2,672 research outputs found
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
VLA H92 Alpha and H53 Alpha Radio Recombination Line Observations of M82
We present high angular resolution (0.6'') observations made with the VLA of
the radio continuum at 8.3 and 43 GHz as well as H92 Alpha and H53 Alpha radio
recombination lines from the nearby (3 Mpc) starburst galaxy M82. In the
continuum we report 19 newly identified sources at 8.3 GHz and 5 at 43 GHz that
were unknown previously. The spatial distribution of the H92 Alpha line is
inhomogeneous; we identify 27 features. The line and continuum emission are
modeled using a collection of HII regions at different distances from the
nucleus assuming a single-density component and two-density components. The
high-density component has a density of 4 X 10^{4} cm^{-3}. However, the bulk
of the ionization is in regions with densities which are typically a factor 10
lower. The gas kinematics, using the H92 Alpha line, confirms the presence of
steep velocity gradient (26 km s^{-1} arcsec^{-1}) in the nuclear region. As
this steep gradient is observed not only on the major axis but also at large
distances along a band of PA of 150 degrees, the interpretation in terms of x2
orbits elongated along the minor axis of the bar, which would be observed at an
angle close to the inclination of the main disk, seems inadequate. Ad-hoc
radial motions must be introduced to reproduce the pattern of the velocity
field. Different families of orbits are indicated as we detect a signature in
the kinematics at the transition between the two plateaus observed in the NIR
light distribution. The H92 Alpha line also reveals the base of the outflow
where the injection towards the halo on the Northern side occurs. The
kinematical pattern suggests a connection between the gas flowing in the plane
of M82 towards the center; this behavior most likely originates due to the
presence of a bar and the outflow out of the plane.Comment: 45 pages, 11 figures and 6 tables. Accepted for publication in Ap
On the Nature of Soft X-ray Weak Quasi-Stellar Objects
Recent studies of QSOs with ROSAT suggest the existence of a significant
population of Soft X-ray Weak QSOs (SXW QSOs) where the soft X-ray flux is ~
10-30 times smaller than in typical QSOs. As a first step in a systematic study
of these objects, we establish a well-defined sample of SXW QSOs which includes
all alpha_ox<=-2 QSOs from the Boroson & Green (1992) sample of 87 BQS QSOs.
SXW QSOs comprise about 11% of this optically selected QSO sample. From an
analysis of CIV absorption in the 55 BG92 QSOs with available CIV data, we find
a remarkably strong correlation between alpha_ox and the CIV absorption
equivalent width. This correlation suggests that absorption is the primary
cause of soft X-ray weakness in QSOs, and it reveals a continuum of absorption
properties connecting unabsorbed QSOs, X-ray warm absorber QSOs, SXW QSOs and
BAL QSOs. From a practical point of view, our correlation demonstrates that
selection by soft X-ray weakness is an effective (>=80% successful) and
observationally inexpensive way to find low-redshift QSOs with strong and
interesting ultraviolet absorption. We have also identified several notable
differences between the optical emission-line properties of SXW QSOs and those
of the other BG92 QSOs. SXW QSOs show systematically low [O III] luminosities
as well as distinctive H-beta profiles. They tend to lie toward the weak-[O
III] end of BG92 eigenvector 1, as do many low-ionization BAL QSOs. Unabsorbed
Seyferts and QSOs with similar values of eigenvector 1 have been suggested to
have extreme values of a primary physical parameter, perhaps mass accretion
rate relative to the Eddington rate (M-dot/M-dot_{Edd}). If these suggestions
are correct, it is likely that SXW QSOs also tend to have generally high values
of (M-dot/M-dot_{Edd}). (Abridged)Comment: 34 pages, ApJ accepted, also available from
http://www.astro.psu.edu/users/niel/papers/papers.htm
Low Temperature Specific Heat of DyTiO in the Kagome Ice State
We report the specific heat of single crystals of the spin ice compound
DyTiO at temperatures down to 100 mK in the so-called Kagome ice
state. In our previous paper, we showed the anisotropic release of residual
entropy in different magnetic field directions and reported new residual
entropy associated with spin frustration in the Kagome slab for field in the
[111] direction. In this paper, we confirm the first-order phase transition
line in the field-temperature phase diagram and the presence of a critical
point at (0.98 T, 400 mK), previously reported from the magnetization and
specific-heat data. We newly found another peak in the specific heat at 1.25 T
below 0.3 K. One possible explanation for the state between 1 T and 1.25 T is
the coexistence of states with different spin configurations including the 2-in
2-out one (Kagome ice state), the 1-in 3-out state (ordered state) and
paramagnetic one (free-spin state).Comment: 14 pages, 7 figure
Jets and QSO Spectra
QSOs' emission lines arise from highest velocity (approximately 10000 km/s),
