119 research outputs found
Ferromagnetism or slow paramagnetic relaxation in Fe-doped LiN?
We report on isothermal magnetization, M\"ossbauer spectroscopy, and
magnetostriction as well as temperature-dependent alternating-current (ac)
susceptibility, specific heat, and thermal expansion of single crystalline and
polycrstalline Li(LiFe)N with and .
Magnetic hysteresis emerges at temperatures below K with
coercivity fields of up to T at K and magnetic
anisotropy energies of K (meV). The ac susceptibility is strongly
frequency dependent (--Hz) and reveals an effective energy
barrier for spin reversal of K. The relaxation times
follow Arrhenius behavior for K. For K, however, the
relaxation times of s are only weakly
temperature-dependent indicating the relevance of a quantum tunneling process
instead of thermal excitations. The magnetic entropy amounts to more than
J molK which significantly exceeds ln2, the
value expected for the entropy of a ground state doublet. Thermal expansion and
magnetostriction indicate a weak magneto-elastic coupling in accordance with
slow relaxation of the magnetization. The classification of
Li(LiFe)N as ferromagnet is stressed and contrasted with highly
anisotropic and slowly relaxing paramagnetic behavior.Comment: 12 pages, 10 figure
Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCoF
We report high-frequency/high-field electron spin resonance (ESR) and
high-field magnetization studies on single crystals of the bond-disordered
pyrochlore NaCaCoF. Frequency- and temperature-dependent ESR
investigations above the freezing temperature K reveal the
coexistence of two distinct magnetic phases. A cooperative paramagnetic phase,
evidenced by a gapless excitation mode, is found as well as a spin-glass phase
developing below 20 K which is associated with a gapped low-energy excitation.
Effective -factors close to 2 are obtained for both modes in line with
pulsed high-field magnetization measurements which show an unsaturated
isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent
magnetization in high magnetic fields, we propose an empirical model accounting
for highly anisotropic ionic -tensors expected for this material and taking
into account the strongly competing interactions between the spins which lead
to a frustrated ground state. As a detailed quantitative relation between
effective -factors as determined from ESR and the local -tensors obtained
by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still
sought after, our work motivates further theoretical investigations of the
low-energy excitations in bond-disordered pyrochlores.Comment: 9 pages, 6 figure
Search for chameleons with CAST
In this work we present a search for (solar) chameleons with the CERN Axion
Solar Telescope (CAST). This novel experimental technique, in the field of dark
energy research, exploits both the chameleon coupling to matter () and to photons () via the Primakoff effect. By reducing
the X-ray detection energy threshold used for axions from 1keV to 400eV
CAST became sensitive to the converted solar chameleon spectrum which peaks
around 600eV. Even though we have not observed any excess above background,
we can provide a 95% C.L. limit for the coupling strength of chameleons to
photons of for .Comment: 8 pages, 12 figure
Solar axion search with the CAST experiment
The CAST (CERN Axion Solar Telescope) experiment is searching for solar
axions by their conversion into photons inside the magnet pipe of an LHC
dipole. The analysis of the data recorded during the first phase of the
experiment with vacuum in the magnet pipes has resulted in the most restrictive
experimental limit on the coupling constant of axions to photons. In the second
phase, CAST is operating with a buffer gas inside the magnet pipes in order to
extent the sensitivity of the experiment to higher axion masses. We will
present the first results on the data taking as well as the
system upgrades that have been operated in the last year in order to adapt the
experiment for the data taking. Expected sensitivities on the
coupling constant of axions to photons will be given for the recent run just started in March 2008.Comment: Proceedings of the ICHEP 2008 conferenc
First results from the CERN Axion Solar Telescope (CAST)
Hypothetical axion-like particles with a two-photon interaction would be
produced in the Sun by the Primakoff process. In a laboratory magnetic field
(``axion helioscope'') they would be transformed into X-rays with energies of a
few keV. Using a decommissioned LHC test magnet, CAST has been running for
about 6 months during 2003. The first results from the analysis of these data
are presented here. No signal above background was observed, implying an upper
limit to the axion-photon coupling < 1.16 10^{-10} GeV^-1 at 95% CL for m_a
<~0.02 eV. This limit is comparable to the limit from stellar energy-loss
arguments and considerably more restrictive than any previous experiment in
this axion mass range.Comment: 4 pages, accepted by PRL. Final version after the referees comment
Results and perspectives of the solar axion search with the CAST experiment
The status of the solar axion search with the CERN Axion Solar Telescope
(CAST) will be presented. Recent results obtained by the use of He as a
buffer gas has allowed us to extend our sensitivity to higher axion masses than
our previous measurements with He. With about 1 h of data taking at each of
252 different pressure settings we have scanned the axion mass range 0.39 eV 0.64 eV. From the absence of an excess of x rays when the
magnet was pointing to the Sun we set a typical upper limit on the axion-photon
coupling of g GeV at 95% C.L., the
exact value depending on the pressure setting. CAST published results represent
the best experimental limit on the photon couplings to axions and other similar
exotic particles dubbed WISPs (Weakly Interacting Slim Particles) in the
considered mass range and for the first time the limit enters the region
favored by QCD axion models. Preliminary sensitivities for axion masses up to
1.16 eV will also be shown reaching mean upper limits on the axion-photon
coupling of g GeV at 95% C.L.
