5,023 research outputs found
Optical phonons in new ordered perovskite Sr2Cu(Re0.69Ca0.31) Oy system observed by infrared reflectance spectroscopy
We report infrared reflectivity spectra for a new correlated cupric oxide
system Sr2Cu(Re0.69Ca0.31)Oy with y ~ 0.6 at several temperatures ranging
between 8 and 380 K. The reflectivity spectrum at 300 K comprises of several
optical phonons. A couple of residual bands located around 315 and 653 cm-1
exhibit exceptionally large intensity as compared to the other ones. The
overall reflectivity spectrum lifts up slightly with increasing temperature.
The energy and damping factor of transverse-optical phonons are determined by
fitting the imaginary dielectric constant by Lorentz oscillator model and
discussed as a function of temperature in terms of lattice anharmonicity.Comment: 9 pages, 3 figures, presented at ISS2005, to appear in Physica
Electronic structure of the incommensurate compound
We extracted, from strongly-correlated ab-initio calculations, a complete
model for the chain subsystem of the
incommensurate compound. A second neighbor model has been determined as
a function of the fourth crystallographic parameter , for both low and
room temperature crystallographic structures. The analysis of the obtained
model shows the crucial importance of the structural modulations on the
electronic structure through the on-site energies and the magnetic
interactions. The structural distortions are characterized by their long range
effect on the cited parameters that hinder the reliability of analyses such as
BVS. One of the most striking results is the existence of antiferromagnetic
nearest-neighbor interactions for metal-ligand-metal angles of . A
detailed analysis of the electron localization and spin arrangement is
presented as a function of the chain to ladder hole transfer and of the
temperature. The obtained spin arrangement is in agreement with
antiferromagnetic correlations in the chain direction at low temperature
Fast Ion-Driven MHD Instabilities and Consequent Fast Ion Losses in the Compact Helical System
Impacts of fast-ion-driven MHD modes on fast ion transport have been investigated in a medium size helical device CHS having negative magnetic shear over an entire region of plasma. Recently, it was observed that bursting toroidicity-induced Alfven eigenmodes (TAEs) excited by co-circulating beam ions enhance beam ion loss at low magnetic field strength of 0.9 T. Flux of escaping beam ions steeply increases as the magnetic fluctuation level increases. Excitation of energetic particle modes (EPMs) due to co-circulating beam ions has been also observed in CHS, of which frequency range is located appreciably below the TAE gap frequency. Bursts of EPMs and TAEs recur periodically accompanied with periodically increased beam ion loss. Fast particle diagnostics indicate that beam ions transported by these modes are promptly lost to the large major radius side of CHS plasma in the horizontally elongated section. Particle simulation suggests that a perturbed magnetic field whose amplitude is consistent with the CHS experiment can rapidly increase the fast ion loss
Evidence of non-thermal X-ray emission from radio lobes of Cygnus A
Using deep Chandra ACIS observation data for Cygnus A, we report evidence of
non-thermal X-ray emission from radio lobes surrounded by a rich intra-cluster
medium (ICM). The diffuse X-ray emission, which are associated with the eastern
and western radio lobes, were observed in a 0.7--7 keV Chandra$ ACIS image. The
lobe spectra are reproduced with not only a single-temperature Mekal model,
such as that of the surrounding ICM component, but also an additional power-law
(PL) model. The X-ray flux densities of PL components for the eastern and
western lobes at 1 keV are derived as 77.7^{+28.9}_{-31.9} nJy and
52.4^{+42.9}_{-42.4} nJy, respectively, and the photon indices are
1.69^{+0.07}_{-0.13} and 1.84^{+2.90}_{-0.12}, respectively. The non-thermal
component is considered to be produced via the inverse Compton (IC) process, as
is often seen in the X-ray emission from radio lobes. From a re-analysis of
radio observation data, the multiwavelength spectra strongly suggest that the
seed photon source of the IC X-rays includes both cosmic microwave background
radiation and synchrotron radiation from the lobes. The derived parameters
indicate significant dominance of the electron energy density over the magnetic
field energy density in the Cygnus A lobes under the rich ICM environment.Comment: 8 pages, 5 figures, accepted for publication in Ap
Finite Temperature Effects in One-dimensional Mott-Hubbard Insulator: Angle-Resolved Photoemission Study of Na_{0.96}V_{2}O_{5}
We have made an angle-resolved photoemission study of a one-dimensional (1D)
Mott-Hubbard insulator Na_{0.96}V_{2}O_{5} and found that the spectra of the V
3d lower Hubbard band are strongly dependent on the temperature. We have
