548 research outputs found
Friction force on slow charges moving over supported graphene
We provide a theoretical model that describes the dielectric coupling of a 2D
layer of graphene, represented by a polarization function in the Random Phase
Approximation, and a semi-infinite 3D substrate, represented by a surface
response function in a non-local formulation. We concentrate on the role of the
dynamic response of the substrate for low-frequency excitations of the combined
graphene-substrate system, which give rise to the stopping force on slowly
moving charges above graphene. A comparison of the dielectric loss function
with experimental HREELS data for graphene on a SiC substrate is used to
estimate the damping rate in graphene and to reveal the importance of phonon
excitations in an insulating substrate. A signature of the hybridization
between graphene's pi plasmon and the substrate's phonon is found in the
stopping force. A friction coefficient that is calculated for slow charges
moving above graphene on a metallic substrate shows an interplay between the
low-energy single-particle excitations in both systems.Comment: 13 pages, 5 figures, submitted to Nanotechnology for a special issue
related to the NGC 2009 conference (http://asdn.net/ngc2009/index.shtml
Knowledge and expectations of labour among primigravid women in the public health sector
Objectives. We analysed knowledge and expectations of the process and pain of labour in primigravidas attending a local midwifery obstetric unit (MOU). It was anticipated that the results of this study could inform the development of interventions aimed at improving the analgesic care of women delivering at primary health care obstetric units. Design. Qualitative analysis of data obtained from in-depth semi-structured interviews. Setting. A Cape Town MOU. Subjects. 30 black African, Xhosa-speaking primigravidas. Outcome measures. An open-ended interview guide was developed. The themes explored included previous painful experiences, knowledge of labour, expectations of and attitudes towards labour pain, and knowledge of biomedical analgesia. Results. Patients were poorly informed about the process and pain of labour. Most women appeared highly motivated concerning their ability to cope with labour. Most expected pain, but had no concept of the severity or duration of the pain, and knew very little concerning methods available for pain relief in labour. Conclusion. Women at this MOU were poorly prepared for the experience of delivery. Antenatal programmes should incorporate sensitive education concerning the process and pain of labour and the methods available to alleviate pain
The Effects of Next-Nearest-Neighbor Interactions on the Orientation Dependence of Step Stiffness: Reconciling Theory with Experiment for Cu(001)
Within the solid-on-solid (SOS) approximation, we carry out a calculation of
the orientational dependence of the step stiffness on a square lattice with
nearest and next-nearest neighbor interactions. At low temperature our result
reduces to a simple, transparent expression. The effect of the strongest trio
(three-site, non pairwise) interaction can easily be incorporated by modifying
the interpretation of the two pairwise energies. The work is motivated by a
calculation based on nearest neighbors that underestimates the stiffness by a
factor of 4 in directions away from close-packed directions, and a subsequent
estimate of the stiffness in the two high-symmetry directions alone that
suggested that inclusion of next-nearest-neighbor attractions could fully
explain the discrepancy. As in these earlier papers, the discussion focuses on
Cu(001).Comment: 8 pages, 3 figures, submitted to Phys. Rev.
