2,211 research outputs found
Mutual first order coherence of phase-locked lasers
We argue that (first-order) coherence is a relative, and not an absolute,
property. It is shown how feedforward or feedback can be employed to make two
(or more) lasers relatively coherent. We also show that after the relative
coherence is established, the two lasers will stay relatively coherent for some
time even if the feedforward or feedback loop has been turned off, enabling,
e.g., demonstration of unconditional quantum teleportation using lasers.Comment: 9 pages, 6 figure
Antiferromagnetic S=1/2 Heisenberg Chain and the Two-flavor Massless Schwinger Model
An antiferromagnetic S=1/2 Heisenberg chain is mapped to the two-flavor
massless Schwinger model at \theta=\pi. The electromagnetic coupling constant
and velocity of light in the Schwinger model are determined in terms of the
Heisenberg coupling and lattice spacing in the spin chain system.Comment: 3 pages. LaTex2
Spin Density wave instability in a ferromagnet
Ferromagnetic (FM) and incommensurate spin-density wave (ISDW) states are an
unusual set of competing magnetic orders that are seldom observed in the same
material without application of a polarizing magnetic field. We report, for the
first time, the discovery of an ISDW state that is derived from a FM ground
state through a Fermi surface (FS) instability in FeGa. This was
achieved by combining neutron scattering experiments with first principles
simulations. Neutron diffraction demonstrates that FeGa is in an ISDW
state at intermediate temperatures and that there is a conspicuous re-emergence
of ferromagnetism above 360 K. First principles calculations show that the ISDW
ordering wavevector is in excellent agreement with a prominent nesting
condition in the spin-majority FS demonstrating the discovery of a novel
instability for FM metals; ISDW formation due to Fermi surface nesting in a
spin-polarized Fermi surface.Comment: 6 pages with 4 figures. Supplemental Materials Include
Evidence for a colour dependence in the size distribution of main belt asteroids
We present the results of a project to detect small (~1 km) main-belt
asteroids with the 3.6 meter Canada-France-Hawaii Telescope (CFHT). We observed
in 2 filters (MegaPrime g' and r') in order to compare the results in each
band. Owing to the observational cadence we did not observe the same asteroids
through each filter and thus do not have true colour information. However
strong differences in the size distributions as seen in the two filters point
to a colour-dependence at these sizes, perhaps to be expected in this regime
where asteroid cohesiveness begins to be dominated by physical strength and
composition rather than by gravity. The best fit slopes of the cumulative size
distributions (CSDs) in both filters tend towards lower values for smaller
asteroids, consistent with the results of previous studies. In addition to this
trend, the size distributions seen in the two filters are distinctly different,
with steeper slopes in r' than in g'. Breaking our sample up according to
semimajor axis, the difference between the filters in the inner belt is found
to be somewhat less pronounced than in the middle and outer belt, but the CSD
of those asteroids seen in the r' filter is consistently and significantly
steeper than in g' throughout. The CSD slopes also show variations with
semimajor axis within a given filter, particularly in r'. We conclude that the
size distribution of main belt asteroids is likely to be colour dependent at
kilometer sizes and that this dependence may vary across the belt.Comment: 28 pages, 5 figures, submitted to the Astronomical Journa
Phase diagrams of the generalized spin-1/2 ladder under staggered field and dimerization: A renormalization group study
In the weak-coupling regime of the continuous theories, two sets of one-loop
renormalization group equations are derived and solved to disclose the phase
diagrams of the antiferromagnetic generalized two-leg spin-1/2 ladder under the
effect of (I) a staggered external magnetic field and (II) an explicit
dimerization. In model (I), the splitting of the SU(2) critical line into
U(1) and Z critical surfaces is observed; while in model (II), two critical
surfaces arising from their underlying critical lines with SU(2) and Z
characteristics merge into an SU(2) critical surface on the line where the
model attains its highest symmetry.Comment: 10 pages, 9 figure
Effective descriptions of branes on non-geometric tori
We investigate the low-energy effective description of non-geometric
compactifications constructed by T-dualizing two or three of the directions of
a T^3 with non-vanishing H-flux. Our approach is to introduce a D3-brane in
these geometries and to take an appropriate decoupling limit. In the case of
two T-dualities, we find at low energies a non-commutative T^2 fibered
non-trivially over an S^1. In the UV this theory is still decoupled from
gravity, but is dual to a little string theory with flavor. For the case of
three T-dualities, we do not find a sensible decoupling limit, casting doubt on
this geometry as a low-energy effective notion in critical string theory.
