84 research outputs found
Thermodynamics of the (1,1/2) Ferrimagnet in Finite Magnetic Fields
We investigate the specific heat and magnetisation of a ferrimagnet with gS=1
and S=1/2 spins in a finite magnetic field using the transfer matrix DMRG down
to T=0.025J. Ferromagnetic gapless and antiferromagnetic gapped excitations for
H=0 lead to rich thermodynamics for H > 0. While the specific heat is
characterized by a generic double peak structure, magnetisation reveals two
critical fields, Hc1=1.76(1) and Hc2=3.00(1) with square-root behaviour in the
T=0 magnetisation. Simple analytical arguments allow to understand these
experimentally accessible findings.Comment: 5 pages, 7 eps figures, uses RevTeX, submitted to PR
Searching for non-Gaussianity in the VSA data
We have tested Very Small Array (VSA) observations of three regions of sky
for the presence of non-Gaussianity, using high-order cumulants, Minkowski
functionals, a wavelet-based test and a Bayesian joint power
spectrum/non-Gaussianity analysis. We find the data from two regions to be
consistent with Gaussianity. In the third region, we obtain a 96.7% detection
of non-Gaussianity using the wavelet test. We perform simulations to
characterise the tests, and conclude that this is consistent with expected
residual point source contamination. There is therefore no evidence that this
detection is of cosmological origin. Our simulations show that the tests would
be sensitive to any residual point sources above the data's source subtraction
level of 20 mJy. The tests are also sensitive to cosmic string networks at an
rms fluctuation level of (i.e. equivalent to the best-fit observed
value). They are not sensitive to string-induced fluctuations if an equal rms
of Gaussian CDM fluctuations is added, thereby reducing the fluctuations due to
the strings network to rms . We especially highlight the usefulness
of non-Gaussianity testing in eliminating systematic effects from our data.Comment: Minor corrections; accepted for publication to MNRA
Elevated Atmospheric {CO}2 Modifies Mostly the Metabolic Active Rhizosphere Soil Microbiome in the Giessen {FACE} Experiment
Elevated levels of atmospheric CO2 lead to the increase of plant photosynthetic rates, carbon inputs into soil and root exudation. In this work, the effects of rising atmospheric CO2 levels on the metabolic active soil microbiome have been investigated at the Giessen free-air CO2 enrichment (Gi-FACE) experiment on a permanent grassland site near Giessen, Germany. The aim was to assess the effects of increased C supply into the soil, due to elevated CO2, on the active soil microbiome composition. RNA extraction and 16S rRNA (cDNA) metabarcoding sequencing were performed from bulk and rhizosphere soils, and the obtained data were processed for a compositional data analysis calculating diversity indices and differential abundance analyses. The structure of the metabolic active microbiome in the rhizospheric soil showed a clear separation between elevated and ambient CO2 (p = 0.002); increased atmospheric CO2 concentration exerted a significant influence on the microbiomes differentiation (p = 0.01). In contrast, elevated CO2 had no major influence on the structure of the bulk soil microbiome (p = 0.097). Differential abundance results demonstrated that 42 bacterial genera were stimulated under elevated CO2. The RNA-based metabarcoding approach used in this research showed that the ongoing atmospheric CO2 increase of climate change will significantly shift the microbiome structure in the rhizosphere
The Tenerife Cosmic Microwave Background Maps: Observations and First Analysis
The results of the Tenerife Cosmic Microwave Background (CMB) experiments are
presented. These observations cover 5000 and 6500 square degrees on the sky at
10 and 15 GHz respectively centred around Dec.~ +35 degrees. The experiments
are sensitive to multipoles l=10-30 which corresponds to the Sachs-Wolfe
plateau of the CMB power spectra. The sensitivity of the results are ~31 and
\~12 microK at 10 and 15 GHz respectively in a beam-size region (5 degrees
FWHM). The data at 15 GHz show clear detection of structure at high Galactic
latitude; the results at 10 GHz are compatible with these, but at lower
significance. A likelihood analysis of the 10 and 15 GHz data at high Galactic
latitude, assuming a flat CMB band power spectra gives a signal Delta
T_l=30+10-8 microK (68 % C.L.). Including the possible contaminating effect due
to the diffuse Galactic component, the CMB signal is Delta T_l=30+15-11 microK.