Descriptive analysis of the aripiprazole arm in the risperidone long-acting injectable vs. quetiapine relapse prevention trial (constaTRE)
Geometry and quantum delocalization of interstitial oxygen in silicon
The problem of the geometry of interstitial oxygen in silicon is settled by
proper consideration of the quantum delocalization of the oxygen atom around
the bond-center position. The calculated infrared absorption spectrum accounts
for the 517 and 1136 cm bands in their position, character, and isotope
shifts. The asymmetric lineshape of the 517 cm peak is also well
reproduced. A new, non-infrared-active, symmetric-stretching mode is found at
596 cm. First-principles calculations are presented supporting the
nontrivial quantum delocalization of the oxygen atom.Comment: uuencoded, compressed postscript file for the whole. 4 pages (figures
included), accepted in PR
Adsorbate-induced substrate relaxation and the adsorbate-adsorbate interaction
We formulate the theory of the perturbation caused by an adsorbate upon the
substrate lattice in terms of a local modification of the interatomic potential
energy around the adsorption site, which leads to the relaxation of substrate
atoms. We apply the approach to CO chemisorption on close-packed metal
surfaces, and show that the adsorbate-adsorbate interaction and a variety of
other properties can be well described by a simple model.Comment: 4 pages, 2 eps figures, RevTex, submitted to PR
Variable Curvature Slab Molecular Dynamics as a Method to Determine Surface Stress
A thin plate or slab, prepared so that opposite faces have different surface
stresses, will bend as a result of the stress difference. We have developed a
classical molecular dynamics (MD) formulation where (similar in spirit to
constant-pressure MD) the curvature of the slab enters as an additional
dynamical degree of freedom. The equations of motion of the atoms have been
modified according to a variable metric, and an additional equation of motion
for the curvature is introduced. We demonstrate the method to Au surfaces, both
clean and covered with Pb adsorbates, using many-body glue potentials.
Applications to stepped surfaces, deconstruction and other surface phenomena
are under study.Comment: 16 pages, 8 figures, REVTeX, submitted to Physical Review
Knowledge and expectations of labour among primigravid women in the public health sector
Objectives. We analysed knowledge and expectations of the
process and pain of labour in primigravidas attending a
local midwifery obstetric unit (MOU). It was anticipated that
the results of this study could inform the development of
interventions aimed at improving the analgesic care of women
delivering at primary health care obstetric units.
Design. Qualitative analysis of data obtained from in-depth
semi-structured interviews.
Setting. A Cape Town MOU.
Subjects. 30 black African, Xhosa-speaking primigravidas.
Outcome measures. An open-ended interview guide was
developed. The themes explored included previous painful
experiences, knowledge of labour, expectations of and attitudes
towards labour pain, and knowledge of biomedical analgesia.
Results. Patients were poorly informed about the process
and pain of labour. Most women appeared highly motivated
concerning their ability to cope with labour. Most expected
pain, but had no concept of the severity or duration of the pain,
and knew very little concerning methods available for pain
relief in labour.
Conclusion. Women at this MOU were poorly prepared for
the experience of delivery. Antenatal programmes should
incorporate sensitive education concerning the process and
pain of labour and the methods available to alleviate pain. South African Medical Journal Vol. 97 (6) 2007: pp. 461-46
Relaxation and reconstruction on (111) surfaces of Au, Pt, and Cu
We have theoretically studied the stability and reconstruction of (111)
surfaces of Au, Pt, and Cu. We have calculated the surface energy, surface
stress, interatomic force constants, and other relevant quantities by ab initio
electronic structure calculations using the density functional theory (DFT), in
a slab geometry with periodic boundary conditions. We have estimated the
stability towards a quasi-one-dimensional reconstruction by using the
calculated quantities as parameters in a one-dimensional Frenkel-Kontorova
model. On all surfaces we have found an intrinsic tensile stress. This stress
is large enough on Au and Pt surfaces to lead to a reconstruction in which a
denser surface layer is formed, in agreement with experiment. The
experimentally observed differences between the dense reconstruction pattern on
Au(111) and a sparse structure of stripes on Pt(111) are attributed to the
details of the interaction potential between the first layer of atoms and the
substrate.Comment: 8 pages, 3 figures, submitted to Physical Review
Linewidth of single photon transitions in Mn-acetate
We use time-domain terahertz spectroscopy to measure the position and
linewidth of single photon transitions in Mn-acetate. This linewidth is
compared to the linewidth measured in tunneling experiments. We conclude that
local magnetic fields (due to dipole or hyperfine interactions) cannot be
responsible for the observed linewidth, and suggest that the linewidth is due
to variations in the anisotropy constants for different clusters. We also
calculate a lower limit on the dipole field distribution that would be expected
due to random orientations of clusters and find that collective effects must
narrow this distribution in tunneling measurements.Comment: 5 pages, accepted to Physical Review
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