calculated the one-particle spectral function of the one-dimensional t-J model
at finite temperatures by exact diagonalization and compared them with the
experimental results. Good overall agreement is obtained between experiment and
theory. The strong finite temperature effects are discussed in terms of the
existence of the ``Fermi surface'' of the spinon band.Comment: 4 pages, 3 figure
First evidence for charge ordering in NaVO from Raman spectroscopy
We argue on the basis of symmetry selection rules and Raman scattering
spectra that NaVO undergoes a charge ordering phase transition at
T=34 K. Such a transition is characterized by the redistribution of the
charges at the phase transition and corresponds to the change of the vanadium
ions, from uniform V to two different V and V states. In
the low temperature phase the V ions are forming a "zig-zag" ladder
structure along the {\bf b}-axis, consistent with the symmetry of the P2/b
space group.Comment: to be published in solid state communication
Anomalous thermal conductivity of NaV2O5 as compared to conventional spin-Peierls system CuGeO3
A huge increase of thermal conductivity k is observed at the phase transition
in stoichiometric NaV2O5. This anomaly decreases and gradually disappears with
deviation from stoichiometry in Na(1-x}V2O5 (x = 0.01, 0.02, 0.03, and 0.04).
This behavior is compared with that of pure and Zn-doped CuGeO3 where only
modest kinks in the k(T) curves are observed at the spin-Peierls transition.
The change of k at critical temperature Tc could be partially attributed to the
opening of an energy gap in the magnetic excitation spectrum excluding the
scattering of thermal phonons on spin fluctuations. However, the reason for
such a strong anomaly in the k(T) may lie not only in the different energy
scales of CuGeO3 and NaV2O5, but also in the different character of the phase
transition in NaV2O5 which can have largely a structural origin, e.g. connected
with the charge ordering.Comment: PostScript 4 pages, 4 PostScript pictures. Submitted to Physical
Review Letter
The Magnetic Rayleigh-Taylor Instability in Three Dimensions
We study the magnetic Rayleigh-Taylor instability in three dimensions, with
focus on the nonlinear structure and evolution that results from different
initial field configurations. We study strong fields in the sense that the
critical wavelength l_c at which perturbations along the field are stable is a
large fraction of the size of the computational domain. We consider magnetic
fields which are initially parallel to the interface, but have a variety of
configurations, including uniform everywhere, uniform in the light fluid only,
and fields which change direction at the interface. Strong magnetic fields do
not suppress instability, in fact by inhibiting secondary shear instabilities,
they reduce mixing between the heavy and light fluid, and cause the rate of
growth of bubbles and fingers to increase in comparison to hydrodynamics.
Fields parallel to, but whose direction changes at, the interface produce long,
isolated fingers separated by the critical wavelength l_c, which may be
relevant to the morphology of the optical filaments in the Crab nebula.Comment: 14 pages, 9 pages, accepted by Ap
Small coupling limit and multiple solutions to the Dirichlet Problem for Yang Mills connections in 4 dimensions - Part I
In this paper (Part I) and its sequels (Part II and Part III), we analyze the
structure of the space of solutions to the epsilon-Dirichlet problem for the
Yang-Mills equations on the 4-dimensional disk, for small values of the
coupling constant epsilon. These are in one-to-one correspondence with
solutions to the Dirichlet problem for the Yang Mills equations, for small
boundary data. We prove the existence of multiple solutions, and, in
particular, non minimal ones, and establish a Morse Theory for this non-compact
variational problem. In part I, we describe the problem, state the main
theorems and do the first part of the proof. This consists in transforming the
problem into a finite dimensional problem, by seeking solutions that are
approximated by the connected sum of a minimal solution with an instanton, plus
a correction term due to the boundary. An auxiliary equation is introduced that
allows us to solve the problem orthogonally to the tangent space to the space
of approximate solutions. In Part II, the finite dimensional problem is solved
via the Ljusternik-Schirelman theory, and the existence proofs are completed.
In Part III, we prove that the space of gauge equivalence classes of Sobolev
connections with prescribed boundary value is a smooth manifold, as well as
some technical lemmas used in Part I. The methods employed still work when the
4-dimensional disk is replaced by a more general compact manifold with
boundary, and SU(2) is replaced by any compact Lie group
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