884 research outputs found
A possible mechanism for superconductivity in doped SrTiO3
The soft ferro-electric phonon in SrTiO3 observed with optical spectroscopy
has an extraordinary strong spectral weight which is much stronger than
expected in the limit of a perfectly ionic compound. The "charged phonon" in
SrTiO3 is caused by the close-to-covalent character of the Ti-O ionic bond and
implies a strong coupling between the soft ferro-electric phonon and the inter
band transitions across the 3 eV gap of SrTiO3. We demonstrate that this
coupling leads, in addition to the charged phonon effect, to a pairing
interaction involving the exchange of two transverse optical phonons. This
process owes its relevance to the strong electron-phonon coupling and to the
fact that the interaction mediated by a single transverse optical phonon
vanishes at low electron density. We use the experimental soft phonon spectral
weight to calculate the strength of the bi-phonon mediated pairing interaction
in the electron doped material and show that it is of the correct magnitude
when compared to the experimental value of the superconducting critical
temperature.Comment: Missing factors corrected in Eqs. 6 and
The fate of quasiparticles in the superconducting state
Quasiparticle properties in the superconducting state are masked by the
superfluid and are not directly accessible to infrared spectroscopy. We show
how one can use a Kramers--Kronig transformation to separate the quasiparticle
from superfluid response and extract intrinsic quasiparticle properties in the
superconducting state. We also address the issue of a narrow quasiparticle peak
observed in microwave measurements, and demonstrate how it can be combined with
infrared measurements to obtain unified picture of electrodynamic properties of
cuprate superconductors
Optical Response of SrRuO Reveals Universal Fermi-liquid Scaling and Quasiparticles Beyond Landau Theory
We report optical measurements demonstrating that the low-energy relaxation
rate () of the conduction electrons in SrRuO obeys scaling
relations for its frequency () and temperature () dependence in
accordance with Fermi-liquid theory. In the thermal relaxation regime,
1/\tau\propto (\hbar\omega)^2 + (p\pi\kB T)^2 with , and
scaling applies. Many-body electronic structure calculations using dynamical
mean-field theory confirm the low-energy Fermi-liquid scaling, and provide
quantitative understanding of the deviations from Fermi-liquid behavior at
higher energy and temperature. The excess optical spectral weight in this
regime provides evidence for strongly dispersing "resilient" quasiparticle
excitations above the Fermi energy
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
An Intermediate-Mass Black Hole in the Globular Cluster G1: Improved Significance from New Keck and Hubble Space Telescope Observations
We present dynamical models for the massive globular cluster G1. The goal is
to measure or place a significant upper limit on the mass of any central black
hole. Whether or not globular clusters contain central massive black holes has
important consequences for a variety of studies. We use new kinematic data
obtained with Keck and new photometry from the Hubble Space Telescope. The Keck
spectra allow us to obtain kinematics out to large radii that are required to
pin down the mass-to-light ratio of the dynamical model and the orbital
structure. The Hubble Space Telescope observations give us a factor of two
better spatial resolution for the surface brightness profile. By fitting
non-parametric, spherical, isotropic models we find a best-fit black hole mass
of 1.7(+-0.3)e4 Msun. Fully general axisymmetric orbit-based models give
similar results, with a black hole mass of 1.8(+-0.5)e4 Msun. The no-black hole
model has Delta_chi^2=5 (marginalized over mass-to-light ratio), implying less
than 3% significance. We have taken into account any change in the
mass-to-light ratio in the center due to stellar remnants. These results are
consistent with our previous estimate in Gebhardt, Rich & Ho (2002), and
inconsistent with the analysis of Baumgardt et al. (2003) who claim that G1
does not show evidence for a black hole. These new results make G1 the best
example of a cluster that contains an intermediate-mass black hole.Comment: accepted for publication in the Astrophysical Journa
CO-bandhead spectroscopy of IC 342: mass and age of the nuclear star cluster
We have used the NASA Infra-Red Telescope Facility (IRTF) to observe the
nuclear stellar cluster in the nearby, face-on, giant Scd spiral IC 342. From
high resolution (R = 21500) spectra at the 12CO (2-0) bandhead at 2.3 micron we
derive a line-of-sight stellar velocity dispersion sigma = (33 +- 3) km/s.
To interpret this observation we construct dynamical models based on the
Jeans equation for a spherical system. The light distribution of the cluster is
modeled using an isophotal analysis of an HST V-band image from the HST Data
Archive, combined with new ground-based K-band imaging. Under the assumption of
an isotropic velocity distribution, the observed kinematics imply a K-band
mass-to-light ratio M/L_K = 0.05, and a cluster mass M ~ 6 times 10^6 Msun. We
model the mass-to-light ratio with the `starburst99' stellar population
synthesis models of Leitherer and collaborators, and infer a best-fitting
cluster age in the range 63-630 Myears. Although this result depends somewhat
on a number of uncertainties in the modeling (e.g., the assumed extinction
along the line-of-sight towards the nucleus, the IMF of the stellar population
model, and the velocity dispersion anisotropy of the cluster), none of these
can be plausibly modified to yield a significantly larger age. We discuss the
implications of this result on possible scenarios for the frequency of nuclear
starbursts and their impact on secular evolution of spiral galaxy nuclei.
As a byproduct of our analysis, we infer that IC 342 cannot have any central
black hole more massive than 0.5 million solar masses. This is ~ 6 times less
massive than the black hole inferred to exist in our Galaxy, consistent with
the accumulating evidence that galaxies with less massive bulges harbor less
massive black holes.Comment: 27 pages, incl. 9 figures, submitted to The Astronomical Journa
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
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