5,855 research outputs found
Influence of Mg, Ag and Al substitutions on the magnetic excitations in the triangular-lattice antiferromagnet CuCrO2
Magnetic excitations in CuCrO, CuCrMgO,
CuAgCrO, and CuCrAlO have been
studied by powder inelastic neutron scattering to elucidate the element
substitution effects on the spin dynamics in the Heisenberg triangular-lattice
antiferromagnet CuCrO. The magnetic excitations in
CuCrMgO consist of a dispersive component and a flat
component. Though this feature is apparently similar to CuCrO, the energy
structure of the excitation spectrum shows some difference from that in
CuCrO. On the other hand, in CuAgCrO and
CuCrAlO the flat components are much reduced, the
low-energy parts of the excitation spectra become intense, and additional
low-energy diffusive spin fluctuations are induced. We argued the origins of
these changes in the magnetic excitations are ascribed to effects of the doped
holes or change of the dimensionality in the magnetic correlations.Comment: 7 pages, 5 figure
Molecular kinetic analysis of a finite-time Carnot cycle
We study the efficiency at the maximal power of a
finite-time Carnot cycle of a weakly interacting gas which we can reagard as a
nearly ideal gas. In several systems interacting with the hot and cold
reservoirs of the temperatures and , respectively,
it is known that which
is often called the Curzon-Ahlborn (CA) efficiency . For the
first time numerical experiments to verify the validity of
are performed by means of molecular dynamics simulations and reveal that our
does not always agree with , but
approaches in the limit of .
Our molecular kinetic analysis explains the above facts theoretically by using
only elementary arithmetic.Comment: 6 pages, 4 figure
Super-critical Accretion Flows around Black Holes: Two-dimensional, Radiation-pressure-dominated Disks with Photon-trapping
The quasi-steady structure of super-critical accretion flows around a black
hole is studied based on the two-dimensional radiation-hydrodynamical (2D-RHD)
simulations. The super-critical flow is composed of two parts: the disk region
and the outflow regions above and below the disk. Within the disk region the
circular motion as well as the patchy density structure are observed, which is
caused by Kelvin-Helmholtz instability and probably by convection. The
mass-accretion rate decreases inward, roughly in proportion to the radius, and
the remaining part of the disk material leaves the disk to form outflow because
of strong radiation pressure force. We confirm that photon trapping plays an
important role within the disk. Thus, matter can fall onto the black hole at a
rate exceeding the Eddington rate. The emission is highly anisotropic and
moderately collimated so that the apparent luminosity can exceed the Eddington
luminosity by a factor of a few in the face-on view. The mass-accretion rate
onto the black hole increases with increase of the absorption opacity
(metalicity) of the accreting matter. This implies that the black hole tends to
grow up faster in the metal rich regions as in starburst galaxies or
star-forming regions.Comment: 16 pages, 12 figures, accepted for publication in ApJ (Volume 628,
July 20, 2005 issue
Bubbling Calabi-Yau geometry from matrix models
We study bubbling geometry in topological string theory. Specifically, we
analyse Chern-Simons theory on both the 3-sphere and lens spaces in the
presence of a Wilson loop insertion of an arbitrary representation. For each of
these three manifolds we formulate a multi-matrix model whose partition
function is the vev of the Wilson loop and compute the spectral curve. This
spectral curve is the reduction to two dimensions of the mirror to a Calabi-Yau
threefold which is the gravitational dual of the Wilson loop insertion. For
lens spaces the dual geometries are new. We comment on a similar matrix model
which appears in the context of Wilson loops in AdS/CFT.Comment: 30 pages; v.2 reference added, minor correction
Enhancement of Superconducting Transition Temperature due to the strong Antiferromagnetic Spin Fluctuations in Non-centrosymmetric Heavy-fermion Superconductor CeIrSi3 :A 29Si-NMR Study under Pressure
We report a 29Si-NMR study on the pressure-induced superconductivity (SC) in
an antiferromagnetic (AFM) heavy-fermion compound CeIrSi3 without inversion
symmetry. In the SC state at P=2.7-2.8 GPa, the temperature dependence of the
nuclear-spin lattice relaxation rate 1/T_1 below Tc exhibits a T^3 behavior
without any coherence peak just below Tc, revealing the presence of line nodes
in the SC gap. In the normal state, 1/T_1 follows a \sqrt{T}-like behavior,
suggesting that the SC emerges under the non-Fermi liquid state dominated by
AFM spin fluctuations enhanced around quantum critical point (QCP). The reason
why the maximum Tc in CeIrSi3 is relatively high among the Ce-based
heavy-fermion superconductors may be the existence of the strong AFM spin
fluctuations. We discuss the comparison with the other Ce-based heavy-fermion
superconductors.Comment: 4 pages, 5 figures, To be published in Phys. Rev. Let
Optical identification of ISO far-infrared sources in the Lockman Hole using a deep VLA 1.4 GHz continuum survey
By exploiting the far-infrared(FIR) and radio correlation, we have performed
the Likelihood-Ratio analysis to identify optical counterparts to the
far-infrared sources in the Lockman Hole. Using the likelihood ratio analysis
and the associated reliability, 44 FIR sources have been identified with radio
sources. Redshifts have been obtained for 29 out of 44 identified sources. One
hyper-luminous infrared galaxy (HyLIRG) with and four ultraluminous infrared
galaxies (ULIRGs) are identified in our sample. The space density of the FIR
sources at z = 0.3-0.6 is 4.6\times 10^{-5}Mpc^{-3}, implying a rapid evolution
of the ULIRG population. Most of \ISO FIR sources have their FIR-radio ratios
similar to star-forming galaxies ARP 220 and M82. At least seven of our FIR
sources show evidence for the presence of an active galactic nucleus (AGN) in
optical emission lines, radio continuum excess, or X-ray activity. Three out of
five (60%) of the ULIRG/HyLIRGs are AGN galaxies. Five of the seven AGN
galaxies are within the ROSAT X-ray survey field, and two are within the
XMM-Newton survey fields. X-ray emission has been detected in only one source,
1EX030, which is optically classified as a quasar. The non-detection in the
XMM-Newton 2-10 keV band suggests a very thick absorption obscuring the central
source of the two AGN galaxies. Several sources have an extreme FIR luminosity
relative to the optical R-band, L(90\mu\mathrm{m})/L(R) > 500, which is rare
even among the local ULIRG population. While source confusion or blending might
offer an explanation in some cases, they may represent a new population of
galaxies with an extreme activity of star formation in an undeveloped stellar
system -- i.e., formation of bulges or young ellipticals.Comment: 55 pages, 16 figures. To appear in A
Theory of Low Temperature Electron Spin Resonance in Half-integer Spin Antiferromagnetic Chains
A theory of low temperature (T) electron spin resonance (ESR) in half-integer
spin antiferromagnetic chains is developed using field theory methods and
avoiding previous approximations. It is compared to experiments on Cu benzoate.
Power laws are predicted for the line-width broadening due to various types of
anisotropy. At T -> 0, zero width absorption peaks occur in some cases. The
second ESR peak in Cu benzoate, observed at T<.76K, is argued not to indicate
Neel order as previously claimed, but to correspond to a sine-Gordon "breather"
excitation.Comment: 4 pages, REVTEX, 3 PostScript figures embedded in tex
Amelioration of normothermic canine liver ischemia with prostacyclin.
A model of hepatic ischemia was developed in dogs using a pump-driven splanchnic-to-jugular vein bypass during crossclamping of the portal triad. An LD50 was established with three hours of ischemia. PGI2 given for one hour before the ischemic insult ameliorated the ischemic injury and increased survival
- …