22,543 research outputs found
Multiepoch Radial Velocity Observations of L Dwarfs
We report on the development of a technique for precise radial-velocity
measurements of cool stars and brown dwarfs in the near infrared. Our technique
is analogous to the Iodine (I2) absorption cell method that has proven so
successful in the optical regime. We rely on telluric CH4 absorption features
to serve as a wavelength reference, relative to which we measure Doppler shifts
of the CO and H2O features in the spectra of our targets. We apply this
technique to high-resolution (R~50,000) spectra near 2.3 micron of nine L
dwarfs taken with the Phoenix instrument on Gemini-South and demonstrate a
typical precision of 300 m/s. We conduct simulations to estimate our expected
precision and show our performance is currently limited by the signal-to-noise
of our data. We present estimates of the rotational velocities and systemic
velocities of our targets. With our current data, we are sensitive to
companions with M sin i > 2MJ in orbits with periods less than three days. We
identify no companions in our current data set. Future observations with
improved signal-to-noise should result in radial-velocity precision of 100 m/s
for L dwarfs.Comment: Accepted for publication in ApJ, 24 pages, 7 figure
Searching For Integrated Sachs-Wolfe Effect Beyond Temperature Anisotropies: CMB E-mode Polarization-Galaxy Cross Correlation
The cross-correlation between cosmic microwave background (CMB) temperature
anisotropies and the large scale structure (LSS) traced by the galaxy
distribution, or sources at different wavelengths, is now well known. This
correlation results from the integrated Sachs-Wolfe (ISW) effect in CMB
anisotropies generated at late times due to the dark energy component of the
Universe. In a reionized universe, the ISW quadrupole rescatters and
contributes to the large-scale polarization signal. Thus, in principle, the
large-scale polarization bump in the E-mode should also be correlated with the
galaxy distribution. Unlike CMB temperature-LSS correlation that peaks for
tracers at low redshifts this correlation peaks mostly at redshifts between 1
and 3. Under certain conditions, mostly involving a low optical depth to
reionization, if the Universe reionized at a redshift around 6, the cross
polarization-source signal is marginally detectable, though challenging as it
requires all-sky maps of the large scale structure at redshifts between 1 and
3. If the Universe reionized at a redshift higher than 10, it is unlikely that
this correlation will be detectable even with no instrumental noise all-sky
maps. While our estimates do not guarantee a detection unknown physics related
to the dark energy as well as still uncertain issues related to the large
angular scale CMB and polarization anisotropies may motivate attempts to
measure this correlation using upcoming CMB polarization E-mode maps.Comment: 13 pages; 3 figure panels, JCAP submitte
Theoretical and numerical studies of wave-packet propagation in tokamak plasmas
Theoretical and numerical studies of wave-packet propagation are presented to
analyze the time varying 2D mode structures of electrostatic fluctuations in
tokamak plasmas, using general flux coordinates. Instead of solving the 2D wave
equations directly, the solution of the initial value problem is used to obtain
the 2D mode structure, following the propagation of wave-packets generated by a
source and reconstructing the time varying field. As application, the 2D WKB
method is applied to investigate the shaping effects (elongation and
triangularity) of tokamak geometry on the lower hybrid wave propagation and
absorbtion. Meanwhile, the Mode Structure Decomposition (MSD) method is used to
handle the boundary conditions and simplify the 2D problem to two nested 1D
problems. The MSD method is related to that discussed earlier by Zonca and Chen
[Phys. Fluids B 5, 3668 (1993)], and reduces to the well-known "ballooning
formalism" [J. W. Connor, R. J. Hastie, and J. B. Taylor, Phys. Rev. Lett. 40,
396 (1978)], when spatial scale separation applies. This method is used to
investigate the time varying 2D electrostatic ITG mode structure with a mixed
WKB-full-wave technique. The time varying field pattern is reconstructed and
the time asymptotic structure of the wave-packet propagation gives the 2D
eigenmode and the corresponding eigenvalue. As a general approach to
investigate 2D mode structures in tokamak plasmas, our method also applies for
