677 research outputs found
Star Formation in the Circumnuclear Environment of NGC1068
We present near-infrared emission line images of the circumnuclear ring in
NGC1068. We have measured the Br_gamma fluxes in a number of star forming
complexes and derived extinctions for each of these by comparison with H_alpha.
We investigate the star forming histories of these regions and find that a
short burst of star formation occured co-evally throughout the ring within the
last 30-40 Myr, and perhaps as recently as 4-7 Myr ago. The 1-0 S(1) flux and
S(1)/Br_gamma ratios indicate that as well as fluorescence, shock excited H_2
emission contributes to the total flux. There is excess H_2 flux to the
North-West where the ionisation cone crosses the ring, and we have shown it is
possible that the non-stellar continuum from the Seyfert nucleus which produces
the high excitation lines could also be causing fluorescence at the edges of
molecular clouds in the ring. The nuclear 1-0 S(1) is more extended than
previously realised but only along the bar's major axis, and we consider
mechanisms for its excitation.Comment: 10 pages, 4 figures, LaTeX (mn.sty & psfig.sty). Accepted for
publication in MNRA
Controlled vaporization of the superconducting condensate in cuprate superconductors sheds light on the pairing boson
We use ultrashort intense laser pulses to study superconducting state
vaporization dynamics in La(2-x)Sr(x)CuO4 (x=0.1 and 0.15) on the femtosecond
timescale. We find that the energy density required to vaporize the
superconducting state is 2+- 0.8 K/Cu and 2.6 +- 1 K/Cu for x=0.1 and 0.15
respectively. This is significantly greater than the condensation energy
density, indicating that the quasiparticles share a large amount of energy with
the boson glue bath on this timescale. Considering in detail both spin and
lattice energy relaxation pathways which take place on the relevant timescale
of picoseconds, we rule out purely spin-mediated pair-breaking in favor of
phonon-mediated mechanisms, effectively ruling out spin-mediated pairing in
cuprates as a consequence.Comment: 5 pages of article plus 4 pages of supplementary materia
High-Tc superconductivity in entirely end-bonded multi-walled carbon nanotubes
We report that entirely end-bonded multi-walled carbon nanotubes (MWNTs) can
show superconductivity with the transition temperature Tc as high as 12K that
is approximately 40-times larger than those reported in ropes of single-walled
nanotubes. We find that emergence of this superconductivity is very sensitive
to junction structures of Au electrode/MWNTs. This reveals that only MWNTs with
optimal numbers of electrically activated shells, which are realized by the
end-bonding, can allow the superconductivity due to inter shell effects.Comment: 5 page
1-1.4 Micron Spectral Atlas of Stars
We present a catalog of J-band (1.08 um to 1.35 um) stellar spectra at low
resolution (R ~ 400). The targets consist of 105 stars ranging in spectral type
from O9.5 to M7 and luminosity classes I through V. The relatively featureless
spectra of hot stars, earlier than A4, can be used to remove the atmospheric
features which dominate ground-based J-band spectroscopy. We measure equivalent
widths for three absorption lines and nine blended features which we identify
in the spectra. Using detailed comparison with higher resolution spectra, we
demonstrate that low resolution data can be used for stellar classification,
since several features depend on the effective temperature and gravity. For
example The CN index (1.096 - 1.104 um) decreases with temperature, but the
strength of a blended feature at 1.28 um (consisting of primarily P beta)
increases. The slope of a star's spectrum can also be used to estimate its
effective temperature. The luminosity class of a star correlates with the ratio
of the Mg I (1.1831 um) line to a blend of several species at 1.16 um. Using
these indicators, a star can be classified to within several subclasses.
Fifteen stars with particularly high and low metal abundances are included in
the catalog and some spectral dependence on metal abundance is also found.Comment: 35 pages, 10 figures (3a-e are in gif format. For complete high
resolution figures, go to http://www.astro.ucla.edu/~malkan/newjspec/) ;
Accepted for published in ApJS; For associated spectra files, see
http://www.astro.ucla.edu/~malkan/newjspec
Lattice dynamics and the electron-phonon interaction in CaRuO
We present a Raman scattering study of CaRuO, in which we investigate
the temperature-dependence of the lattice dynamics and the electron-phonon
interaction below the metal-insulator transition temperature ({\it T}). Raman spectra obtained in a backscattering geometry with light polarized
in the ab-plane reveal 9 B phonon modes (140, 215, 265, 269, 292, 388,
459, 534, and 683 cm) and 9 A phonon modes (126, 192, 204, 251, 304,
322, 356, 395, and 607 cm) for the orthorhombic crystal structure
(PbcaD). With increasing temperature toward {\it T},
the observed phonon modes shift to lower energies and exhibit reduced spectral
weights, reflecting structural changes associated with the elongation of the
RuO octahedra. Interestingly, the phonons exhibit significant increases in
linewidths and asymmetries for {\it T} {\it T}. These results
indicate that there is an increase in the effective number of electrons and the
electron-phonon interaction strengths as the temperature is raised through {\it
T}, suggesting the presence of orbital fluctuations in the
temperature regime {\it T} {\it T} {\it T}.Comment: 6 pages, 4 figure
Vibrational signature of broken chemical order in a GeS2 glass: a molecular dynamics simulation
Using density functional molecular dynamics simulations, we analyze the
broken chemical order in a GeS glass and its impact on the dynamical
properties of the glass through the in-depth study of the vibrational
eigenvectors. We find homopolar bonds and the frequencies of the corresponding
modes are in agreement with experimental data. Localized S-S modes and 3-fold
coordinated sulfur atoms are found to be at the origin of specific Raman peaks
whose origin was not previously clear. Through the ring size statistics we
find, during the glass formation, a conversion of 3-membered rings into larger
units but also into 2-membered rings whose vibrational signature is in
agreement with experiments.Comment: 11 pages, 8 figures; to appear in Phys. Rev.
Resonant Two-Magnon Raman Scattering in Cuprate Antiferromagnetic Insulators
We present results of low-temperature two-magnon resonance Raman excitation
profile measurements for single layer Sr_2CuO_2Cl_2 and bilayer YBa_2Cu_3O_{6 +
\delta} antiferromagnets over the excitation region from 1.65 to 3.05 eV. These
data reveal composite structure of the two-magnon line shape and strong
nonmonotic dependence of the scattering intensity on excitation energy. We
analyze these data using the triple resonance theory of Chubukov and Frenkel
(Phys. Rev. Lett., 74, 3057 (1995)) and deduce information about magnetic
interaction and band parameters in these materials.Comment: REVTeX, 4 pages + 2 PostScript (compressed) figure
Resonant two-magnon Raman scattering in antiferromagnetic insulators
We propose a theory of two-magnon {\it resonant\/} Raman scattering from
antiferromagnetic insulators, which contains information both on the magnetism
and the carrier properties in the lighly doped phases. We argue that the
conventional theory does not work in the resonant regime, in which the energy
of the incident photon is close to the gap between the conduction and valence
bands. We identify the diagram which gives the dominant contribution to Raman
intensity in this regime and show that it can explain the unusual features in
the two-magnon profile and in the two-magnon peak intensity dependence on the
incoming photon frequency.Comment: 11 pages (REVTeX) + 3 figures in a single postscript file are
appended in uuencoded format, preprint TCSUH-94:09
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