97,134 research outputs found
Further Series Studies of the Spin-1/2 Heisenberg Antiferromagnet at T=0: Magnon Dispersion and Structure Factors
We have extended our previous series studies of quantum antiferromagnets at
zero temperature by computing the one-magnon dispersion curves and various
structure factors for the linear chain, square and simple cubic lattices. Many
of these results are new; others are a substantial extension of previous work.
These results are directly comparable with neutron scattering experiments and
we make such comparisons where possible.Comment: 15 pages, 12 figures, revised versio
Phase Diagram of the J1, J2, J3 Heisenberg Models on the Honeycomb Lattice: A Series Expansion Study
We study magnetically ordered phases and their phase boundaries in the
Heisenberg models on the honeycomb lattice using series
expansions around N\'eel and different colinear and non-colinear magnetic
states. An Ising anisotropy () is introduced and
ground state energy and magnetization order parameter are calculated as a power
seies expansion in . Series extrapolation methods are used to study
properties for the Heisenberg model (). We find that at large
() there is a first order transition between N\'eel and columnar states,
in agreement with the classical answer. For , we find that the N\'eel
phase extends beyond the region of classical stability. We also find that
spiral phases are stabilized over large parameter regions, although their
spiral angles can be substantially renormalized with respect to the classical
values. Our study also shows a magnetically disordered region at intermedaite
and values.Comment: 6 pages, 9 figure
Ground state properties, excitation spectra and phase transitions in the and bilayer Heisenberg models on the honeycomb Lattice
Motivated by the observation of a disordered spin ground state in the
material BiMnONO, we study the ground state properties and
excitation spectra of the (and for comparison ) bilayer
Heisenberg model on the honeycomb lattice, with and without frustrating further
neighbor interactions. We use series expansions around the N\'eel state to
calculate properties of the magnetically ordered phase. Furthermore, series
expansions in , where is an in-plane exchange
constant and is the exchange constant between the layers are used to
study properties of the spin singlet phase. For the unfrustrated case, our
results for the phase transitions are in very good agreement with recent
Quantum Monte Carlo studies. We also obtain the excitation spectra in the
disordered phase and study the change in the critical when
frustrating exchange interactions are added to the system and find a
rapid suppression of the ordered phase with frustration. Implications for the
material BiMnONO are discussed.Comment: 5 pages, 6 figure
A computer program for analyzing unresolved Mossbauer hyperfine spectra
The program for analyzing unresolved Mossbauer hyperfine spectra was written in FORTRAN 4 language for the Control Data CYBER 170 series digital computer system with network operating system 1.1. With the present dimensions, the program requires approximately 36,000 octal locations of core storage. A typical case involving two innermost coordination shells in which the amplitudes and the peak positions of all three components were estimated in 25 iterations requires 30 seconds on CYBER 173. The program was applied to determine the effects of various near neighbor impurity shells on hyperfine fields in dilute FeAl alloys
Spin-wave excitation spectra and spectral weights in square lattice antiferromagnets
Using a recently developed method for calculating series expansions of the
excitation spectra of quantum lattice models, we obtain the spin-wave spectra
for square lattice, Heisenberg-Ising antiferromagnets. The calculated
spin-wave spectrum for the Heisenberg model is close to but noticeably
different from a uniformly renormalized classical (large-) spectrum with the
renormalization for the spin-wave velocity of approximately . The
relative weights of the single-magnon and multi-magnon contributions to neutron
scattering spectra are obtained for wavevectors throughout the Brillouin zone.Comment: Two postscript figures, 4 two-column page
Low-lying dipole strengths for probable -wave one-neutron halos in the medium mass region
The one-neutron halos lying in the island of inversion around has
provided the podium, to study the variation of total low-lying dipole strength
with the neutron separation energy. We study three probable p-wave one-neutron
halo candidates 31Ne and 34Na and 37Mg lying in the island of inversion. A
simple analytic model has been used for the calculation of the total low-lying
dipole strength for the medium mass p-wave one-neutron halos. A correction
factor to this analytical model has been estimated with a realistic Woods-Saxon
potential. A comparison of these analytic calculations has been made with the
those performed by a finite-range distorted-wave Born approximation theory of
the Coulomb dissociation. We also make an estimate of the one-neutron
separation energies of 31Ne, 34Na and 37Mg.Comment: 7 pages latex, 4 figures, to appear in EPJ
New technique for calibrating hydrocarbon gas flowmeters
A technique for measuring calibration correction factors for hydrocarbon mass flowmeters is described. It is based on the Nernst theorem for matching the partial pressure of oxygen in the combustion products of the test hydrocarbon, burned in oxygen-enriched air, with that in normal air. It is applied to a widely used type of commercial thermal mass flowmeter for a number of hydrocarbons. The calibration correction factors measured using this technique are in good agreement with the values obtained by other independent procedures. The technique is successfully applied to the measurement of differences as low as one percent of the effective hydrocarbon content of the natural gas test samples
A new technique for measuring gas conversion factors for hydrocarbon mass flowmeters
A technique for measuring calibration conversion factors for hydrocarbon mass flowmeters was developed. It was applied to a widely used type of commercial thermal mass flowmeter for hydrocarbon gases. The values of conversion factors for two common hydrocarbons measured using this technique are in good agreement with the empirical values cited by the manufacturer. Similar agreements can be expected for all other hydrocarbons. The technique is based on Nernst theorem for matching the partial pressure of oxygen in the combustion product gases with that in normal air. It is simple, quick and relatively safe--particularly for toxic/poisonous hydrocarbons
Aircraft cockpit vision: Math model
A mathematical model was developed to describe the field of vision of a pilot seated in an aircraft. Given the position and orientation of the aircraft, along with the geometrical configuration of its windows, and the location of an object, the model determines whether the object would be within the pilot's external vision envelope provided by the aircraft's windows. The computer program using this model was implemented and is described
Use of low energy hydrogen ion implants in high efficiency crystalline silicon solar cells
This program is a study of the use of low energy hydrogen ion implantation for high efficiency crystalline silicon solar cells. The first quarterly report focuses on two tasks of this program: (1) an examination of the effects of low energy hydrogen implants on surface recombination speed; and (2) an examination of the effects of hydrogen on silicon regrowth and diffusion in silicon. The first part of the project focussed on the measurement of surface properties of hydrogen implanted silicon. Low energy hydrogen ions when bombarded on the silicon surface will create structural damage at the surface, deactivate dopants and introduce recombination centers. At the same time the electrically active centers such as dangling bonds will be passivated by these hydrogen ions. Thus hydrogen is expected to alter properties such as the surface recombination velocity, dopant profiles on the emitter, etc. In this report the surface recombination velocity of a hydrogen emplanted emitter was measured
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