674 research outputs found
Correlations, compressibility, and capacitance in double-quantum-well systems in the quantum Hall regime
In the quantum Hall regime, electronic correlations in double-layer
two-dimensional electron systems are strong because the kinetic energy is
quenched by Landau quantization. In this article we point out that these
correlations are reflected in the way the partitioning of charge between the
two-layers responds to a bias potential. We report on illustrative calculations
based on an unrestricted Hartree-Fock approximation which allows for
spontaneous inter-layer phase coherence. The possibility of studying
inter-layer correlations by capacitive coupling to separately contacted
two-dimensional layers is discussed in detail.Comment: RevTex style, 21 pages, 6 postscript figures in a separate file;
Phys. Rev. B (in press
Low-field magnetoresistance in GaAs 2D holes
We report low-field magnetotransport data in two-dimensional hole systems in
GaAs/AlGaAs heterostructures and quantum wells, in a large density range, cm, with primary focus on
samples grown on (311)A GaAs substrates. At high densities, cm, we observe a remarkably strong positive magnetoresistance.
It appears in samples with an anisotropic in-plane mobility and predominantly
along the low-mobility direction, and is strongly dependent on the
perpendicular electric field and the resulting spin-orbit interaction induced
spin-subband population difference. A careful examination of the data reveals
that the magnetoresistance must result from a combination of factors including
the presence of two spin-subbands, a corrugated quantum well interface which
leads to the mobility anisotropy, and possibly weak anti-localization. None of
these factors can alone account for the observed positive magnetoresistance. We
also present the evolution of the data with density: the magnitude of the
positive magnetoresistance decreases with decreasing density until, at the
lowest density studied ( cm), it vanishes and is
replaced by a weak negative magnetoresistance.Comment: 8 pages, 8 figure
Learning from the experience of peer support facilitators and study nurses in diabetes peer support: a qualitative study
Objectives
We report on the experiences of peer support facilitators and study nurses who participated in a large trial of peer support for type 2 diabetes. The support was led by volunteer peer support facilitators, who were trained in overcoming barriers to diabetes care, motivational interviewing, listening skills and setting up and running group support sessions. There is currently a distinct lack of qualitative evidence on what works in peer support.
Methods
The peer support facilitators and study nurses completed open-answer questionnaire items on what worked well and less well, problems encountered and how they were resolved, group dynamics and suggestions for improvement. We also collected data from end-of-study meetings. Inductive thematic analysis was used to allow the emergent themes to be strongly based in the data.
Findings: We find that process factors, peer support facilitator and peer characteristics, their relationships with each other and group dynamics are all fundamental for effective peer support. Sustaining and ending support also emerged as a key theme.
Discussion
Given the increasing interest in peer support, these findings will be useful to those interested in running groups in the future. Training programmes should help peer support facilitators develop confidence whilst emphasising that peer support ideally entails an equal, democratic dynamic. More attention is needed on to how to end groups appropriately
Magnetoresistance and electronic structure of asymmetric GaAs/AlGaAs double quantum wells in the in-plane/tilted magnetic field
Bilayer two-dimensional electron systems formed by a thin barrier in the GaAs
buffer of a standard heterostructure were investigated by magnetotransport
measurements. In magnetic fields oriented parallel to the electron layers, the
magnetoresistance exhibits an oscillation associated with the depopulation of
the higher occupied subband and the field-induced transition into a decoupled
bilayer. Shubnikov-de Haas oscillations in slightly tilted magnetic fields
allow to reconstruct the evolution of the electron concentration in the
individual subbands as a function of the in-plane magnetic field. The
characteristics of the system derived experimentally are in quantitative
agreement with numerical self-consistent-field calculations of the electronic
structure.Comment: 6 pages, 5 figure
Compressibility of a two-dimensional hole gas in tilted magnetic field
We have measured compressibility of a two-dimensional hole gas in
p-GaAs/AlGaAs heterostructure, grown on a (100) surface, in the presence of a
tilted magnetic field. It turns out that the parallel component of magnetic
field affects neither the spin splitting nor the density of states. We conclude
that: (a) g-factor in the parallel magnetic field is nearly zero in this
system; and (b) the level of the disorder potential is not sensitive to the
parallel component of the magnetic field
Interatomic potentials for atomistic simulations of the Ti-Al system
Semi-empirical interatomic potentials have been developed for Al, alpha-Ti,
and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large
database of experimental as well as ab-initio data. The ab-initio calculations
were performed by the linear augmented plane wave (LAPW) method within the
density functional theory to obtain the equations of state for a number of
crystal structures of the Ti-Al system. Some of the calculated LAPW energies
were used for fitting the potentials while others for examining their quality.
The potentials correctly predict the equilibrium crystal structures of the
phases and accurately reproduce their basic lattice properties. The potentials
are applied to calculate the energies of point defects, surfaces, planar faults
in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al
system, the proposed potentials provide reasonable description of the lattice
thermal expansion, demonstrating their usefulness in the molecular dynamics or
Monte Carlo studies at high temperatures. The energy along the tetragonal
deformation path (Bain transformation) in gamma-TiAl calculated with the EAM
potential is in a fairly good agreement with LAPW calculations. Equilibrium
point defect concentrations in gamma-TiAl are studied using the EAM potential.
It is found that antisite defects strongly dominate over vacancies at all
compositions around stoichiometry, indicating that gamm-TiAl is an antisite
disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press
Association studies of polybrominated biphenyls in aquatic systems
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47983/1/128_2005_Article_BF01606089.pd
Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal
Defects are believed to play a fundamental role in the supersolid state of
4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at
zero temperature of the properties of solid 4He in presence of many vacancies,
up to 30 in two dimensions (2D). In all studied cases the crystalline order is
stable at least as long as the concentration of vacancies is below 2.5%. In the
2D system for a small number, n_v, of vacancies such defects can be identified
in the crystalline lattice and are strongly correlated with an attractive
interaction. On the contrary when n_v~10 vacancies in the relaxed system
disappear and in their place one finds dislocations and a revival of the
Bose-Einstein condensation. Thus, should zero-point motion defects be present
in solid 4He, such defects would be dislocations and not vacancies, at least in
2D. In order to avoid using periodic boundary conditions we have studied the
exact ground state of solid 4He confined in a circular region by an external
potential. We find that defects tend to be localized in an interfacial region
of width of about 15 A. Our computation allows to put as upper bound limit to
zero--point defects the concentration 0.003 in the 2D system close to melting
density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special
Issue on Supersolid
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