259 research outputs found
Impurity Energy Level Within The Haldane Gap
An impurity bond in a periodic 1D antiferromagnetic, spin 1 chain with
exchange is considered. Using the numerical density matrix renormalization
group method, we find an impurity energy level in the Haldane gap,
corresponding to a bound state near the impurity bond. When the level
changes gradually from the edge of the Haldane gap to the ground state energy
as the deviation changes from 0 to 1. It seems that there is
no threshold. Yet, there is a threshold when . The impurity level
appears only when the deviation is greater than ,
which is near 0.3 in our calculation.Comment: Latex file,9 pages uuencoded compressed postscript including 4
figure
Guiding the Way to Gamma-Ray Sources: X-ray Studies of Supernova Remnants
Supernova remnants have long been suggested as a class of potential
counterparts to unidentified gamma-ray sources. The mechanisms by which such
gamma-rays can arise may include emission from a pulsar associated with a
remnant, or a variety of processes associated with energetic particles
accelerated by the SNR shock. Imaging and spectral observations in the X-ray
band can be used to identify properties of the remnants that lead to gamma-ray
emission, including the presence of pulsar-driven nebulae, nonthermal X-ray
emission from the SNR shells, and the interaction of SNRs with dense
surrounding material.Comment: 16 pages, 11 figures, To appear in the proceedings of the workshop:
"The Nature of the Unidentified Galactic Gamma-Ray Sources" held at INAOE,
Mexico, October 2000, (A.Carraminana, O. Reiner and D. Thompson, eds.
On the Application of the Non Linear Sigma Model to Spin Chains and Spin Ladders
We review the non linear sigma model approach (NLSM) to spin chains and spin
ladders, presenting new results. The generalization of the Haldane's map to
ladders in the Hamiltonian approach, give rise to different values of the
parameter depending on the spin S, the number of legs and
the choice of blocks needed to built up the NLSM fields. For rectangular blocks
we obtain or depending on wether , is even or
odd, while for diagonal blocks we obtain . Both
results agree modulo , and yield the same prediction, namely that even (
resp. odd) ladders are gapped (resp. gapless). For even legged ladders we show
that the spin gap collapses exponentially with and we propose a
finite size correction to the gap formula recently derived by Chakravarty using
the 2+1 NSLM, which gives a good fit of numerical results. We show the
existence of a Haldane phase in the two legged ladder using diagonal blocks and
finally we consider the phase diagram of dimerized ladders.Comment: 25 pages, Latex, 7 figures in postscript files, Proc. of the 1996 El
Escorial Summer School on "Strongly Correlated Magnetic and Superconducting
Systems". Some more references are adde
Use of the Generalized Gradient Approximation in Pseudopotential Calculations of Solids
We present a study of the equilibrium properties of -bonded solids within
the pseudopotential approach, employing recently proposed generalized gradient
approximation (GGA) exchange correlation functionals. We analyze the effects of
the gradient corrections on the behavior of the pseudopotentials and discuss
possible approaches for constructing pseudopotentials self-consistently in the
context of gradient corrected functionals. The calculated equilibrium
properties of solids using the GGA functionals are compared to the ones
obtained through the local density approximation (LDA) and to experimental
data. A significant improvement over the LDA results is achieved with the use
of the GGA functionals for cohesive energies. For the lattice constant, the
same accuracy as in LDA can be obtained when the nonlinear coupling between
core and valence electrons introduced by the exchange correlation functionals
is properly taken into account. However, GGA functionals give bulk moduli that
are too small compared to experiment.Comment: 15 pages, latex, no figure
Thin-film cryogenic accelerator targets
Thin-film accelerator targets (0.1 mg/cm2 x 2) produced by condensation of various gases (Ar, Kr, Xe, N2, etc.) onto thin, cryogenically cooled substrates (T = 20 K to 80 K) have been developed and tested in-beam with 35 MeV 4He.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23775/1/0000013.pd
Constraining warm dark matter with cosmic shear power spectra
We investigate potential constraints from cosmic shear on the dark matter
particle mass, assuming all dark matter is made up of light thermal relic
particles. Given the theoretical uncertainties involved in making cosmological
predictions in such warm dark matter scenarios we use analytical fits to linear
warm dark matter power spectra and compare (i) the halo model using a mass
function evaluated from these linear power spectra and (ii) an analytical fit
to the non-linear evolution of the linear power spectra. We optimistically
ignore the competing effect of baryons for this work. We find approach (ii) to
be conservative compared to approach (i). We evaluate cosmological constraints
using these methods, marginalising over four other cosmological parameters.
Using the more conservative method we find that a Euclid-like weak lensing
survey together with constraints from the Planck cosmic microwave background
mission primary anisotropies could achieve a lower limit on the particle mass
of 2.5 keV.Comment: 26 pages, 9 figures, minor changes to match the version accepted for
publication in JCA
The New Urban Revival in the United States
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68657/2/10.1080_00420989320081901.pd
1D Frustrated Ferromagnetic Model with Added Dzyaloshinskii-Moriya Interaction
The one-dimensional (1D) isotropic frustrated ferromagnetic spin-1/2 model is
considered. Classical and quantum effects of adding a Dzyaloshinskii-Moriya
(DM) interaction on the ground state of the system is studied using the
analytical cluster method and numerical Lanczos technique. Cluster method
results, show that the classical ground state magnetic phase diagram consists
of only one single phase: "chiral". The quantum corrections are determined by
means of the Lanczos method and a rich quantum phase diagram including the
gapless Luttinger liquid, the gapped chiral and dimer orders is obtained.
Moreover, next nearest neighbors will be entangled by increasing DM interaction
and for open chains, end-spins are entangled which shows the long distance
entanglement (LDE) feature that can be controlled by DM interaction.Comment: 8 pages, 9 figure
HI in the Outskirts of Nearby Galaxies
The HI in disk galaxies frequently extends beyond the optical image, and can
trace the dark matter there. I briefly highlight the history of high spatial
resolution HI imaging, the contribution it made to the dark matter problem, and
the current tension between several dynamical methods to break the disk-halo
degeneracy. I then turn to the flaring problem, which could in principle probe
the shape of the dark halo. Instead, however, a lot of attention is now devoted
to understanding the role of gas accretion via galactic fountains. The current
cold dark matter theory has problems on galactic scales, such as
the core-cusp problem, which can be addressed with HI observations of dwarf
galaxies. For a similar range in rotation velocities, galaxies of type Sd have
thin disks, while those of type Im are much thicker. After a few comments on
modified Newtonian dynamics and on irregular galaxies, I close with statistics
on the HI extent of galaxies.Comment: 38 pages, 17 figures, invited review, book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
Immigration and Internal Migration 'Flight' from US Metropolitan Areas: Toward a New Demographic Balkanisation
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68704/2/10.1080_00420989550012861.pd
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