dense gas within approximately 0.1 parsec of the central engine, out to
low-velocity, low-density gas at great distances from the host galaxy. In
radio-loud QSOs there are clear indications that the distribution and
kinematics of emission-line gas are related to the symmetry axis of the central
engine, as defined by the radio jet. These jets originate at nuclear distances
< 0.1 pc --- similar to the highest-velocity emission line gas. There are two
ways we can investigate the different environments of radio-loud and
radio-quiet QSOs, i.e., those with and without powerful radio jets. One is to
look for optical-UV spectroscopic differences between radio-loud and
radio-quiet QSOs. The other is to investigate dependences of spectroscopic
properties on properties of the powerful jets in radio-loud QSOs. Here we
summarize the spectroscopic differences between the two classes, and present
known dependences of spectra on radio core-dominance, which we interpret as
dependences on the angle of the central engine to the line-of-sight. We
speculate on what some of the differences may mean.Comment: 10 pages, plus 2 postscript figure files not included in tex file. In
addition there is a Springer style file, lamuphys.sty. Paper to appear with
the two related papers submitted here by the same author(s), in Springer
Lecture Notes series: Jets from Stars and Galactic Nuclei, ed. W. Kund
Spin dynamics and disorder effects in the S=1/2 kagome Heisenberg spin liquid phase of kapellasite
We report Cl NMR, ESR, SR and specific heat measurements on the
frustrated kagom\'e magnet kapellasite,
CuZn(OH)Cl, where a gapless spin liquid phase is
stabilized by a set of competing exchange interactions. Our measurements
confirm the ferromagnetic character of the nearest-neighbour exchange
interaction and give an energy scale for the competing interactions K. The study of the temperature-dependent ESR lineshift reveals a
moderate symmetric exchange anisotropy term , with %. These
findings validate a posteriori the use of the Heisenberg
model to describe the magnetic properties of kapellasite [Bernu et al., Phys.
Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this
model is the severe random depletion of the magnetic kagom\'e lattice by 27%,
due to Cu/Zn site mixing, and specifically address the effect of this disorder
by Cl NMR, performed on an oriented polycrystalline sample.
Surprisingly, while being very sensitive to local structural deformations, our
NMR measurements demonstrate that the system remains homogeneous with a unique
spin susceptibility at high temperature, despite a variety of magnetic
environments. Unconventional spin dynamics is further revealed by NMR and
SR in the low-, correlated, spin liquid regime, where a broad
distribution of spin-lattice relaxation times is observed. We ascribe this to
the presence of local low-energy modes.Comment: 15 pages, 11 figures. To appear in Phys. Rev.
Strongly Correlated Cerium Systems: Non-Kondo Mechanism for Moment Collapse
We present an ab initio based method which gives clear insight into the
interplay between the hybridization, the coulomb exchange, and the
crystal-field interactions, as the degree of 4f localization is varied across a
series of strongly correlated cerium systems. The results for the ordered
magnetic moments, magnetic structure, and ordering temperatures are in
excellent agreement with experiment, including the occurence of a moment
collapse of non-Kondo origin. In contrast, standard ab initio density
functional calculations fail to predict, even qualitatively, the trend of the
unusual magentic properties.Comment: A shorter version of this has been submitted to PR
A Comprehensive Economic Stimulus for our Failing Economy
This paper presents a comprehensive plan to fix the ailing American economy, through a five-step approach. First, the Federal Reserve must continue to broaden the scope of monetary policy, by purchasing and selling long-term securities. Manipulating expectations through FOMC statements is another tool at the Federal Reserve’s disposal. Secondly, the government must enact fiscal stimulus to stabilize the economy in the short and medium runs, through investment in infrastructure projects, green technology, fusion technology, and science education. Additionally, the new fiscal policy must tackle the mortgage meltdown, which is weighing down the entire economy. Third, the regulatory system must be changed to reduce the likelihood of another financial collapse, starting with the nationalization of the ratings agencies. Ratings should be updated faster, with a numeric grading system rather than the pre-existing letter grades. Fourth, our globalized economy insures that a coordinated globalized response is necessary to recover. Global cooperation to reduce inflation and avoid protectionist policies is vital. Finally, the American bailout policy must be made clear, only giving bailouts to companies that are sound but financially strapped and those that are too big to fail
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