33,633 research outputs found
Magnetic Excitations of Stripes Near a Quantum Critical Point
We calculate the dynamical spin structure factor of spin waves for weakly
coupled stripes. At low energy, the spin wave cone intensity is strongly peaked
on the inner branches. As energy is increased, there is a saddlepoint followed
by a square-shaped continuum rotated 45 degree from the low energy peaks. This
is reminiscent of recent high energy neutron scattering data on the cuprates.
The similarity at high energy between this semiclassical treatment and quantum
fluctuations in spin ladders may be attributed to the proximity of a quantum
critical point with a small critical exponent .Comment: 4+ pages, 5 figures, published versio
Magnetic Excitations of Stripes and Checkerboards in the Cuprates
We discuss the magnetic excitations of well-ordered stripe and checkerboard
phases, including the high energy magnetic excitations of recent interest and
possible connections to the "resonance peak" in cuprate superconductors. Using
a suitably parametrized Heisenberg model and spin wave theory, we study a
variety of magnetically ordered configurations, including vertical and diagonal
site- and bond-centered stripes and simple checkerboards. We calculate the
expected neutron scattering intensities as a function of energy and momentum.
At zero frequency, the satellite peaks of even square-wave stripes are
suppressed by as much as a factor of 34 below the intensity of the main
incommensurate peaks. We further find that at low energy, spin wave cones may
not always be resolvable experimentally. Rather, the intensity as a function of
position around the cone depends strongly on the coupling across the stripe
domain walls. At intermediate energy, we find a saddlepoint at for
a range of couplings, and discuss its possible connection to the "resonance
peak" observed in neutron scattering experiments on cuprate superconductors. At
high energy, various structures are possible as a function of coupling strength
and configuration, including a high energy square-shaped continuum originally
attributed to the quantum excitations of spin ladders. On the other hand, we
find that simple checkerboard patterns are inconsistent with experimental
results from neutron scattering.Comment: 11 pages, 13 figures, for high-res figs, see
http://physics.bu.edu/~yaodx/spinwave2/spinw2.htm
Final Report: Wall Effects in Cavity Flows
The wall effects in cavity flows past an arbitrary two-dimensional body is investigated for both pure-drag and lifting cases based on an inviscid nonlinear flow theory. The over-all features of various theoretical flow models for inviscid cavity flows under the wall effects are discussed from the general momentum consideration in comparison with typical viscous, incompressible wake flows in a channel. In the case of pure drag cavity flows, three theoretical models in common use, namely, the open-wake, Riabouchinsky and re-entrant jet models, are applied to evaluate the solution. Methods of numerical computation are discussed for bodies of arbitrary shape, and are carried out in detail for wedges of all angles. The final numerical results are compared between the different flow models, and the differences pointed out. Further analysis of the results has led to development of several useful formulas for correcting the wall effect. In the lifting flow case, the wall effect on the pressure and hydrodynamic forces acting on arbitrary body is formulated for the choked cavity flow in a closed water tunnel of arbitrary shape, and computed for the flat plate with a finite cavity in a straight tunnel
Reply to Comment on "Quantum phase transition in the four-spin exchange antiferromagnet"
We argue that our analysis of the J-Q model, presented in Phys. Rev. B 80,
174403 (2009), and based on a field-theory description of coupled dimers,
captures properly the strong quantum fluctuations tendencies, and the
objections outlined by L. Isaev, G. Ortiz, and J. Dukelsky, arXiv:1003.5205,
are misplaced
A Practical Guide for X-Ray Diffraction Characterization of Ga(Al, In)N Alloys
Ga(In, Al)N alloys are used as an active layer or cladding layer in light
emitting diodes and laser diodes. x-ray diffraction is extensively used to
evaluate the crystalline quality, the chemical composition and the residual
strain in Ga(Al,In)N thin films, which directly determine the emission
wavelength and the device performance. Due to the minor mismatch in lattice
parameters between Ga(Al, In)N alloy and a GaN virtual substrate, x-ray
diffraction comes to a problem to separate the signal from Ga(Al,In)N alloy and
GaN. We give a detailed comparison on different diffraction planes. In order to
balance the intensity and peak separation between Ga(Al,In)N alloy and GaN,
(0004) and (1015) planes make the best choice for symmetric scan and asymmetric
scan, respectively.Comment: 9 pages, 5 figure
Baryon enhancement in high-density QCD and relativistic heavy ion collisions
We argue that the collinear factorization of the fragmentation functions in
high energy nuclear collisions breaks down at transverse momenta due to high parton densities in the colliding hadrons and/or nuclei. We
find that gluon recombination dominates in that region. We calculate the
inclusive cross-section for meson and nucleon production using the low
energy theorems for the scale anomaly in QCD, and compare our quantitative
baryon-to-meson ratio to the RHIC data.Comment: 4 pages, 2 figure; Contribution to Quark Matter 2008 in Jaipur,
India; submitted to J. Phys.