These values are highly stable against the Galactic cut chosen. Assuming a
Harrison-Zeldovich spectrum for the primordial fluctuations, the above values
imply an expected quadrupole Q_RMS-PS=20+10-7 microK which confirms previous
results from these experiments, and which are compatible with the COBE DMR.Comment: 17 pages, 7 figures. Submitted to Ap
Specific Heat Study on a Novel Spin-Gapped System : (CH_3)_2NH_2CuCl_3
Specific heat measurements down to 120mK have been performed on a
quasi-one-dimensional spin-gapped system (CH)NHCuCl in
a magnetic field up to 8 T. This compound has a characteristic magnetization
curve which shows a gapless ground state and a plateau at 1/2 of the saturation
value. We have observed a spontaneous antiferromagnetic ordering and a
field-induced one below and above the 1/2 plateau field range, respectively.
The field versus temperature phase diagram is quite unusual and completely
different from those of the other quantum spin systems investigated so far. In
the plateau field range, a double-structure in the specific heat is observed,
reflecting the coexistence of ferromagnetic and antiferromagnetic excitations.
These behaviors are discussed on the basis of a recently proposed novel quantum
spin chain model consisting of weakly coupled ferromagnetic and
antiferromagnetic dimers.Comment: 4 pages, 3 figures, submitted to J. Phys. Soc. Jp
Weak Lensing Mass Reconstruction using Wavelets
This paper presents a new method for the reconstruction of weak lensing mass
maps. It uses the multiscale entropy concept, which is based on wavelets, and
the False Discovery Rate which allows us to derive robust detection levels in
wavelet space. We show that this new restoration approach outperforms several
standard techniques currently used for weak shear mass reconstruction. This
method can also be used to separate E and B modes in the shear field, and thus
test for the presence of residual systematic effects. We concentrate on large
blind cosmic shear surveys, and illustrate our results using simulated shear
maps derived from N-Body Lambda-CDM simulations with added noise corresponding
to both ground-based and space-based observations.Comment: Accepted manuscript with all figures can be downloaded at:
http://jstarck.free.fr/aa_wlens05.pdf and software can be downloaded at
http://jstarck.free.fr/mrlens.htm
Maximum Entropy for Gravitational Wave Data Analysis: Inferring the Physical Parameters of Core-Collapse Supernovae
The gravitational wave signal arising from the collapsing iron core of a Type
II supernova progenitor star carries with it the imprint of the progenitor's
mass, rotation rate, degree of differential rotation, and the bounce depth.
Here, we show how to infer the gravitational radiation waveform of a core
collapse event from noisy observations in a network of two or more LIGO-like
gravitational wave detectors and, from the recovered signal, constrain these
source properties. Using these techniques, predictions from recent core
collapse modeling efforts, and the LIGO performance during its S4 science run,
we also show that gravitational wave observations by LIGO might have been
sufficient to provide reasonable estimates of the progenitor mass, angular
momentum and differential angular momentum, and depth of the core at bounce,
for a rotating core collapse event at a distance of a few kpc.Comment: 44 pages, 12 figures; accepted version scheduled to appear in Ap J 1
April 200
Nuclear spin relaxation in ordered bimetallic chain compounds
A theoretical interpretation is given to recent proton spin relaxation-time
(T_1) measurements on NiCu(C_7H_6N_2O_6)(H_2O)_32H_2O, which is an ideal
one-dimensional ferrimagnetic Heisenberg model system of alternating spins 1
and 1/2. The relaxation rate T_1^{-1} is formulated in temrs of the spin-wave
theory and is evaluated by the use of a quantum Monte Carlo method.
Calculations of the temperature and applied-field (H) dependences of T_1^{-1}
are in total agreement with the experimental findings. T_1 behaves as
, which turns out an indirect observation of the
quadratic dispersion relations dominating the low-energy physics of quantum
ferrimagnets.Comment: 5 pages, 4 figures embedded, to appear in Phys. Lett.
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