However, by studying a topological toy model in this background, we find a
non-associative geometry similar to one found by Bouwknegt, Hannabuss, and
Mathai.Comment: 22 pages, 4 figures, references adde
Heterotic-type IIA duality with fluxes
In this paper we study a possible non-perturbative dual of the heterotic
string compactified on K3 x T^2 in the presence of background fluxes. We show
that type IIA string theory compactified on manifolds with SU(3) structure can
account for a subset of the possible heterotic fluxes. This extends our
previous analysis to a case of a non-perturbative duality with fluxes.Comment: 26 pages, minor corrections; version to appear in JHE
Graph products of spheres, associative graded algebras and Hilbert series
Given a finite, simple, vertex-weighted graph, we construct a graded
associative (non-commutative) algebra, whose generators correspond to vertices
and whose ideal of relations has generators that are graded commutators
corresponding to edges. We show that the Hilbert series of this algebra is the
inverse of the clique polynomial of the graph. Using this result it easy to
recognize if the ideal is inert, from which strong results on the algebra
follow. Non-commutative Grobner bases play an important role in our proof.
There is an interesting application to toric topology. This algebra arises
naturally from a partial product of spheres, which is a special case of a
generalized moment-angle complex. We apply our result to the loop-space
homology of this space.Comment: 19 pages, v3: elaborated on connections to related work, added more
citations, to appear in Mathematische Zeitschrif
Supernova Remnants in the Magellanic Clouds. IV. X-Ray Emission from the Largest SNR in the LMC
We present the first X-ray detection of SNR 0450-70.9 the largest known
supernova remnant (SNR) in the Large Magellanic Cloud. To study the physical
conditions of this SNR, we have obtained XMM-Newton X-ray observations, optical
images and high-dispersion spectra, and radio continuum maps. Optical images of
SNR 0450-70.9 show a large, irregular elliptical shell with bright filaments
along the eastern and western rims and within the shell interior. The interior
filaments have higher [S II]/Halpha ratios and form an apparent inner shell
morphology. The X-ray emission region is smaller than the full extent of the
optical shell, with the brightest X-ray emission found within the small
interior shell and on the western rim of the large shell. The expansion
velocity of the small shell is ~220 km/s, while the large shell is ~120 km/s.
The radio image shows central brightening and a fairly flat radio spectral
index over the SNR. However, no point X-ray or radio source corresponding to a
pulsar is detected and the X-ray emission is predominantly thermal. Therefore,
these phenomena can be most reasonably explained in terms of the advanced age
of the large SNR. Using hydrodynamic models combined with a nonequilibrium
ionization model for thermal X-ray emission, we derived a lower limit on the
SNR age of about 45,000 yr, well into the later stages of SNR evolution.
Despite this, the temperature and density derived from spectral fits to the
X-ray emission indicate that the remnant is still overpressured, and thus that
the development is largely driven by hot gas in the SNR interior.Comment: Accepted for publication in The Astrophysical Journa
Measuring the Polarization of Boosted Hadronic Tops
We propose a new technique for measuring the polarization of hadronically
decaying boosted top quarks. In particular, we apply a subjet-based technique
to events where the decay products of the top are clustered within a single
jet. The technique requires neither b-tagging nor W-reconstruction, and does
not rely on assumptions about either the top production mechanism or the
sources of missing energy in the event. We include results for various new
physics scenarios made with different Monte Carlo generators to demonstrate the
robustness of the technique.Comment: v2: version accepted for publication in JHE
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