1,505 research outputs found
Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr
We have performed time-of-flight neutron scattering measurements on powder
samples of the unconventional spin-Peierls compound TiOBr using the
fine-resolution Fermi chopper spectrometer (SEQUOIA) at the SNS. These
measurements reveal two branches of magnetic excitations within the
commensurate and incommensurate spin-Peierls phases, which we associate with n
= 1 and n = 2 triplet excitations out of the singlet ground state. These
measurements represent the first direct measure of the singlet-triplet energy
gap in TiOBr, which is determined to be Eg = 21.2 +/- 1.0 meV.Comment: 5 pages, 4 figures, submitted for publicatio
Eigenvalue correlations in non-Hermitean symplectic random matrices
Correlation function of complex eigenvalues of N by N random matrices drawn
from non-Hermitean random matrix ensemble of symplectic symmetry is given in
terms of a quaternion determinant. Spectral properties of Gaussian ensembles
are studied in detail in the regimes of weak and strong non-Hermiticity.Comment: 14 page
Origin of the large phonon band-gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskite: First principles lattice dynamics and inelastic neutron scattering of PbTiO3, BaTiO3 and SrTiO3
We report first principles density functional perturbation theory
calculations and inelastic neutron scattering measurements of the phonon
density of states, dispersion relations and electromechanical response of
PbTiO3, BaTiO3 and SrTiO3. The phonon density-of-states of the quantum
paraelectric SrTiO3 is found to be fundamentally distinct from that of
ferroelectric PbTiO3 and BaTiO3 with a large 70-90 meV phonon band-gap. The
phonon dispersion and electromechanical response of PbTiO3 reveal giant
anisotropies. The interplay of covalent bonding and ferroelectricity, strongly
modulates the electromechanical response and give rise to spectacular
signatures in the phonon spectra. The computed charge densities have been used
to study the bonding in these perovskites. Distinct bonding characteristics in
the ferroelectric and paraelectric phases give rise to spectacular vibrational
signatures. While a large phonon band-gap in ATiO3 perovskites seems a
characteristic of quantum paraelectrics, anisotropy of the phonon spectra
correlates well with ferroelectric strength. These correlations between the
phonon spectra and ferroelectricity, can guide future efforts at custom
designing still more effective piezoelectrics for applications. These results
suggest that vibrational spectroscopy can help design novel materials.Comment: 11 pages, 4 color figures and 2 Table
Speeding up Simplification of Polygonal Curves using Nested Approximations
We develop a multiresolution approach to the problem of polygonal curve
approximation. We show theoretically and experimentally that, if the
simplification algorithm A used between any two successive levels of resolution
satisfies some conditions, the multiresolution algorithm MR will have a
complexity lower than the complexity of A. In particular, we show that if A has
a O(N2/K) complexity (the complexity of a reduced search dynamic solution
approach), where N and K are respectively the initial and the final number of
segments, the complexity of MR is in O(N).We experimentally compare the
outcomes of MR with those of the optimal "full search" dynamic programming
solution and of classical merge and split approaches. The experimental
evaluations confirm the theoretical derivations and show that the proposed
approach evaluated on 2D coastal maps either shows a lower complexity or
provides polygonal approximations closer to the initial curves.Comment: 12 pages + figure
Positronic lithium, an electronically stable Li-e ground state
Calculations of the positron-Li system were performed using the Stochastic
Variational Method and yielded a minimum energy of -7.53208 Hartree for the L=0
ground state. Unlike previous calculations of this system, the system was found
to be stable against dissociation into the Ps + Li channel with a binding
energy of 0.00217 Hartree and is therefore electronically stable. This is the
first instance of a rigorous calculation predicting that it is possible to
combine a positron with a neutral atom and form an electronically stable bound
state.Comment: 11 pages, 2 tables. To be published in Phys.Rev.Let
Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB samples
The real and imaginary parts of the surface impedance
in polycrystalline MgB samples of different density
with the critical temperature K are measured at the frequency
of 9.4 GHz and in the temperature range K. The normal skin-effect
condition at holds only for the samples of the
highest density with roughness sizes not more than 0.1 m. For such samples
extrapolation of the linear at temperature dependences
and results in values of the London
penetration depth \AA and residual surface resistance
m. In the entire temperature range the dependences
and are well described by the modified two-fluid model.Comment: 7 pages, 3 figures. Europhysics Letters, accepted for publicatio
Measurement of the Cross Section Asymmetry of the Reaction gp-->pi0p in the Resonance Energy Region Eg = 0.5 - 1.1 GeV
The cross section asymmetry Sigma has been measured for the photoproduction
of pi0-mesons off protons, using polarized photons in the energy range Eg = 0.5
- 1.1 GeV. The CM angular coverage is Theta = 85 - 125 deg with energy and
angle steps of 25 MeV and 5 deg, respectively. The obtained Sigma data, which
cover the second and third resonance regions, are compared with existing
experimental data and recent phenomenological analyses. The influence of these
measurements on such analyses is also considered
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