5,325 research outputs found
Symmetry breaking and other phenomena in the optimization of eigenvalues for composite membranes
We consider the following eigenvalue optimization problem: Given a bounded
domain and numbers , ,
find a subset of area for which the first Dirichlet
eigenvalue of the operator is as small as possible.
We prove existence of solutions and investigate their qualitative properties.
For example, we show that for some symmetric domains (thin annuli and dumbbells
with narrow handle) optimal solutions must possess fewer symmetries than
; on the other hand, for convex reflection symmetries are
preserved.
Also, we present numerical results and formulate some conjectures suggested
by them.Comment: 24 pages; 3 figures (as separate files); (shortened previous
version); to appear in Comm. Math. Phy
Hall effect in superconducting Fe(Se0.5Te0.5) thin films
The Hall effect is investigated for eight superconducting Fe(Se_0.5_Te_0.5_)
thin films grown on MgO and LaSrAlO_4_ substrates with different transition
temperatures (T_c_). The normal Hall coefficients (R_H_) have positive values
with magnitude of 1 - 1.5 x 10^-3^ cm^3^/C at room temperature for the all
samples. With decreasing temperature, we find two characteristic types of
behavior in R_H_(T) depending on T_c_. For thin films with lower T_c_
(typically T_c_ < 5 K), R_H_ start decreasing approximately below T = 250 K
toward a negative side, some of which shows sign reversal at T = 50 - 60 K, but
turns positive toward T = 0 K. On the other hand for the films with higher T_c_
(typically T_c_ > 9 K), R_ H_ leaves almost unchanged down to T = 100 K, and
then starts decreasing toward a negative side. Around the temperatures when
R_H_ changes its sign from positive to negative, obvious nonlinearity is
observed in the field-dependence of Hall resistance as to keep the low-field
R_H_ positive while the high-field R_H_ negative. Thus the electronic state
just above T_c_ is characterized by n_e_ (electron density) > n_h_ (hole
density) with keeping \mu_e_ < \mu_h_. These results suggest the dominance of
electron density to the hole density is an essential factor for the occurence
of superconductivity in Fe-chalcogenide superconductors.Comment: 11 pages, 4 figures, revised version for Physical Review B. accepted
for publication in Physical Review
Electronic Structures of CaAlSi with Different Stacking AlSi Layers by First-Principles Calculations
The full-potential linear augmented plane-wave calculations have been applied
to investigate the systematic change of electronic structures in CaAlSi due to
different stacking sequences of AlSi layers. The present ab-initio calculations
have revealed that the multistacking, buckling and 60 degrees rotation of AlSi
layer affect the electronic band structure in this system. In particular, such
a structural perturbation gives rise to the disconnected and cylindrical Fermi
surface along the M-L lines of the hexagonal Brillouin zone. This means that
multistacked CaAlSi with the buckling AlSi layers increases degree of
two-dimensional electronic characters, and it gives us qualitative
understanding for the quite different upper critical field anisotropy between
specimens with and without superstructure as reported previously.Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jp
SiO Maser Survey of the Large-Amplitude Variables in the Galactic Center
We have surveyed ~400 known large-amplitude variables within 15' of the
galactic center in the SiO J=1--0 v=1 and 2 maser lines at 43 GHz, resulting in
179 detections. SiO lines were also detected from 16 other resulting in 180
detections. SiO lines were also detected from 16 other sources, which are
located within 20" (the telescope half beamwidth) of the program objects. The
detection rate of 48 percent is comparable to that obtained in Bulge IRAS
source surveys. Among the SiO detections, five stars have radial velocities
greater than 200 km/s. The SiO detection rate increases steeply with the period
of light variation, particularly for stars with P>500 d, where it exceeds 80%.
We found that, at a given period, the SiO detection rate is approximately three
times that for OH. These facts suggest that the large-amplitude variables in
the Nuclear Disk region are AGB stars similar in their overall properties to
the inner and outer Bulge IRAS/SiO sources. From the set of radial velocity
data, the mass distribution within 30 pc of the galactic center is derived by a
new method which is based on the collisionless Boltzmann equation integrated
along the line of sight. The mass within 30 pc is about 6.4 [\pm 0.7] \times
10^7 M_{\odot} and the mass of the central black hole is 2.7 [\pm 1.3] \times
10^6 M_{\odot}. Consideration of the line-of-sight velocity of each star and
its potential energy leads to the conclusion that the five high-velocity stars
come from galactocentric distances as high as 300 pc. The high-velocity
subsample of stars with negative radial velocities exhibits a tendency to have
brighter K magnitudes than the subsample of stars with positive velocities. The
origin of these high-velocity stars is discussed.Comment: Hires. figures are available as No.604 of NRO report at
http://www.nro.nao.ac.jp/library/report/list.html . PASJ 56 (april 28 issue)
in pres
Anomalous magnetic properties near Mott transition in Kagom\'e lattice Hubbard model
We investigate the characteristics of the metallic phase near the Mott
transition in the Kagom\'e lattice Hubbard model using the cellular dynamical
mean field theory. By calculating the specific heat and spin correlation
functions, we demonstrate that the quasiparticles show anomalous properties in
the metallic phase close to the Mott transition. We find clear evidence for the
multi-band heavy quasiparticles in the specific heat, which gives rise to
unusual temperature dependence of the spin correlation functions.Comment: 2 pages, 3 figures, accepted for publication in J. Mag. Mag. Mater.
