4,803 research outputs found
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
Lattice distortion in hcp rare gas solids
The lattice distortion parameter has been
calculated as a function of molar volume for the hcp phases of He, Ar, Kr and
Xe. Results from both semi-empirical potentials and density functional theory
are presented. Our study shows that is negative for helium in the
entire pressure range. For Ar, Kr and Xe, however, changes sign from
negative to positive as the pressure increases, growing rapidly in magnitude at
higher pressures.Comment: Submitted to Low. Temp. Phys., 14 pages, 5 figure
Emission lines and optical continuum in low-luminosity radio galaxies
We present spectroscopic observations of a complete sub-sample of 13
low-luminosity radio galaxies selected from the 2Jy sample. The underlying
continuum in these sources is carefully modelled in order to make a much-needed
comparison between the emission line and continuum properties of FRIs with
those of other classes of radio sources. We find that 5 galaxies in the sample
show a measurable UV excess: 2 of the these sources are BL Lacs and in the
remaining 3 galaxies we argue that the most likely contributor to the UV excess
is a young stellar component. Excluding the BL Lacs, we therefore find that
\~30% of the sample show evidence for young stars, which is similar to the
results obtained for higher luminosity samples. We compare our results with
far-infrared measurements in order to investigate the far-infrared-starburst
link. The nature of the optical-radio correlations is investigated in light of
this new available data and, in contrast to previous studies, we find that the
FRI sources follow the correlations with a similar slope to that found for the
FRIIs. Finally, we compare the luminosity of the emission lines in the FRI and
BL Lac sources and find a significant difference in the [OIII] line
luminosities of the two groups. Our results are discussed in the context of the
unified schemes.Comment: 18 pages, 31 figures, MNRAS in press, (all enquiries to Clive
Tadhunter ([email protected])
Eddington Accretion and QSO Emission Lines at z ~ 2
Broad Absorption Line (BAL) QSOs have been suggested to be youthful
super-accretors based on their powerful radiatively driven absorbing outflows
and often reddened continua. To test this hypothesis, we observed near IR
spectra of the H region for 11 bright BAL QSOs at redshift z ~ 2. We
measured these and literature spectra for 6 BAL QSOs, 13 radio-loud and 7
radio-quiet non-BAL QSOs. Using the luminosity and H broad line width to
derive black hole mass and accretion rate, we find that both BAL and non-BAL
QSOs at z ~ 2 tend to have higher than those at low z -- probably a
result of selecting the brightest QSOs. However, we find that the high z QSOs,
in particular the BAL QSOs, have extremely strong Fe II and very weak [O III],
extending the inverse relationship found for low z QSOs. This suggests that,
even while radiating near , the BAL QSOs have a more plentiful fuel
supply than non-BAL QSOs. Comparison with low z QSOs shows for the first time
that the inverse Fe II -- [O III] relationship is indeed related to
, rather than black hole mass.Comment: 18 pages including 5 figures and 1 table. Accepted by the
Astrophysical Journal Letter
- …