6,167 research outputs found
Crystal Structure and Magnetism of the Linear-Chain Copper Oxides Sr5Pb3-xBixCuO12
The title quasi-1D copper oxides (0=< x =<0.4) were investigated by neutron
diffraction and magnetic susceptibility studies. Polyhedral CuO4 units in the
compounds were found to comprise linear-chains at inter-chain distance of
approximately 10 A. The parent chain compound (x = 0), however, shows less
anisotropic magnetic behavior above 2 K, although it is of substantially
antiferromagnetic (mu_{eff}= 1.85 mu_{B} and Theta_{W} = -46.4 K) spin-chain
system. A magnetic cusp gradually appears at about 100 K in T vs chi with the
Bi substitution. The cusp (x = 0.4) is fairly characterized by and therefore
suggests the spin gap nature at Delta/k_{B} ~ 80 K. The chain compounds hold
electrically insulating in the composition range.Comment: To be published in PR
Stability of temporal statistics in Transition Path Theory with sparse data
Transition Path Theory (TPT) provides a rigorous statistical characterization
of the ensemble of trajectories connecting directly, i.e., without detours, two
disconnected (sets of) states in a Markov chain, a stochastic process that
undergoes transitions from one state to another with probability depending on
the state attained in the previous step. Markov chains can be constructed using
trajectory data via counting of transitions between cells covering the domain
spanned by trajectories. With sparse trajectory data, the use of regular cells
is observed to result in unstable estimates of the total duration of transition
paths. Using Voronoi cells resulting from k-means clustering of the trajectory
data, we obtain stable estimates of this TPT statistic, which is generalized to
frame the remaining duration of transition paths, a new TPT statistic suitable
for investigating connectivity.Comment: Submitted to Chaos. Comments welcomed
Resonance production from jet fragmentation
Short lived resonances are sensitive to the medium properties in heavy-ion
collisions. Heavy hadrons have larger probability to be produced within the
quark gluon plasma phase due to their short formation times. Therefore heavy
mass resonances are more likely to be affected by the medium, and the
identification of early produced resonances from jet fragmentation might be a
viable option to study chirality. The high momentum resonances on the away-side
of a triggered di-jet are likely to be the most modified by the partonic or
early hadronic medium. We will discuss first results of triggered
hadron-resonance correlations in Cu+Cu heavy ion collisions.Comment: Hot Quarks Colorado 2008 Proceedings, 4 pages 5 figure
Efficient Comparison of Massive Graphs Through The Use Of 'Graph Fingerprints'
The problem of how to compare empirical graphs is an area of great interest within the field of network science. The ability to accurately but efficiently compare graphs has a significant impact in such areas as temporal graph evolution, anomaly detection and protein comparison. The comparison problem is compounded when working with graphs containing millions of anonymous, i.e. unlabelled, vertices and edges. Comparison of two or more graphs is highly computationally expensive. Thus reducing a graph to a much smaller feature set – called a fingerprint, which accurately captures the essence of the graph would be highly desirable. Such an approach would have potential applications outside of graph comparisons, especially in the area of machine learning. This paper introduces a feature extraction based approach for the efficient comparison of large topologically similar, but order varying, unlabelled graph datasets. The approach acts by producing a ‘Graph Fingerprint’ which represents both vertex level and global level topological features from a graph. The approach is shown to be efficient when comparing graphs which are highly topologically similar but order varying. The approach scales linearly with the size and complexity of the graphs being fingerprinted
Exact diagonalization of the S=1/2 Heisenberg antiferromagnet on finite bcc lattices to estimate properties on the infinite lattice
Here we generate finite bipartite body-centred cubic lattices up to 32
vertices. We have studied the spin one half Heisenberg antiferromagnet by
diagonalizing its Hamiltonian on each of the finite lattices and hence
computing its ground state properties. By extrapolation of these data we obtain
estimates of the T = 0 properties on the infinite bcc lattice. Our estimate of
the T = 0 energy agrees to five parts in ten thousand with third order spin
wave and series expansion method estimates, while our estimate of the staggered
magnetization agrees with the spin wave estimate to within a quarter of one
percent.Comment: 16 pages, LaTeX, 1 ps figure, to appear in J.Phys.
Magnetic Properties of (VO)_2P_2O_7 from Frustrated Interchain Coupling
Neutron-scattering experiments on (VO)_2P_2O_7 reveal both a gapped magnon
dispersion and an unexpected, low-lying second mode. The proximity and
intensity of these modes suggest a frustrated coupling between the alternating
spin chains. We deduce the minimal model containing such a frustration, and
show that it gives an excellent account of the magnon dispersion, static
susceptibility and electron spin resonance absorption. We consider two-magnon
states which bind due to frustration, and demonstrate that these may provide a
consistent explanation for the second mode.Comment: RevTeX, 5 pages, 6 figures, compressed from first versio
Spin gap in the Quasi-One-Dimensional S=1/2 Antiferromagnet: Cu2(1,4-diazacycloheptane)2Cl4
Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2
Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutron
scattering measurements on this material. The magnetic susceptibility,
, shows a rounded maximum at T = 8 K indicative of a low dimensional
antiferromagnet with no zero field magnetic phase transition. We compare the
data to exact diagonalization results for various one dimensional
spin Hamiltonians and find excellent agreement for a spin ladder with
intra-rung coupling meV and two mutually frustrating
inter-rung interactions: meV and meV. The
specific heat in zero field is exponentially activated with an activation
energy meV. A spin gap is also found through inelastic
neutron scattering on powder samples which identify a band of magnetic
excitations for meV. Using sum-rules we derive an
expression for the dynamic spin correlation function associated with
non-interacting propagating triplets in a spin ladder. The van-Hove
singularities of such a model are not observed in our scattering data
indicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4}
are more complicated. For magnetic fields above T specific
heat data versus temperature show anomalies indicating a phase transition to an
ordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail
Comments to [email protected]
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