles
scattering on quenched impurities. The impurities introduce an external
scattering potential, which is either nearly local in space or it can acquire a
substantial momentum dependence due to a possible strong momentum dependence of
the electronic screening near a charge modulation instability. The key new
effect that we introduce is an additional mesoscopic disorder aiming to
reproduce the inhomogeneities experimentally observed in scanning tunnelling
microscopy. The crucial effect of this mesoscopic disorder is to give rise to
point-like spectroscopic features, to be contrasted with the curve-like shape
of the spectra previously calculated within the interfering-quasiparticle
schemes. It is also found that stripe-like charge modulations play a relevant
role to correctly reproduce all the spectral features of the experiments.Comment: 11 pages and 5 figure
Effective medium theory for superconducting layers: A systematic analysis including space correlation effects
We investigate the effects of mesoscopic inhomogeneities on the
metal-superconductor transition occurring in several two-dimensional electron
systems. Specifically, as a model of systems with mesoscopic inhomogeneities,
we consider a random-resistor network, which we solve both with an exact
numerical approach and by the effective medium theory. We find that the width
of the transition in these two-dimensional superconductors is mainly ruled by
disorder rather than by fluctuations. We also find that "tail" features in
resistivity curves of interfaces between LaAlO3 or LaTiO3 and SrTiO3 can arise
from a bimodal distribution of mesoscopic local Tc's and/or substantial space
correlations between the mesoscopic domains.Comment: 12 pages, 10 figure
- âŠ