450 research outputs found
Electron energy loss spectroscopy with parallel readout of energy and momentum
We introduce a high energy resolution electron source that matches the
requirements for parallel readout of energy and momentum of modern
hemispherical electron energy analyzers. The system is designed as an add-on
device to typical photoemission chambers. Due to the multiplex gain, a complete
phonon dispersion of a Cu(111) surface was measured in seven minutes with 4 meV
energy resolution
The Neverout March : and Two step
https://digitalcommons.library.umaine.edu/mmb-ps/2001/thumbnail.jp
Parental presence at induction of anaesthesia: who benefits?
No Abstract. Southern African Journal of Anaesthesia and Analgesia Vol. 12(1) 2006: 2
Effective mass in quasi two-dimensional systems
The effective mass of the quasiparticle excitations in quasi two-dimensional
systems is calculated analytically. It is shown that the effective mass
increases sharply when the density approaches the critical one of
metal-insulator transition. This suggests a Mott type of transition rather than
an Anderson like transition.Comment: 3 pages 3 figure
Dynamic polarization of graphene by moving external charges: random phase approximation
We evaluate the stopping and image forces on a charged particle moving
parallel to a doped sheet of graphene by using the dielectric response
formalism for graphene's -electron bands in the random phase approximation
(RPA). The forces are presented as functions of the particle speed and the
particle distance for a broad range of charge-carrier densities in graphene. A
detailed comparison with the results from a kinetic equation model reveal the
importance of inter-band single-particle excitations in the RPA model for high
particle speeds. We also consider the effects of a finite gap between graphene
and a supporting substrate, as well as the effects of a finite damping rate
that is included through the use of Mermin's procedure. The damping rate is
estimated from a tentative comparison of the Mermin loss function with a HREELS
experiment. In the limit of low particle speeds, several analytical results are
obtained for the friction coefficient that show an intricate relationship
between the charge-carrier density, the damping rate, and the particle
distance, which may be relevant to surface processes and electrochemistry
involving graphene.Comment: 14 pages, 10 figures, accepted for publication in Phys. Rev.
Twisted-light-induced optical transitions in semiconductors: Free-carrier quantum kinetics
We theoretically investigate the interband transitions and quantum kinetics
induced by light carrying orbital angular momentum, or twisted light, in bulk
semiconductors. We pose the problem in terms of the Heisenberg equations of
motion of the electron populations, and inter- and intra-band coherences. Our
theory extends the free-carrier Semiconductor Bloch Equations to the case of
photo-excitation by twisted light. The theory is formulated using cylindrical
coordinates, which are better suited to describe the interaction with twisted
light than the usual cartesian coordinates used to study regular optical
excitation. We solve the equations of motion in the low excitation regime, and
obtain analytical expressions for the coherences and populations; with these,
we calculate the orbital angular momentum transferred from the light to the
electrons and the paramagnetic and diamagnetic electric current densities.Comment: 11 pages, 3 figure
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
A novel high-current, high-resolution, low-kinetic-energy electron source for inverse photoemission spectroscopy
A high-current electron source for inverse photoemission spectroscopy (IPES)
is described. The source comprises a thermal cathode electron emission system,
an electrostatic deflector-monochromator, and a lens system for variable
kinetic energy (1.6 - 20 eV) at the target. When scaled to the energy
resolution, the electron current is an order of magnitude higher than that of
previously described electron sources developed in the context of electron
energy loss spectroscopy. Surprisingly, the experimentally measured energy
resolution turned out to be significantly better than calculated by standard
programs, which include the electron-electron repulsion in the continuum
approximation. The achieved currents are also significantly higher than
predicted. We attribute this "inverse Boersch-effect" to a mechanism of
velocity selection in the forward direction by binary electron-electron
collisions
Bent surface free energy differences from simulation
We present a calculation of the change of free energy of a solid surface upon
bending of the solid. It is based on extracting the surface stress through a
molecular dynamics simulation of a bent slab by using a generalized stress
theorem formula, and subsequent integration of the stress with respect to
strain as a function of bending curvature. The method is exemplified by
obtaining and comparing free energy changes with curvature of various
reconstructed Au(001) surfaces.Comment: 14 pages, 2 figures, accepted for publication in Surface Science
(ECOSS-19
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