The Flexible Group Spatial Keyword Query
We present a new class of service for location based social networks, called
the Flexible Group Spatial Keyword Query, which enables a group of users to
collectively find a point of interest (POI) that optimizes an aggregate cost
function combining both spatial distances and keyword similarities. In
addition, our query service allows users to consider the tradeoffs between
obtaining a sub-optimal solution for the entire group and obtaining an
optimimized solution but only for a subgroup.
We propose algorithms to process three variants of the query: (i) the group
nearest neighbor with keywords query, which finds a POI that optimizes the
aggregate cost function for the whole group of size n, (ii) the subgroup
nearest neighbor with keywords query, which finds the optimal subgroup and a
POI that optimizes the aggregate cost function for a given subgroup size m (m
<= n), and (iii) the multiple subgroup nearest neighbor with keywords query,
which finds optimal subgroups and corresponding POIs for each of the subgroup
sizes in the range [m, n]. We design query processing algorithms based on
branch-and-bound and best-first paradigms. Finally, we provide theoretical
bounds and conduct extensive experiments with two real datasets which verify
the effectiveness and efficiency of the proposed algorithms.Comment: 12 page
The Class 0 Protostar BHR71: Herschel Observations and Dust Continuum Models
We use Herschel spectrophotometry of BHR71, an embedded Class 0 protostar, to
provide new constraints on its physical properties. We detect 645 (non-unique)
spectral lines amongst all spatial pixels. At least 61 different spectral lines
originate from the central region. A CO rotational diagram analysis shows four
excitation temperature components, 43 K, 197 K, 397 K, and 1057 K. Low-J CO
lines trace the outflow while the high-J CO lines are centered on the infrared
source. The low-excitation emission lines of H2O trace the large-scale outflow,
while the high-excitation emission lines trace a small-scale distribution
around the equatorial plane. We model the envelope structure using the dust
radiative transfer code, Hyperion, incorporating rotational collapse, an outer
static envelope, outflow cavity, and disk. The evolution of a rotating
collapsing envelope can be constrained by the far-infrared/millimeter SED along
with the azimuthally-averaged radial intensity profile, and the structure of
the outflow cavity plays a critical role at shorter wavelengths. Emission at
20-40 um requires a cavity with a constant-density inner region and a power-law
density outer region. The best fit model has an envelope mass of 19 solar mass
inside a radius of 0.315 pc and a central luminosity of 18.8 solar luminosity.
The time since collapse began is 24630-44000 yr, most likely around 36000 yr.
The corresponding mass infall rate in the envelope (1.2x10 solar mass
per year) is comparable to the stellar mass accretion rate, while the mass loss
rate estimated from the CO outflow is 20% of the stellar mass accretion rate.
We find no evidence for episodic accretion.Comment: Accepted for publication in ApJ. 33 pages; 34 figures; 4 table
Direct Measurement of the Fermi Energy in Graphene Using a Double Layer Structure
We describe a technique which allows a direct measurement of the relative
Fermi energy in an electron system using a double layer structure, where
graphene is one of the two layers. We illustrate this method by probing the
Fermi energy as a function of density in a graphene monolayer, at zero and in
high magnetic fields. This technique allows us to determine the Fermi velocity,
Landau level spacing, and Landau level broadening in graphene. We find that the
N=0 Landau level broadening is larger by comparison to the broadening of upper
and lower Landau levels.Comment: 5 pages, 4 figure
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