5,150 research outputs found
Chern number spins of Mn acceptor magnets in GaAs
We determine the effective total spin of local moments formed from
acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern
numbers. We find that when individual Mn atoms are close to the sample surface,
the total spin changes from to , due to quenching of the
acceptor orbital moment. For Mn pairs in bulk, the total depends on the
pair orientation in the GaAs lattice and on the separation between the Mn
atoms. We point out that Berry curvature variation as a function of local
moment orientation can profoundly influence the quantum spin dynamics of these
magnetic entities.Comment: 4 pages, 3 figure
Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layers
Motivated by recent STM experiments, we present a theoretical study of the
electronic and magnetic properties of the Mn-induced acceptor level obtained by
substituting a single Ga atom in the (110) surface layer of GaAs or in one of
the atoms layers below the surface. We employ a kinetic-exchange tight-binding
model in which the relaxation of the (110) surface is taken into account. The
acceptor wave function is strongly anisotropic in space and its detailed
features depend on the depth of the sublayer in which the Mn atom is located.
The local-density-of-states (LDOS) on the (110) surface associated with the
acceptor level is more sensitive to the direction of the Mn magnetic moment
when the Mn atom is located further below the surface. We show that the total
magnetic anisotropy energy of the system is due almost entirely to the
dependence of the acceptor level energy on Mn spin orientation, and that this
quantity is strongly dependent on the depth of the Mn atom.Comment: 14 pages, 13 figure
Magnetic interactions of substitutional Mn pairs in GaAs
We employ a kinetic-exchange tight-binding model to calculate the magnetic
interaction and anisotropy energies of a pair of substitutional Mn atoms in
GaAs as a function of their separation distance and direction. We find that the
most energetically stable configuration is usually one in which the spins are
ferromagnetically aligned along the vector connecting the Mn atoms. The
ferromagnetic configuration is characterized by a splitting of the topmost
unoccupied acceptor levels, which is visible in scanning tunneling microscope
studies when the pair is close to the surface and is strongly dependent on pair
orientation. The largest acceptor splittings occur when the Mn pair is oriented
along the symmetry direction, and the smallest when they are oriented
along . We show explicitly that the acceptor splitting is not simply
related to the effective exchange interaction between the Mn local moments. The
exchange interaction constant is instead more directly related to the width of
the distribution of all impurity levels -- occupied and unoccupied. When the Mn
pair is at the (110) GaAs surface, both acceptor splitting and effective
exchange interaction are very small except for the smallest possible Mn
separation.Comment: 25 figure
Magnetic Anisotropy of Isolated Cobalt Nanoplatelets
Motivated in part by experiments performed by M.H. Pan et al. (nanoletters,
v.5, p 83, 2005), we have undertaken a theoretical study of the the magnetic
properties of two-monolayer thick Co nanoplatelets with an equilateral
triangular shape. The analysis is carried out using a microscopic Slater-Koster
tight-binding model with atomic exchange and spin-orbit interactions designed
to realistically capture the salient magnetic features of large nanoclusters
containing up to 350 atoms. Two different truncations of the FCC lattice are
studied, in which the nanoplatelet surface is aligned parallel to the FCC (111)
and (001)crystal planes respectively. We find that the higher coordination
number in the (111) truncated crystal is more likely to reproduce the
perpendicular easy direction found in experiment. Qualitatively, the most
important parameter governing the anisotropy of the model is found to be the
value of the intra-atomic exchange integral J. If we set the value of J near
the bulk value in order to reproduce the experimentally observed magnitude of
the magnetic moments, we find both quasi-easy-planes and perpendicular easy
directions. At larger values of J we find that the easy-axis of magnetization
is perpendicular to the surface, and the value of the magnetic anisotropy
energy per atom is larger. The possible role of hybridization with substrate
surface states in the experimental systems is discussed.Comment: 15 pages, 13 figure
Status of Geriatrics in 22 Countries
The aim of this article is to describe the current status of geriatrics and position of geriatricians in 22 countries of three continents, and to portray their attitudes towards and resources allocated to geriatrics. An electronic survey was delivered to a convenience sample of 22 geriatricians in leading positions of their countries. The time required in post graduation specialist training to become a geriatrician varied from one year (subspecialty in the USA) to six years (independent specialty in Belgium). The number in the population aged 80+ per geriatrician varied from 450 (Austria) to 25,000 (Turkey). Of respondents, 55% reported that geriatrics is not a popular specialty in their country. Acute geriatric wards, rehabilitation and outpatient clinics were the most common working places for geriatricians. Nearly half of the respondents had an opinion that older patients who were acutely ill, were receiving subacute rehabilitation or had dementia should be cared for by geriatricians whereas half of the respondents would place geriatricians also in charge of nursing home and orthogeriatric patients. The biggest problems affecting older people's clinical care in their countries were: lack of geriatric knowledge, lack of geriatricians, and attitudes towards older people. Half of respondents thought that older people's health promotion and comprehensive geriatric assessment were not well implemented in their countries, although a majority felt that they could promote good geriatric care in their present position as a geriatrician. The position of geriatric, geriatricians' training and contents of work has wide international variety.Peer reviewe
Measurements of proton induced reaction cross sections on 120Te for the astrophysical p-process
The total cross sections for the 120Te(p,gamma)121I and 120Te(p,n)120I
reactions have been measured by the activation method in the effective
center-of-mass energies between 2.47 MeV and 7.93 MeV. The targets were
prepared by evaporation of 99.4 % isotopically enriched 120Te on Aluminum and
Carbon backing foils, and bombarded with proton beams provided by the FN tandem
accelerator at the University of Notre Dame. The cross sections and factors
were deduced from the observed gamma ray activity, which was detected off-line
by two Clover HPGe detectors mounted in close geometry. The results are
presented and compared with the predictions of statistical model calculations
using the codes NON-SMOKER and TALYS.Comment: 17 pages, 5 figures, 5 tables, regular articl
Comparison of Oxygen Adsorption and Platinum Dissolution in Acid and Alkaline Solutions Using Electrochemical Quartz Crystal Microbalance
Platinum (Pt) is a widely used electrocatalyst material in fuel cells and electrolysers. Proton exchange membrane (PEM) fuel cells and electrolysis operate under highly acidic conditions whereas the more recently developed anion exchange membrane (AEM) processes take place under alkaline conditions. Pt dissolution and Pt oxidation during operation and varying potentials has been studied mainly for the acidic PEM and less for the alkaline AEM. This study presents a comparison of Pt dissolution and Pt oxidation in 0.5 M H2SO4 and 1 M KOH using electrochemical quartz crystal microbalance (EQCM) on Pt thin films. Physical characterisation using electron microscopy and atomic force microscopy (AFM) revealed small, yet significant differences in the Pt film surface structure, which is related to differences in measured electrochemical surface area (ECSA). The mass increase from adsorption of oxygenated species and Pt oxidation is higher in alkaline conditions compared to in acid while dissolution of Pt is similar
Microwave Spectroscopy
Contains research objectives and reports on four research projects.Contract DA36-039 sc-73014Department of the Arm
Microwave Spectroscopy
Contains research objectives and reports on four research projects.Signal Corps Contract DA36-039-sc-7489
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