53,989 research outputs found
Implications of the isotope effects on the magnetization, magnetic torque and susceptibility
We analyze the magnetization, magnetic torque and susceptibility data of
La2-xSrxCu(16,18)O4 and YBa2(63,65)CuO7-x near Tc in terms of the universal
3D-XY scaling relations. It is shown that the isotope effect on Tc mirrors that
on the anisotropy. Invoking the generic behavior of the anisotropy the doping
dependence of the isotope effects on the critical properties, including Tc,
correlation lengths and magnetic penetration depths are traced back to a change
of the mobile carrier concentration.Comment: 5 pages, 3 figure
Angular Radii of Stars via Microlensing
We outline a method by which the angular radii of giant and main sequence
stars in the Galactic bulge can be measured to a few percent accuracy. The
method combines ground-based photometry of caustic-crossing bulge microlensing
events, with a handful of precise astrometric measurements of the lensed star
during the event, to measure the angular radius of the source, theta_*. Dense
photometric coverage of one caustic crossing yields the crossing timescale dt.
Less frequent coverage of the entire event yields the Einstein timescale t_E
and the angle phi of source trajectory with respect to the caustic. The
photometric light curve solution predicts the motion of the source centroid up
to an orientation on the sky and overall scale. A few precise astrometric
measurements therefore yield theta_E, the angular Einstein ring radius. Then
the angular radius of the source is obtained by theta_*=theta_E(dt/t_E)
sin(phi). We argue that theta_* should be measurable to a few percent accuracy
for Galactic bulge giant stars using ground-based photometry from a network of
small (1m-class) telescopes, combined with astrometric observations with a
precision of ~10 microarcsec to measure theta_E. We find that a factor of ~50
times fewer photons are required to measure theta_E to a given precision for
binary-lens events than single-lens events. Adopting parameters appropriate to
the Space Interferometry Mission (SIM), ~7 min of SIM time is required to
measure theta_E to ~5% accuracy for giant sources in the bulge. For
main-sequence sources, theta_E can be measured to ~15% accuracy in ~1.4 hours.
With 10 hrs of SIM time, it should be possible to measure theta_* to ~5% for
\~80 giant stars, or to 15% for ~7 main sequence stars. A byproduct of such a
campaign is a significant sample of precise binary-lens mass measurements.Comment: 13 pages, 3 figures. Revised version, minor changes, required SIM
integration times revised upward by ~60%. Accepted to ApJ, to appear in the
March 20, 2003 issue (v586
Exact gravitational lensing and rotation curve
Based on the geodesic equation in a static spherically symmetric metric we
discuss the rotation curve and gravitational lensing. The rotation curve
determines one function in the metric without assuming Einstein's equations.
Then lensing is considered in the weak field approximation of general
relativity. From the null geodesics we derive the lensing equation and
corrections to it.Comment: 12 pages, 1 figur
Local pressure-induced metallization of a semiconducting carbon nanotube in a crossed junction
The electronic and vibrational density of states of a semiconducting carbon
nanotube in a crossed junction was investigated by elastic and inelastic
scanning tunneling spectroscopy. The strong radial compression of the nanotube
at the junction induces local metallization spatially confined to a few nm. The
local electronic modifications are correlated with the observed changes in the
radial breathing and G-band phonon modes, which react very sensitively to local
mechanical deformation. In addition, the experiments reveal the crucial
contribution of the image charges to the contact potential at nanotube-metal
interfaces
Geometry of Universal Magnification Invariants
Recent work in gravitational lensing and catastrophe theory has shown that
the sum of the signed magnifications of images near folds, cusps and also
higher catastrophes is zero. Here, it is discussed how Lefschetz fixed point
theory can be used to interpret this result geometrically. It is shown for the
generic case as well as for elliptic and hyperbolic umbilics in gravitational
lensing.Comment: RevTEX4, 13 pages, submitted to J. Math. Phy
Statistics of Weak Gravitational Lensing in Cold Dark Matter Models; Magnification Bias on Quasar Luminosity Functions
We compute statistical properties of weak gravitational lensing by
large-scale structure in three Cold Dark Matter models. We use a PM
-body code to simulate the formation and evolution of large-scale structure
in the universe. We perform ray-tracing experiments for each
model using the multiple lens-plane algorithm. From the results of these
experiments, we calculate the probability distribution functions (PDF) of the
convergences, shears, and magnifications, and their root-mean-square (rms)
values. We find that the rms values of the convergence and shear agree with the
predictions of a nonlinear analytical model. We also find that the PDFs of the
magnifications have a peak at values slightly smaller than , and
are strongly skewed toward large magnifications. In particular, for the
high-density model, a power-law tail appears in the magnification distribution
at large magnifications for sources at redshifts . The rms values of the
magnifications essentially agree with the nonlinear analytical predictions for
sources at low redshift, but exceed these predictions for high redshift
sources, once the power-law tail appears.
We study the effect of magnification bias on the luminosity functions of
high-redshift quasars, using the calculated PDFs of the magnifications. We show
that the magnification bias is moderate in the absence of the power-law tail in
the magnification distribution, but depends strongly on the value of the
density parameter. In presence of the power-law tail, the bias becomes
considerable, especially at the bright end of the luminosity functions.Comment: 24 pages, 9 figures, LaTex using epsfig.sty. Submitted to the The
Astrophysical Journa
The Born and Lens-Lens Corrections to Weak Gravitational Lensing Angular Power Spectra
We revisit the estimation of higher order corrections to the angular power
spectra of weak gravitational lensing. Extending a previous calculation of
Cooray and Hu, we find two additional terms to the fourth order in potential
perturbations of large-scale structure corresponding to corrections associated
with the Born approximation and the neglect of line-of-sight coupling of two
foreground lenses in the standard first order result. These terms alter the
convergence (), the lensing shear E-mode (),
and their cross-correlation () power spectra on large angular
scales, but leave the power spectra of the lensing shear B-mode ()
and rotational () component unchanged as compared to previous
estimates. The new terms complete the calculation of corrections to weak
lensing angular power spectra associated with both the Born approximation and
the lens-lens coupling to an order in which the contributions are most
significant. Taking these features together, we find that these corrections are
unimportant for any weak lensing survey, including for a full sky survey
limited by cosmic variance.Comment: Added references, minor changes to text. 9 pages, 2 figure
- …