1,785 research outputs found
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
Positive contraction mappings for classical and quantum Schrodinger systems
The classical Schrodinger bridge seeks the most likely probability law for a
diffusion process, in path space, that matches marginals at two end points in
time; the likelihood is quantified by the relative entropy between the sought
law and a prior, and the law dictates a controlled path that abides by the
specified marginals. Schrodinger proved that the optimal steering of the
density between the two end points is effected by a multiplicative functional
transformation of the prior; this transformation represents an automorphism on
the space of probability measures and has since been studied by Fortet,
Beurling and others. A similar question can be raised for processes evolving in
a discrete time and space as well as for processes defined over non-commutative
probability spaces. The present paper builds on earlier work by Pavon and
Ticozzi and begins with the problem of steering a Markov chain between given
marginals. Our approach is based on the Hilbert metric and leads to an
alternative proof which, however, is constructive. More specifically, we show
that the solution to the Schrodinger bridge is provided by the fixed point of a
contractive map. We approach in a similar manner the steering of a quantum
system across a quantum channel. We are able to establish existence of quantum
transitions that are multiplicative functional transformations of a given Kraus
map, but only for the case of uniform marginals. As in the Markov chain case,
and for uniform density matrices, the solution of the quantum bridge can be
constructed from the fixed point of a certain contractive map. For arbitrary
marginal densities, extensive numerical simulations indicate that iteration of
a similar map leads to fixed points from which we can construct a quantum
bridge. For this general case, however, a proof of convergence remains elusive.Comment: 27 page
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
Magnetic excitations and phonons in the spin-chain compound NaCu2O2
We report an inelastic light scattering study of single-crystalline
NaCuO, a spin-chain compound known to exhibit a phase with helical
magnetic order at low temperatures. Phonon excitations were studied as a
function of temperature and light polarization, and the phonon frequencies are
compared to the results of ab-initio lattice dynamical calculations, which are
also reported here. The good agreement between the observed and calculated
modes allows an assignment of the phonon eigenvectors. Two distinct high-energy
two-magnon features as well as a sharp low-energy one-magnon peak were also
observed. These features are discussed in terms of the magnon modes expected in
a helically ordered state. Their polarization dependence provides evidence of
substantial exchange interactions between two closely spaced spin chains within
a unit cell. At high temperatures, the spectral features attributable to
magnetic excitations are replaced by a broad, quasielastic mode due to
overdamped spin excitations
Ground state order and spin-lattice coupling in tetrahedral spin systems Cu2Te2O5X2
High-resolution ac susceptibility and thermal conductivity measurement on
Cu2Te2O5X2(X=Br,Cl) single crystals are reported. For Br-sample, sample
dependence prevents to distinguish between possibilities of magnetically
ordered and spin-singlet ground states. In Cl-sample a three-dimensional
transition at 18.5 K is accompanied by almost isotropic behavior of
susceptibility and almost switching behavior of thermal conductivity. Thermal
conductivity studies suggest the presence of a tremendous spin-lattice coupling
characterizing Cl- but not Br-sample. Below the transition Cl-sample is in a
complex magnetic state involving AF order but also the elements consistent with
the presence of a gap in the excitation spectrum.Comment: version accepted for publication in Phys.Rev.B-Rapid Communicatio
Magnetic bound states in the quarter-filled ladder system }
Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5
shows in the dimerized singlet ground state () an unexpected
sequence of three magnetic bound states. Our results suggest that the recently
proposed mapping onto an effective spin chain for has to be given
up in favor of the full topology and exchange paths of a ladder in the
dimerized phase for . As the new ground state we propose a dynamic
superposition of energetically nearly degenerate dimer configurations on the
ladder.Comment: 5 pages, 4 figures, to be published in PRB, brief reports, Dec. 199
Identifying potentially cost effective chronic care programs for people with COPD
Objective: To review published evidence regarding the cost effectiveness of multi-component COPD programs and to illustrate how potentially cost effective programs can be identified. Methods: Systematic search of Medline and Cochrane databases for evaluations of multicomponent disease management or chronic care programs for adults with COPD, describing process, intermediate, and end results of care. Data were independently extracted by two reviewers and descriptively summarized. Results: Twenty articles describing 17 unique COPD programs were included. There is little evidence for significant improvements in process and intermediate outcomes, except for increased provision of patient self-management education and improved disease-specific knowledge. Overall, the COPD programs generate end results equivalent to usual care, but programs containing ≥3 components show lower relative risks for hospitalization. There is limited scope for programs to break-even or save money. Conclusion: Identifying cost effective multi-component COPD programs remains a challenge due to scarce methodologically sound studies that demonstrate significant improvements on process, intermediate and end results of care. Estimations of potential cost effectiveness of specific programs illustrated in this paper can, in the absence of 'perfect data', support timely decision-making regarding these programs. Nevertheless, well-designed health economic studies are needed to decrease the current decision uncertainty
Magnon-magnon interactions in the Spin-Peierls compound CuGeO_3
In a magnetic substance the gap in the Raman spectrum, Delta_R, is
approximatively twice the value of the neutron scattering gap, Delta_S, if the
the magnetic excitations (magnons) are only weakly interacting.
But for CuGeO_3 the experimentally observed ratio Delta_R/Delta_S is
approximatively 1.49-1.78, indicating attractive magnon-magnon interactions in
the quasi-1D Spin-Peierls compound CuGe_3.
We present numerical estimates for Delta_R/Delta_S from exact diagonalization
studies for finite chains and find agreement with experiment for intermediate
values of the frustration parameter alpha.
An analysis of the numerical Raman intensity leads us to postulate a
continuum of two-magnon bound states in the Spin-Peierls phase. We discuss in
detail the numerical method used, the dependence of the results on the model
parameters and a novel matrix-element effect due to the dimerization of the
Raman-operator in the Spin-Peierls phase.Comment: submitted to PRB, Phys. Rev. B, in pres
Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho
We report muon spin relaxation measurements on the magnetic structures of
RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one
ferrimagnetic and two antiferromagnetic, are identified below 300 K. They
consist of different ordered spin state arrangements of high-, intermediate-,
and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated
regions with different spin state order is observed in the antiferromagnetic
phases. The unusual strongly anisotropic magnetoresistance and its onset at the
FM-AFM phase boundary is explained.Comment: 4 pages, accepted for publication in Phys. Rev. Let
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