(Proceedings of the ICM, Kyoto, Japan, August 2006
NMR relaxation in half-integer antiferromagnetic spin chains
Nuclear relaxation in half-integer spin chains at low temperatures (T << J,
the antiferromagnetic exchange constant) is dominated by dissipation from a gas
of thermally-excited, overdamped, spinons. The universal low temperature
dependence of the relaxation rates and is computed.Comment: 7 pages, 1 uuencoded postscript figure appende
Timing of sea ice retreat can alter phytoplankton community structure in the western Arctic Ocean
This study assesses the response of phytoplankton assemblages to recent
climate change, especially with regard to the shrinking of sea ice in the
northern Chukchi Sea of the western Arctic Ocean. Distribution patterns of
phytoplankton groups in the late summers of 2008â2010 were analysed based
on HPLC pigment signatures and, the following four major algal groups were
inferred via multiple regression and cluster analyses: prasinophytes,
diatoms, haptophytes and dinoflagellates. A remarkable interannual
difference in the distribution pattern of the groups was found in the
northern basin area. Haptophytes dominated and dispersed widely in warm
surface waters in 2008, whereas prasinophytes dominated in cold water in
2009 and 2010. A difference in the onset date of sea ice retreat was evident
among yearsâthe sea ice retreat in 2008 was 1â2 months earlier than in
2009 and 2010. The spatial distribution of early sea ice retreat matched the
areas in which a shift in algal community composition was observed.
Steel-Dwass's multiple comparison tests were used to assess the physical,
chemical and biological parameters of the four clusters. We found a
statistically significant difference in temperature between the
haptophyte-dominated cluster and the other clusters, suggesting that the
change in the phytoplankton communities was related to the earlier sea ice
retreat in 2008 and the corollary increase in sea surface temperatures.
Longer periods of open water during the summer, which are expected in the
future, may affect food webs and biogeochemical cycles in the western Arctic
due to shifts in phytoplankton community structure
First-Principles Computation of YVO3; Combining Path-Integral Renormalization Group with Density-Functional Approach
We investigate the electronic structure of the transition-metal oxide YVO3 by
a hybrid first-principles scheme. The density-functional theory with the
local-density-approximation by using the local muffin-tin orbital basis is
applied to derive the whole band structure. The electron degrees of freedom far
from the Fermi level are eliminated by a downfolding procedure leaving only the
V 3d t2g Wannier band as the low-energy degrees of freedom, for which a
low-energy effective model is constructed. This low-energy effective
Hamiltonian is solved exactly by the path-integral renormalization group
method. It is shown that the ground state has the G-type spin and the C-type
orbital ordering in agreement with experimental indications. The indirect
charge gap is estimated to be around 0.7 eV, which prominently improves the
previous estimates by other conventional methods
SiO Maser Survey of IRAS Sources in the Inner Galactic Disk
We have surveyed 401 color selected IRAS sources in the Galactic disk in the
SiO J=1--0 v= 1 and 2 maser lines at 43 GHz, resulting in 254 (239 new)
detections. The observed sources lie mostly in a strip of the inner Galactic
disk with boundaries -10<l<40 deg and |b|<3 deg. This survey provides radial
velocities of inner-disk stars for which optical measurements cannot be made
due to interstellar extinction. The SiO -- diagram in the area lv$ map does, indicating a slight difference of stellar
type between SiO and OH emitting stars. After identifying all of the SiO
detected sources in the 2MASS near-infrared catalog, we computed their
luminosity distances based on the infrared fluxes. We then mapped these objects
onto the first quadrant of the Galactic plane. Combining the distances with the
SiO radial-velocities, we obtained a pattern speed for SiO maser sources,
Omega_P=21 (+- 13) km s^{-1} kpc^{-1}, between the distances 1 and 5.5 kpc,
without the use of any dynamical models. The increase of the pattern speed
toward the Galactic center (up to 60 km s^{-1} kpc^{-1} between the distances,
5.5 and 7 kpc) suggests the presence of two pattern speeds in the Galaxy.Comment: 38 page 9 figures, high res. eps files are available as NRO report
No. 608 (http://www.nro.nao.ac.jp/library/report/list.html). PASJ 56 No. 4 in
pres
Detections of SiO Masers from the Large-Amplitude Variables in the Galactic Nuclear Disk
We have surveyed known large-amplitude variables within 15' of the Galactic
center in the SiO J=1-0 v=1 and 2 maser lines at 43 GHz, resulting in 79
detections and 58 non-detections. The detection rate of 58 percent is
comparable to that obtained in Bulge IRAS source surveys. SiO lines were also
detected from four other sources near the program objects. The SiO detection
rate increases steeply with the period, particularly for stars with P>500 d,
where it exceeds 80%. We found at a given period that the SiO detection rate is
approximately double that for OH. These facts suggest that the large-amplitude
variables in the Nuclear Disk region are AGB stars similar in their overall
properties to the inner and outer bulge IRAS/SiO sources.Comment: 5 pages, 2 figures, 1 Table. PASJ 54, No 2 April 25 issue in pres
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