1,178 research outputs found
Infrared spectroscopic study of phonons coupled to charge excitations in FeSi
From an investigation of the optical conductivity of FeSi single crystals
using FTIR spectroscopy in the frequency range from 30 to 20000 wavenumbers we
conclude that the transverse effective charge of the Fe and Si ions is
approximately 4e. Of the five optical phonons which are allowed by symmetry we
observe only four, three of which have a Fano line shape presumably resulting
from an interaction of these modes with the electronic continuum. We show that
the large oscillator strength of the phonons results from a relatively weak
coupling (lambda of the order of 0.1) of the lattice degrees of freedom to an
electronic resonance above the semiconductor gap, which is also responsible for
the large electronic polarizability of the medium.Comment: Revtex, 10 pages, 2 postscript pictures, to be published in Phys.
Rev. B Rapid Comm
Hybridization gap and anisotropic far-infrared optical conductivity of URu2Si2
We performed far-infrared optical spectroscopy measurements on the heavy
fermion compound URu 2 Si 2 as a function of temperature. The light's
electric-field was applied along the a-axis or the c-axis of the tetragonal
structure. We show that in addition to a pronounced anisotropy, the optical
conductivity exhibits for both axis a partial suppression of spectral weight
around 12 meV and below 30 K. We attribute these observations to a change in
the bandstructure below 30 K. However, since these changes have no noticeable
impact on the entropy nor on the DC transport properties, we suggest that this
is a crossover phenomenon rather than a thermodynamic phase transition.Comment: To be published in Physical Review
The counterrotating core and the black hole mass of IC1459
The E3 giant elliptical galaxy IC1459 is the prototypical galaxy with a fast
counterrotating stellar core. We obtained one HST/STIS long-slit spectrum along
the major axis of this galaxy and CTIO spectra along five position angles. We
present self-consistent three-integral axisymmetric models of the stellar
kinematics, obtained with Schwarzschild's numerical orbit superposition method.
We study the dynamics of the kinematically decoupled core (KDC) in IC1459 and
we find it consists of stars that are well-separated from the rest of the
galaxy in phase space. The stars in the KDC counterrotate in a disk on orbits
that are close to circular. We estimate that the KDC mass is ~0.5% of the total
galaxy mass or ~3*10^9 Msun. We estimate the central black hole mass M_BH of
IC1459 independently from both its stellar and its gaseous kinematics. Some
complications probably explain why we find rather discrepant BH masses with the
different methods. The stellar kinematics suggest that M_BH = (2.6 +/-
1.1)*10^9 Msun (3 sigma error). The gas kinematics suggests that M_BH ~
3.5*10^8 Msun if the gas is assumed to rotate at the circular velocity in a
thin disk. If the observed velocity dispersion of the gas is assumed to be
gravitational, then M_BH could be as high as ~1.0*10^9 Msun. These different
estimates bracket the value M_BH = (1.1 +/- 0.3)*10^9 Msun predicted by the
M_BH-sigma relation. It will be an important goal for future studies to assess
the reliability of black hole mass determinations with either technique. This
is essential if one wants to interpret the correlation between the BH mass and
other global galaxy parameters (e.g. velocity dispersion) and in particular the
scatter in these correlations (believed to be only ~0.3 dex). [Abridged]Comment: 51 pages, LaTeX with 19 PostScript figures. Revised version, with
three new figures and data tables. To appear in The Astrophysical Journal,
578, 2002 October 2
Transverse optical plasmons in layered superconductors
We discuss the possible existance of transverse optical plasma modes in
superlattices consisting of Josephson coupled superconducting layers. These
modes appear as resonances in the current-current correlation function, as
opposed to the usual plasmons which are poles in the density-density channel.
