3,267 research outputs found
Magnetic excitations in the metallic single-layer Ruthenates Ca(2-x)Sr(x)RuO(4) studied by inelastic neutron scattering
By inelastic neutron scattering, we have analyzed the magnetic correlations
in the paramagnetic metallic region of the series Ca(2-x)Sr(x)RuO(4),
0.2<=x<=0.62. We find different contributions that correspond to 2D
ferromagnetic fluctuations and to fluctuations at incommensurate wave vectors
(0.11,0,0), (0.26,0,0) and (0.3,0.3,0). These components constitute the
measured response as function of the Sr-concentration x, of the magnetic field
and of the temperature. A generic model is applicable to metallic
Ca(2-x)Sr(x)RuO(4) close to the Mott transition, in spite of their strongly
varying physical properties. The amplitude, characteristic energy and width of
the incommensurate components vary only little as function of x, but the
ferromagnetic component depends sensitively on concentration, temperature and
magnetic field. While ferromagnetic fluctuations are very strong in
Ca1.38Sr0.62RuO4 with a low characteristic energy of 0.2 meV at T=1.5 K, they
are strongly suppressed in Ca1.8Sr0.2RuO4, but reappear upon the application of
a magnetic field and form a magnon mode above the metamagnetic transition. The
inelastic neutron scattering results document how the competition between
ferromagnetic and incommensurate antiferromagnetic instabilities governs the
physics of this system
Measurements of thermodynamic and transport properties of EuC: a low-temperature analogue of EuO
EuC is a ferromagnet with a Curie-temperature of K. It
is semiconducting with the particularity that the resistivity drops by about 5
orders of magnitude on cooling through , which is therefore called a
metal-insulator transition. In this paper we study the magnetization, specific
heat, thermal expansion, and the resistivity around this ferromagnetic
transition on high-quality EuC samples. At we observe well defined
anomalies in the specific heat and thermal expansion data.
The magnetic contributions of and can satisfactorily be
described within a mean-field theory, taking into account the magnetization
data. In zero magnetic field the magnetic contributions of the specific heat
and thermal expansion fulfill a Gr\"uneisen-scaling, which is not preserved in
finite fields. From an estimation of the pressure dependence of via
Ehrenfest's relation, we expect a considerable increase of under applied
pressure due to a strong spin-lattice coupling. Furthermore the influence of
weak off stoichiometries in EuC was studied. It is
found that strongly affects the resistivity, but hardly changes the
transition temperature. In all these aspects, the behavior of EuC strongly
resembles that of EuO.Comment: 7 pages, 6 figure
A practical high current 11 MeV production of high specific activity 89Zr
Introduction
Zr-89 is a useful radionuclide for radiolabeling proteins and other molecules.1,2 There are many reports of cyclotron production of 89Zr by the 89Y (p,n) reaction. Most irradiations use thin metal backed deposits of Y and irradiation currents up to 100 µA or thicker amounts of Y or Y2O3 with
~ 20 µA irradiations.3,4 We are working to develop high specific activity 89Zr using a low energy 11 MeV cyclotron. We have found that target Y metal contains carrier Zr and higher specific activities are achieved with less Y. The goal of this work was to optimize yield while minimizing the amount of Y that was irradiated.
Material and Methods
All irradiations were done using a Siemens Eclipse 11 MeV proton cyclotron. Y foils were used for the experiments described here. Y2O3 was tried and abandoned due to lower yield and poor heat transfer. Yttrium metal foils from Alfa Aesar, ESPI Metals and Sigma Aldrich, 0.1 to
1 mm in thickness, were tested. Each foil was irradiated for 10 to 15 minutes.
The targets to hold the Y foils were made of aluminum and were designed to fit within the “paper burn” unit of the Siemen’s Eclipse target station, allowing the Y target body to be easily inserted and removed from the system. Several Al targets of 2 cm diam. and 7.6 cm long were tested with the face of the targets from 11, 26 or 90o relative to the beam to vary watts cm−2 on the foil. The front of the foils was cooled by He convection and the foil backs by conduction to the Al target body. The target body was cooled by conduction to the water cooled Al sleeve of the target holder.
Results and Conclusion
The best target was two stacked, 0.25 mm thick, foils to stop beam. 92% of the 89Zr activity was in the front 0.25 mm Y foil. With the greatest slant we could irradiate up to 30 µA of beam on tar-get. However, the 13×30 mm dimensions of the foil was more mass (0.41 g) and lower specific activity than was desired. Redesign of the target gave a target 90o to the beam with 12×12 mm foils (0.15 g/foil) that were undamaged with up to 30 µA irradiation when two foils were used. This design has a reduction in beam at the edges of ~10%. With this design, a single Y foil, 0.25 mm thick sustained over 31 µA of beam and a peak power on target of 270 watts cm−2. The product was radionuclidically pure 89Zr after all 89mZr and small amounts of 13N produced from oxygen at the surface had decayed (TABLE 1).
