6,792 research outputs found
Symmetry and magnetic structure determination: Developments in refinement techniques and examples
Group Theory techniques can aid greatly the determination of magnetic
structures. The integration of their calculations into new and existing
refinement programs is an ongoing development that will simplify and make more
rigorous the analysis of experimental data. This paper presents an overview of
the practical application of symmetry analysis to the determination of magnetic
structures. Details are given of the different programs that perform these
calculations and how refinements can be carried out using their results.
Examples are presented that show how such analysis can be important in the
interpretation of magnetic diffraction data, and to our reasoning of the causes
for the observed ordering.Comment: Proceedings of ICNS 2001. To be published in Applied Physics
Conventional and unconventional orderings in the jarosites
The jarosites make up the most studied family of {\it kagom\'e}
antiferromagnets. The flexibility of the structure to substitution of the A and
B ions allows a wide range of compositions to be synthesised with the general
formula AB3(SO4)2(OH)6 (A = Na, K, Ag, Rb, H3O, NH4,
1/2Ba, and 1/2Pb; B = Fe, Cr, and V).
Additional chemical tuning of the exchange between layers is also possible by
substitution of the (SO4) groups by (SeO4) or (CrO4).
Thus, a variety of S = 5/2, 3/2, and 1 systems can be engineered to allow study
of the effects of frustration in both the classical and more quantum limits.
Within this family both conventional long-ranged magnetic order and more exotic
unconventional orderings have been found. This article reviews the different
types of magnetic orderings that occur and examines some of the parameters that
are their cause.Comment: Proceedings of the conterence "Highly Frustrated Magnetism 2000" . To
be published in the Canadian Journal of Physic
Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, -Cu2(OD)3Cl, a proposed valence bond solid (VBS)
The frustrated magnet clinoatacamite, -Cu(OH)Cl, is
attracting a lot of interest after suggestions that at low temperature it forms
an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised
Cu () spins.\cite{Lee_clinoatacamite} Key to the arguments
surrounding this proposal were suggestions that the kagom\'e planes in the
magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing
the system to behave as a quasi-2-dimensional magnet. This was reasoned from
the near 95 angles made at the bridging oxygens that mediate exchange
between the Cu ions that link the kagom\'e planes.
Recent work pointed out that this exchange model is inappropriate for
-Cu(OH)Cl, where the oxygen is present as a
-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron
powder diffraction to show that the low temperature magnetic structure (
K) was canted and involved significant spin ordering on all the Cu
spins, which is incompatible with the interpretation of simultaneous VBS and
N\'eel ordering. Correspondingly, clinoatacamite is best considered a distorted
pyrochlore magnet. In this report we show detailed inelastic neutron scattering
spectra and revisit the responses of this frustrated quantum magnet.Comment: Proceedings of The International Conference on Highly Frustrated
Magnetism 2008 (HFM2008
Toward Perfection: Kapellasite, Cu3Zn(OH)6Cl2, a New Model S = 1/2 Kagome Antiferromagnet
The search for the resonating valence bond (RVB) state continues to underpin
many areas of condensed matter research. The RVB is made from the dimerisation
of spins on different sites into fluctuating singlets, and was proposed by
Anderson to be the reference state from which the transition to BCS
superconductivity occurs. Little is known about the state experimentally, due
to the scarcity of model materials. Theoretical work has put forward the S =
1/2 kagome antiferromagnet (KAFM) as a good candidate for the realization of
the RVB state. In this paper we introduce a new model system, the S = 1/2 KAFM
Kapellasite, Cu3Zn(OH)6Cl2. We show that its crystal structure is a good
approximation to a 2-dimensional kagome antiferromagnet and that susceptibility
data indicate a collapse of the magnetic moment below T = 25 K that is
compatible with the spins condensing into the non-magnetic RVB state.Comment: Communication, 3 pages, 3 figure
Slow Relaxation Process in Ising like Heisenberg Kagome Antiferromagnets due to Macroscopic Degeneracy in the Ordered State
We study relaxation phenomena in the ferromagnetically ordered state of the
Ising-like Heisenberg kagome antiferromagnets. We introduce the "weathervane
loop" in order to characterize macroscopic degenerate ordered states and study
the microscopic mechanism of the slow relaxation from a view point of the
dynamics of the weathervane loop configuration. This mechanism may give a
possible origin of the slow relaxation reported in recent experiments.Comment: 6pages, 4figures, HFM2006 proceeding
Spectral Properties From Lyman-alpha to H-alpha For An Essentially Complete Sample of Quasars I: Data
We have obtained quasi-simultaneous ultraviolet-optical spectra for 22 out of
23 quasars in the complete PG-X-ray sample with redshift, z<0.4, and M_B<-23.
The spectra cover rest-frame wavelengths from at least Lyman-alpha to H-alpha.
Here we provide a detailed description of the data, including careful
spectrophotometry and redshift determination. We also present direct
measurements of the continua, strong emission lines and features, including
Lyman-alpha, SiIV+OIV], CIV, CIII], SiIII], MgII, H-beta, [OIII],
He5876+NaI5890,5896, H-alpha, and blended iron emission in the UV and optical.
The widths, asymmetries and velocity shifts of profiles of strong emission
lines show that CIV and Lyman-alpha are very different from H-beta and H-alpha.
This suggests that the motion of the broad line region is related to the
ionization structure, but the data appears not agree with the radially
stratified ionization structure supported by reverberation mapping studies, and
therefore suggest that outflows contribute additional velocity components to
the broad emission line profiles.Comment: 42 pages, 10 figures, 13 tables. Accepted by AJ. Supplemental figures
not included. Full version available at
http://physics.uwyo.edu/~shang/pgxpaper/ShangPaper.pd
Providing value to a business using a lightweight design system to support knowledge reuse by designers
This paper describes an alternative approach to knowledge based systems in engineering than traditional geometry or explicit knowledge focused systems. Past systems have supported product optimisation rather than creative solutions and provide little benefit to businesses for bespoke and low volume products or products which do not benefit from optimisation. The approach here addresses this by supporting the creativity of designers through codified tacit knowledge and encouraging knowledge reuse for bespoke product development, in particular for small to medium sized enterprises. The implementation and evaluation of the approach is described within a company producing bespoke fixtures and tooling in shorter than average lead times. The active support of knowledge management in the company is intended to add value to the business by further reducing the lead times of the designs and creating a positive impact to business processes. The evaluation demonstrates a viable alternative framework to the traditional management of knowledge in engineering, which could be implemented by other small to medium enterprises
- …