We consider both bilayer superlattices, and single layer lattices with a spread
of interlayer Josephson couplings. We show that our model is in quantitative
agreement with the recent experimental observation by a number of groups of a
peak at the Josephson plasma frequency in the optical conductivity of
LaSrCuOComment: Proceedings of LT21, in press, 4 pages, Latex with LTpaper.sty and
epsfig.sty, 2 postscript figure
C-axis Penetration Depth and Inter-layer Conductivity in the Thallium Based Cuprate Superconductors
The c-axis Josephson plasmon in optimally doped single-layer and bi-layer
high Tc cuprates Tl2201 and Tl2212 have been investigated using infrared
spectroscopy. We observed the plasma frequencies for these two compounds at
27.8 and 25.6 cm-1 respectively, which we interpret as a Josephson resonance
across the TlO blocking layers. No maximum in the temperature dependence of the
c-axis conductivity was observed below Tc, indicating that even in the
superconducting state a coherent quasi-particle contribution to the c-axis
conductivity is absent or very weak, in contrast to the behaviour of the
ab-plane conductivity.Comment: 4 pages, 3 figure
Doping Dependence of the Redistribution of Optical Spectral Weight in BiSrCaCuO
We present the ab-plane optical conductivity of four single crystals of
BiSrCaCuO (Bi2212) with different carrier doping
levels from the strongly underdoped to the strongly overdoped range with
=66, 88, 77, and 67 K respectively. We focus on the redistribution of the
low frequency optical spectral weight (SW) in the superconducting and normal
states. The temperature dependence of the low-frequency spectral weight in the
normal state is significantly stronger in the overdoped regime. In agreement
with other studies, the superconducting order is marked by an increase of the
low frequency SW for low doping, while the SW decreases for the highly
overdoped sample. The effect crosses through zero at a doping concentration
=0.19 which is slightly to the right of the maximum of the
superconducting dome. This sign change is not reproduced by the BCS model
calculations, assuming the electron-momentum dispersion known from published
ARPES data. Recent Cluster Dynamical Mean Field Theory (CDMFT) calculations
based on the Hubbard and t-J models, agree in several relevant respects with
the experimental data
Observational Constraints on Disk Heating as a Function of Hubble Type
Current understanding of the secular evolution of galactic disks suggests
that this process is dominated by two or more heating mechanisms, which
increase the random motions of stars in the disk. In particular, the
gravitational influence of giant molecular clouds and irregularities in the
spiral potential have been proposed to explain the observed velocity
dispersions in the solar neighborhood. Each of these mechanisms acts on
different components of the stellar velocities, which affects the ratio of the
vertical and radial components of the stellar velocity dispersion since the
relative strengths of giant molecular clouds and spiral irregularities vary
with Hubble type. A study of this ratio as function of Hubble type has the
potential to provide strong constraints on disk heating mechanisms. We present
major and minor axis stellar kinematics for four spiral galaxies of Hubble type
from Sa to Sbc, and use the data to infer the ratios sigma_z/sigma_R in the
galaxy disks. The results combined with two galaxies studied previously and
with Milky Way data show that the ratio is generally in the range 0.5 - 0.8.
There is a marginally significant trend of decreasing ratio with advancing
Hubble type, consistent with the predictions of disk heating theories. However,
the errors on individual measurements are large, and the absence of any trend
is consistent with the data at the 1-sigma level. As a byproduct of our study,
we find that three of the four galaxies in our sample have a central drop in
their stellar line-of-sight velocity dispersion, a phenomenon that is
increasingly observed in spiral galaxies. [ABRIDGED]Comment: 24 pages, LaTeX, 5 Postscript figures, to appear in AJ (Dec 2003
Suppressed reflectivity due to spin-controlled localization in a magnetic semiconductor
The narrow gap semiconductor FeSi owes its strong paramagnetism to
electron-correlation effects. Partial Co substitution for Fe produces a
spin-polarized doped semiconductor. The spin-polarization causes suppression of
the metallic reflectivity and increased scattering of charge carriers, in
contrast to what happens in other magnetic semiconductors, where magnetic order
reduces the scattering. The loss of metallicity continues progressively even
into the fully polarized state, and entails as much as a 25% reduction in
average mean-free path. We attribute the observed effect to a deepening of the
potential wells presented by the randomly distributed Co atoms to the majority
spin carriers. This mechanism inverts the sequence of steps for dealing with
disorder and interactions from that in the classic Al'tshuler Aronov approach -
where disorder amplifies the Coulomb interaction between carriers - in that
here, the Coulomb interaction leads to spin polarization which in turn
amplifies the disorder-induced scattering.Comment: 6 figures Submitted to PR
Evolution of Hot Gas and Dark Halos in Group-Dominant Elliptical Galaxies: Influence of Cosmic Inflow
We study the complete dynamical evolution of hot interstellar gas in massive
elliptical galaxies born into a simple flat universe beginning with an
overdense perturbation. Within the turn-around radius dark matter flows in a
self-similar fashion into a stationary Navarro-Frenk-White halo and the
baryonic gas shocks. After a few gigayears, when enough gas accumulates within
the accretion shock, the de Vaucouleurs stellar system is constructed and the
energy from Type II supernovae is released. The stars and dark halo are matched
to NGC 4472. Gas continues to enter the galaxy by secondary infall and by
stellar mass loss based on a Salpeter IMF. After about 13 Gyrs the temperature
and density distribution in the hot gas agree quite well with the hot
interstellar gas observed in NGC 4472. As a result of supernova-driven outflow,
the present day baryonic fraction has a deep minimum in the outer galactic
halo. When relatively gas-rich, X-ray luminous models are spatially truncated
at early times, simulating tidal events that may have occurred during galaxy
group dynamics, the current locus of truncated models lies just along the
, X-ray size correlation among well-observed ellipticals, providing
another striking confirmation of our simple model of elliptical evolution.Comment: 16 pages in AASTEX LaTeX with 14 figures; accepted by Astrophysical
Journa
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