1,195 research outputs found
Approaching the ground state of the kagome antiferromagnet
Y{0.5}$Ca{0.5}BaCo4O7 contains kagome layers of Co ions, whose spins are
strongly coupled according to a Curie-Weiss temperature of -2200 K. At low
temperatures, T = 1.2 K, our diffuse neutron scattering study with polarization
analysis reveals characteristic spin correlations close to a predicted
two-dimensional coplanar ground state with staggered chirality. The absence of
three dimensional long-range AF order proves negligible coupling between the
kagome layers. The scattering intensities are consistent with high spin S=3/2
states of Co2+ in the kagome layers and low spin S=0 states for Co3+ ions at
interlayer sites. Our observations agree with previous Monte Carlo simulations
indicating a ground state of only short range chiral order.Comment: 4 pages, 4 figures, contact author: [email protected]
Low Energy Singlets in the Excitation Spectrum of the Spin Tetrahedra System Cu_2Te_2O_5Br_2
Low energy Raman scattering of the s=1/2 spin tetrahedra system
Cu_2Te_2O_5Br_2 is dominated by an excitation at 18 cm^{-1} corresponding to an
energy E_S=0.6\Delta, with \Delta the spin gap of the compound. For elevated
temperatures this mode shows a soft mode-like decrease in energy pointing to an
instability of the system. The isostructural reference system Cu_2Te_2O_5Cl_2
with a presumably larger inter-tetrahedra coupling does not show such a low
energy mode. Instead its excitation spectrum and thermodynamic properties are
compatible with long range Neel-ordering. We discuss the observed effects in
the context of quantum fluctuations and competing ground states.Comment: 5 pages, 2 figures, ISSP-Kashiwa 2001, Conference on Correlated
Electron
Magnetism of a tetrahedral cluster-chain
Magnetic properties of a completely frustrated tetrahedral chain are briefly
summarized. Using exact diagonalization, and bond-operator theory results for
the ground-state phase diagram, the one-triplet excitations and the Raman
spectrum are given. The link to novel tellurate materials is clarified.Comment: 2 pages, 3 figure
Substitution effects on spin fluctuations in the spin-Peierls compound CuGeO_3
Using Raman scattering we studied the effect of substitutions on 1D spin
fluctuations in CuGeO_3 observed as a spinon continuum in frustration induced
exchange scattering. For temperatures below the spin-Peierls transition
(T_{SP}=14K) the intensity of this continuum at 120-500 cm^{-1} is
exponentially suppressed and transferred into a 3D two-magnon density of
states. Besides a spin-Peierls gap-induced mode at 30 cm^{-1} and additional
modes at 105 and 370 cm^{-1} are observed. Substitution of Zn on the Cu-site
and Si on the Ge-site of CuGeO_3 quenches easily the spin-Peierls state.
Consequently a suppression of the spin-Peierls gap observable below T_{SP}=14K
as well as a change of the temperature dependence of the spinon continuum are
observed. These effects are discussed in the context of a dimensional crossover
of this compound below T_{SP} and strong spin-lattice interaction.Comment: 9 pages, 2 eps figures include
Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality
We present a comprehensive study of the coupled tetrahedra-compound
Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report
the observation of a longitudinal magnon in Raman scattering in the ordered
state close to quantum criticality. We show that the excited
tetrahedral-singlet sets the energy scale for the magnetic ordering temperature
T_N. This energy is determined experimentally. The ordering temperature T_N has
an inverse-log dependence on the coupling parameters near quantum criticality
Thermodynamics of Coupled Identical Oscillators within the Path Integral Formalism
A generalization of symmetrized density matrices in combination with the
technique of generating functions allows to calculate the partition function of
identical particles in a parabolic confining well. Harmonic two-body
interactions (repulsive or attractive) are taken into account. Also the
influence of a homogeneous magnetic field, introducing anisotropy in the model,
is examined. Although the theory is developed for fermions and bosons, special
attention is payed to the thermodynamic properties of bosons and their
condensation.Comment: 13 REVTEX pages + 9 postscript figure
Xenogeneic, extracorporeal liver perfusion in primates improves the ratio of branched-chain amino acids to aromatic amino acids (Fischer's ratio)
In fulminant hepatic failure (FHF), the development of hepatic encephalopathy is associated with grossly abnormal concentrations of plasma amino acids (PAA). Normalization of the ratio of branched-chain amino acids to aromatic amino acids (Fischer's ratio) correlates with clinical improvement. This study evaluated changes in PAA metabolism during 4 h of isolated, normothermic extracorporeal liver perfusion using a newly designed system containing human blood and a rhesus monkey liver. Bile and urea production were within the physiological range. Release of the transaminases AST, ALT and LDH were minimal. The ratio of branched (valine, leucine, isoleucine) to aromatic (tyrosine, phenylalanine) amino acids increased significantly. These results indicate that a xenogeneic extracorporeal liver perfusion system is capable of significantly increasing Fischer's ratio and may play a role in treating and bridging patients in FHF in the future
Density and Pair Correlation Function of Confined Identical Particles: the Bose-Einstein Case
Two basic correlation functions are calculated for a model of
harmonically interacting identical particles in a parabolic potential well. The
density and the pair correlation function of the model are investigated for the
boson case. The dependence of these static response properties on the complete
range of the temperature and of the number of particles is obtained. The
calculation technique is based on the path integral approach of symmetrized
density matrices for identical particles in a parabolic confining well.Comment: 8 pages (REVTEX) + 6 figures (postscript
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