Expected sensibilities for the extension of the CAST program up to 2014 will be
presented. Moreover long term options for a new helioscope experiment will be
evoked.Comment: 4 pages, 2 pages, to appear in the proceedings of the 24th Rencontres
de Blois V2 A few affiliations were not corrected in previous version V3
Author adde
Search for low Energy solar Axions with CAST
We have started the development of a detector system, sensitive to single
photons in the eV energy range, to be suitably coupled to one of the CAST
magnet ports. This system should open to CAST a window on possible detection of
low energy Axion Like Particles emitted by the sun. Preliminary tests have
involved a cooled photomultiplier tube coupled to the CAST magnet via a
Galileian telescope and a switched 40 m long optical fiber. This system has
reached the limit background level of the detector alone in ideal conditions,
and two solar tracking runs have been performed with it at CAST. Such a
measurement has never been done before with an axion helioscope. We will
present results from these runs and briefly discuss future detector
developments.Comment: Paper submitted to the proceedings of the "4th Patras Workshop on
Axions, WIMPs and WISPs", DESY, Hamburg Site - Germany, 18-21 June 2008.
Author affiliations are reported on the title page of the paper. In version
2: 1 affiliation change, 3 references adde
Multiscale X ray study of Bacillus subtilis biofilms reveals interlinked structural hierarchy and elemental heterogeneity
Biofilms are multicellular microbial communities that encase themselves in an extracellular matrix ECM of secreted biopolymers and attach to surfaces and interfaces. Bacterial biofilms are detrimental in hospital and industrial settings, but they can be beneficial, for example, in agricultural as well as in food technology contexts. An essential property of biofilms that grants them with increased survival relative to planktonic cells is phenotypic heterogeneity, the division of the biofilm population into functionally distinct subgroups of cells. Phenotypic heterogeneity in biofilms can be traced to the cellular level; however, the molecular structures and elemental distribution across whole biofilms, as well as possible linkages between them, remain unexplored. Mapping X ray diffraction across intact biofilms in time and space, we revealed the dominant structural features in Bacillus subtilis biofilms, stemming from matrix components, spores, and water. By simultaneously following the X ray fluorescence signal of biofilms and isolated matrix components, we discovered that the ECM preferentially binds calcium ions over other metal ions, specifically, zinc, manganese, and iron. These ions, remaining free to flow below macroscopic wrinkles that act as water channels, eventually accumulate and may possibly lead to sporulation. The possible link between ECM properties, regulation of metal ion distribution, and sporulation across whole, intact biofilms unravels the importance of molecular level heterogeneity in shaping biofilm physiology and developmen
Search for solar axion emission from 7Li and D(p,gamma)3He nuclear decays with the CAST gamma-ray calorimeter
We present the results of a search for a high-energy axion emission signal
from 7Li (0.478 MeV) and D(p,gamma)3He (5.5 MeV) nuclear transitions using a
low-background gamma-ray calorimeter during Phase I of the CAST experiment.
These so-called "hadronic axions" could provide a solution to the long-standing
strong-CP problem and can be emitted from the solar core from nuclear M1
transitions. This is the first such search for high-energy pseudoscalar bosons
with couplings to nucleons conducted using a helioscope approach. No excess
signal above background was found.Comment: 20 pages, 8 figures, final version to be published in JCA
Prospects for the CERN Axion Solar Telescope Sensitivity to 14.4 keV Axions
The CERN Axion Solar Telescope (CAST) is searching for solar axions using the
9.0 T strong and 9.26 m long transverse magnetic field of a twin aperture LHC
test magnet, where axions could be converted into X-rays via reverse Primakoff
process. Here we explore the potential of CAST to search for 14.4 keV axions
that could be emitted from the Sun in M1 nuclear transition between the first,
thermally excited state, and the ground state of 57Fe nuclide. Calculations of
the expected signals, with respect to the axion-photon coupling, axion-nucleon
coupling and axion mass, are presented in comparison with the experimental
sensitivity.Comment: 4 pages, 1 figure. Submitted to Nucl. Instr. and Meth.
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