6,680 research outputs found
The spin gap of CaV4O9 revisited
The large-plaquette scenario of the spin gap in CaV4O9 is investigated on the
basis of extensive exact diagonalizations. We confirm the existence of a
large-plaquette phase in a wide range of parameters, and we show that the most
recent neutron scattering data actually require an intra-plaquette second
neighbor exchange integral much larger than the inter-plaquette one, thus
justifying the perturbative calculation used in the interpretation of the
neutron scattering experiments.Comment: 2 pages with 3 figure
Runaway Merging of Black Holes: Analytical Constraint on the Timescale
Following the discovery of a black hole (BH) with a mass of 10^3-10^6 M(sun)
in a starburst galaxy M82, we study formation of such a BH via successive
merging of stellar-mass BHs within a star cluster. The merging has a runaway
characteristic. This is because massive BHs sink into the cluster core and have
a high number density, and because the merging probability is higher for more
massive BHs. We use the Smoluchowski equation to study analytically the
evolution of the BH mass distribution. Under favorable conditions, which are
expected for some star clusters in starburst galaxies, the timescale of the
runaway merging is at most of order 10^7 yr. This is short enough to account
for the presence of a BH heavier than 10^3 M(sun) in an ongoing starburst
region.Comment: 10 pages, no figures, to appear in The Astrophysical Journal
(Letters
Giant Microwave Absorption in Metallic Grains: Relaxation Mechanism
We show that the low frequency microwave absorption of an ensemble of small
metallic grains at low temperatures is dominated by a mesoscopic relaxation
mechanism. Giant positive magnetoresistance and very strong temperature
dependence of the microwave conductivity is predicted.Comment: 4 pages, REVTeX3+mutlticol+epsf, one EPS figur
Tunneling spin valves based on FeGeTe/hBN/FeGeTe van der Waals heterostructures
Thin van der Waals (vdW) layered magnetic materials disclose the possibility
to realize vdW heterostructures with new functionalities. Here we report on the
realization and investigation of tunneling spin valves based on van der Waals
heterostructures consisting of an atomically thin hBN layer acting as tunnel
barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic
electrodes. Low-temperature anomalous Hall effect measurements show that thin
Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to
the layers, and a very sharp magnetization switching at magnetic field values
that depend slightly on their geometry. In Fe3GeTe2/hBN/Fe3GeTe2
heterostructures, we observe a textbook behavior of the tunneling resistance,
which is minimum (maximum) when the magnetization in the two electrodes is
parallel (antiparallel) to each other. The magnetoresistance is 160% at low
temperature, from which we determine the spin polarization of Fe3GeTe2 to be
0.66, corresponding to 83% and 17% of majority and minority carriers,
respectively. The measurements also show that, with increasing temperature, the
evolution of the spin polarization extracted from the tunneling
magnetoresistance is proportional to the temperature dependence of the
magnetization extracted from the analysis of the anomalous Hall conductivity.
This suggests that the magnetic properties of the surface are representative of
those of the bulk, as it may be expected for vdW materials.Comment: 4 figure
An Observational Pursuit for Population III Stars in a Ly_alpha Emitter at z=6.33 through HeII Emission
We present a very deep near-infrared spectroscopic observation of a strong
Ly_alpha emitter at z=6.33, SDF J132440.6+273607, which we used to search for
HeII 1640. This emission line is expected if the target hosts a significant
number of population III stars. Even after 42 ksec of integration with the
Subaru/OHS spectrograph, no emission-line features are detected in the JH band,
which confirms that SDF J132440.6+273607 is neither an active galactic nucleus
nor a low- emission-line object. We obtained a 2sigma upper-limit of
9.06e-18 ergs/s/cm^2 on the HeII 1640 emission line flux, which corresponds to
a luminosity of 4.11e42 ergs/s. This upper-limit on the HeII 1640 luminosity
implies that the upper limit on population III star-formation rate is in the
range 4.9--41.2 M_sun/yr if population III stars suffer no mass loss, and in
the range 1.8--13.2 M_sun/yr if strong mass loss is present. The non-detection
of HeII in SDF J132440.6+273607 at z=6.33 may thus disfavor weak feedback
models for population III stars.Comment: 13 pages, 3 figures, to appear in The Astrophysical Journal Letter
An Origin of the Huge Far-Infrared Luminosity of Starburst Mergers
Recently Taniguchi and Ohyama found that the higher CO to CO
integrated intensity ratios at a transition =1--0, CO)CO) , in a sample of starburst merging
galaxies such as Arp 220 are mainly attributed to the depression of CO
emission with respect to CO. Investigating the same sample of galaxies
analyzed by Taniguchi & Ohyama, we find that there is a tight, almost linear
correlation between the dust mass and CO luminosity. This implies that
dust grains are also depressed in the high- starburst mergers, leading to
the higher dust temperature () in them because of the relative
increase in the radiation density. Nevertheless, the average dust mass () of the high- starburst mergers is higher significantly than that of
non-high galaxies. This is naturally understood because the galaxy mergers
could accumulate a lot of dust grains from their progenitor galaxies together
with supply of dust grains formed newly in the star forming regions. Since
(FIR) given the dust emissivity law, , the increases in both and
explain well why the starburst mergers are so bright in the FIR. We discuss
that the superwind activity plays an important role in destroying dust grains
as well as dense gas clouds in the central region of mergers.Comment: 10 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
Optimal Detection for Diffusion-Based Molecular Timing Channels
This work studies optimal detection for communication over diffusion-based
molecular timing (DBMT) channels. The transmitter simultaneously releases
multiple information particles, where the information is encoded in the time of
release. The receiver decodes the transmitted information based on the random
time of arrival of the information particles, which is modeled as an additive
noise channel. For a DBMT channel without flow, this noise follows the L\'evy
distribution. Under this channel model, the maximum-likelihood (ML) detector is
derived and shown to have high computational complexity. It is also shown that
under ML detection, releasing multiple particles improves performance, while
for any additive channel with -stable noise where (such as
the DBMT channel), under linear processing at the receiver, releasing multiple
particles degrades performance relative to releasing a single particle. Hence,
a new low-complexity detector, which is based on the first arrival (FA) among
all the transmitted particles, is proposed. It is shown that for a small number
of released particles, the performance of the FA detector is very close to that
of the ML detector. On the other hand, error exponent analysis shows that the
performance of the two detectors differ when the number of released particles
is large.Comment: 16 pages, 9 figures. Submitted for publicatio
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