electromagnetic waves with general source/sink terms, either by an
internal/external antenna or nonlinear wave interaction with zonal structures.Comment: 24 pages, 14 figure
Edge states in Open Antiferromagnetic Heisenberg Chains
In this letter we report our results in investigating edge effects of open
antiferromagnetic Heisenberg spin chains with spin magnitudes
using the density-matrix renormalization group (DMRG) method initiated by
White. For integer spin chains, we find that edge states with spin magnitude
exist, in agreement with Valence-Bond-Solid model picture. For
half-integer spin chains, we find that no edge states exist for spin
chain, but edge state exists in spin chain with , in
agreement with previous conjecture by Ng. Strong finite size effects associated
with spin dimmerization in half-integer spin chains will also be discussed.Comment: 4 pages, RevTeX 3.0, 5 figures in a separate uuencoded postscript
file. Replaced once to enlarge the acknowlegement
Vanishing Hall Constant in the Stripe Phase of Cuprates
The Hall constant R_H is considered for the stripe structures. In order to
explain the vanishing of R_H in LNSCO at x = 1/8, we use the relation of R_H to
the Drude weight D as well as direct numerical calculation, to obtain results
within the t-J model, where the stripes are imposed via a charge potential and
a staggered magnetic field. The origin of R_H ~ 0 is related to a maximum in D
and the minimal kinetic energy in stripes with a hole filling ~ 1/2. The same
argument indicates on a possibility of R_H ~ 0 in the whole range of static
stripes for x < 1/8.Comment: RevTeX, 4 pages, 5 figure
Thermodynamic properties of the one-dimensional Kondo insulators studied by the density matrix renormalization group method
Thermodynamic properties of the one-dimensional Kondo lattice model at
half-filling are studied by the density matrix renormalization group method
applied to the quantum transfer matrix. Spin susceptibility, charge
susceptibility, and specific heat are calculated down to T=0.1t for various
exchange constants. The obtained results clearly show crossover behavior from
the high temperature regime of nearly independent localized spins and
conduction electrons to the low temperature regime where the two degrees of
freedom couple strongly. The low temperature energy scales of the charge and
spin susceptibilities are determined and shown to be equal to the quasiparticle
gap and the spin gap, respectively, for weak exchange couplings.Comment: 4 pages, 3 Postscript figures, REVTeX, submitted to J. Phys. Soc. Jp
Observational Tests of the Mass-Temperature Relation for Galaxy Clusters
We examine the relationship between the mass and x-ray gas temperature of
galaxy clusters using data drawn from the literature. Simple theoretical
arguments suggest that the mass of a cluster is related to the x-ray
temperature as . Virial theorem mass estimates based on
cluster galaxy velocity dispersions seem to be accurately described by this
scaling with a normalization consistent with that predicted by the simulations
of Evrard, Metzler, & Navarro (1996). X-ray mass estimates which employ
spatially resolved temperature profiles also follow a scaling
although with a normalization about 40% lower than that of the fit to the
virial masses. However, the isothermal -model and x-ray surface
brightness deprojection masses follow a steeper
scaling. The steepness of the isothermal estimates is due to their implicitly
assumed dark matter density profile of at large radii
while observations and simulations suggest that clusters follow steeper
profiles (e.g., ).Comment: 25 pages, 10 figures, accepted by Ap
Stripes in Doped Antiferromagnets: Single-Particle Spectral Weight
Recent photoemission (ARPES) experiments on cuprate superconductors provide
important guidelines for a theory of electronic excitations in the stripe
phase. Using a cluster perturbation theory, where short-distance effects are
accounted for by exact cluster diagonalization and long-distance effects by
perturbation (in the hopping), we calculate the single-particle Green's
function for a striped t-J model. The data obtained quantitatively reproduce
salient (ARPES-) features and may serve to rule out "bond-centered" in favor of
"site-centered" stripes.Comment: final version as appeared in PRL; (c) 2000 The American Physical
Society; 4 pages, 4 figure
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