Our conclusion is that the optimum target is a single 0.25 mm thick Y foil to obtain the greatest specific activity at this proton energy. This produces 167 MBq of 89Zr at EOB with a 15 minute and 31 µA irradiation. We are continuing to redesign the clamp design to reduce losses at the edge of the beam
Structures and orientational transitions in thin films of tilted hexatic smectics
We present detailed systematic studies of structural transformations in thin
liquid crystal films with the smectic-C to hexatic phase transition. For the
first time all possible structures reported in the literature are observed for
one material (5 O.6) at the variation of temperature and thickness. In unusual
modulated structures the equilibrium period of stripes is twice with respect to
the domain size. We interpret these patterns in the frame work of
phenomenological Landau type theory, as equilibrium phenomena produced by a
natural geometric frustration in a system having spontaneous splay distortion.Comment: 7 pages, 6 figure
A model for scour around bridge piers caused by flood waves
The local scour process during flood waves is modeled introducing the idea of an effective work by the flow on the sediment bed around the pier. Dimensional considerations show different possible formulations for the dimensionless effective flow work W* that, in each case, is shown to be a generalization of the flow intensity concept, commonly used in existing scour formulas. A novel experimental installation able to reproduce any hydrograph with high precision in the laboratory flume is used to carry out steady and unsteady flow experiments in order to calibrate and validate the mathematical model. Results confirm the uniqueness of the relationship between the dimensionless, effective flow work W* and the relative scour depth Z*, highlighting the high predictive capacity of scour depth caused by any hydrograph. The proposed model provides good performances allowing a straightforward prediction of maximum scour depth and its evolution
Mixture models for distance sampling detection functions
Funding: EPSRC DTGWe present a new class of models for the detection function in distance sampling surveys of wildlife populations, based on finite mixtures of simple parametric key functions such as the half-normal. The models share many of the features of the widely-used “key function plus series adjustment” (K+A) formulation: they are flexible, produce plausible shapes with a small number of parameters, allow incorporation of covariates in addition to distance and can be fitted using maximum likelihood. One important advantage over the K+A approach is that the mixtures are automatically monotonic non-increasing and non-negative, so constrained optimization is not required to ensure distance sampling assumptions are honoured. We compare the mixture formulation to the K+A approach using simulations to evaluate its applicability in a wide set of challenging situations. We also re-analyze four previously problematic real-world case studies. We find mixtures outperform K+A methods in many cases, particularly spiked line transect data (i.e., where detectability drops rapidly at small distances) and larger sample sizes. We recommend that current standard model selection methods for distance sampling detection functions are extended to include mixture models in the candidate set.Publisher PDFPeer reviewe
Microscopic theory of quadrupolar ordering in TmTe
We have calculated the crystal electric field of TmTe (T>T_Q) and have
obtained that the ground state of a Tm 4f hole is the doublet in
agreement with Mossbauer experiments. We study the quadrupole interactions
arising from quantum transitions of 4f holes of Tm. An effective attraction is
found at the L point of the Brillouin zone, . Assuming that the
quadrupolar condensation involves a single arm of we show that
there are two variants for quadrupole ordering which are described by the space
groups C2/c and C2/m. The Landau free energy is derived in mean-field theory.
The phase transition is of second order. The corresponding quadrupole order
parameters are combinations of and components. The obtained
domain structure is in agreement with observations from neutron diffraction
studies for TmTe. Calculated lattice distortions are found to be different for
the two variants of quadrupole ordering. We suggest to measure lattice
displacements in order to discriminate between those two structures.Comment: 10 pages, 2 figures, 5 tables; accepted by PR
Magnetic Field and Pressure Phase Diagrams of Uranium Heavy-Fermion Compound UZn
We have performed magnetization measurements at high magnetic fields of up to
53 T on single crystals of a uranium heavy-fermion compound UZn
grown by the Bridgman method. In the antiferromagnetic state below the N\'{e}el
temperature = 9.7 K, a metamagnetic transition is found at
32 T for the field along the [110] direction (-axis). The
magnetic phase diagram for the field along the [110] direction is
given. The magnetization curve shows a nonlinear increase at 35
T in the paramagnetic state above up to a characteristic
temperature where the magnetic susceptibility or
electrical resistivity shows a maximum value. This metamagnetic behavior of the
magnetization at is discussed in comparison with the metamagnetic
magnetism of the heavy-fermion superconductors UPt, URuSi, and
UPdAl. We have also carried out high-pressure resistivity measurement
on UZn using a diamond anvil cell up to 8.7 GPa. Noble gas argon was
used as a pressure-transmitting medium to ensure a good hydrostatic
environment. The N\'{e}el temperature is almost
pressure-independent up to 4.7 GPa and starts to increase in the
higher-pressure region. The pressure dependences of the coefficient of the
term in the electrical resistivity , the antiferromagnetic gap
, and the characteristic temperature are
discussed. It is found that the effect of pressure on the electronic states in
UZn is weak compared with those in the other heavy fermion
compounds
Multipole tensor analysis of the resonant x-ray scattering by quadrupolar and magnetic order in DyB2C2
Resonant x-ray scattering (RXS) experiment has been performed for the (3 0
1.5) superlattice reflection in the antiferroquadrupolar and antiferromagnetic
phase of DyB2C2. Azimuthal-angle dependence of the resonance enhanced
intensities for both dipolar (E1) and quadrupolar (E2) resonant processes has
been measured precisely with polarization analysis. Every scattering channel
exhibits distinctive azimuthal dependence, differently from the symmetric
reflection at (0 0 0.5) which was studied previously. We have analyzed the
results using a theory developed by Lovesey et al., which directly connects
atomic tensors with the cross-section of RXS. The fitting results indicate that
the azimuthal dependences can be explained well by the atomic tensors up to
rank 2. Rank 3 and rank 4 tensors are reflected in the data very little. In
addition, The coupling scheme among the 4f quadrupolar moment, 5d ortitals, and
the lattice has been determined from the interference among the Thomson
scattering from the lattice distortion and the resonant scatterings of E1 and
E2 processes. It has also been established from the RXS of the (3 0 1.5)
reflection that the canting of the 4f quadrupolar moments exists up to T_Q. We
also discuss a possible wavefunction of the ground state from the point-charge
model calculation.Comment: 9 pages, 10 figure
Stigmatising attitude of medical students towards a psychiatry label
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