1,553 research outputs found
Excellent daytime seeing at Dome Fuji on the Antarctic plateau
Context. Dome Fuji, the second highest region on the Antarctic plateau, is
expected to have some of the best astronomical seeing on Earth. However, site
testing at Dome Fuji is still in its very early stages.
Aims. To investigate the astronomical seeing in the free atmosphere above
Dome Fuji, and to determine the height of the surface boundary layer.
Methods. A Differential Image Motion Monitor was used to measure the seeing
in the visible (472 nm) at a height of 11 m above the snow surface at Dome Fuji
during the austral summer of 2012/2013.
Results. Seeing below 0.2'' has been observed. The seeing often has a local
minimum of ~0.3'' near 18 h local time. Some periods of excellent seeing, 0.3''
or smaller, were also observed, sometimes extending for several hours at local
midnight. The median seeing is higher, at 0.52''---this large value is believed
to be caused by periods when the telescope was within the turbulent boundary
layer.
Conclusions. The diurnal variation of the daytime seeing at Dome Fuji is
similar to that reported for Dome C, and the height of the surface boundary
layer is consistent with previous simulations for Dome Fuji. The free
atmosphere seeing is ~0.2'', and the height of the surface boundary layer can
be as low as ~11 m.Comment: 4 pages, 6 figures, Submitted to Astronomy & Astrophysics (letter
Stoner gap in the superconducting ferromagnet UGe2
We report the temperature () dependence of ferromagnetic Bragg peak
intensities and dc magnetization of the superconducting ferromagnet UGe2 under
pressure (). We have found that the low- behavior of the uniform
magnetization can be explained by a conventional Stoner model. A functional
analysis of the data produces the following results: The ferromagnetic state
below a critical pressure can be understood as the perfectly polarized state,
in which heavy quasiparticles occupy only majority spin bands. A Stoner gap
decreases monotonically with increasing pressure and increases
linearly with magnetic field. We show that the present analysis based on the
Stoner model is justified by a consistency check, i.e., comparison of density
of states at the Fermi energy deduced from the analysis with observed
electronic specific heat coeffieients. We also argue the influence of the
ferromagnetism on the superconductivity.Comment: 5 pages, 4 figures. to be published in Phys. Rev.
A radiation driven implosion model for the enhanced luminosity of protostars near HII regions
Context. Molecular clouds near the H II regions tend to harbor more luminous
protostars. Aims. Our aim in this paper is to investigate whether or not
radiation-driven implosion mechanism enhances luminosity of protostars near
regions of high-ionizing fluxes. Methods. We performed numerical simulations to
model collapse of cores exposed to UV radiation from O stars. We investigated
dependence of mass loss rates on the initial density profiles of cores and
variation of UV fluxes. We derived simple analytic estimates of accretion rates
and final masses of protostars. Results. Radiation-driven implosion mechanism
can increase accretion rates of protostars by 1-2 orders of magnitude. On the
other hand, mass loss due to photo-evaporation is not large enough to have a
significant impact on the luminosity. The increase of accretion rate makes
luminosity 1-2 orders higher than those of protostars that form without
external triggering. Conclusions. Radiation-driven implosion can help explain
the observed higher luminosity of protostars in molecular clouds near H II
regions.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Electrical Resistivity and Thermal Expansion Measurements of URu2Si2 under Pressure
We carried out simultaneous measurements of electrical resistivity and
thermal expansion of the heavy-fermion compound URu2Si2 under pressure using a
single crystal. We observed a phase transition anomaly between hidden (HO) and
antiferromagnetic (AFM) ordered states at TM in the temperature dependence of
both measurements. For the electrical resistivity, the anomaly at TM was very
small compared with the distinct hump anomaly at the phase transition
temperature T0 between the paramagnetic state (PM) and HO, and exhibited only a
slight increase and decrease for the I // a-axis and c-axis, respectively. We
estimated each excitation gap of HO, Delta_HO, and AFM, Delta_AFM, from the
temperature dependence of electrical resistivity; Delta_HO and Delta_AFM have
different pressure dependences from each other. On the other hand, the
temperature dependence of thermal expansion exhibited a small anomaly at T0 and
a large anomaly at TM. The pressure dependence of the phase boundaries of T0
and TM indicates that there is no critical end point and the two phase
boundaries meet at the critical point.Comment: 4 pages, 4 figure
Spin correlations in the electron-doped high-transition-temperature superconductor Nd{2-x}Ce{x}CuO{4+/-delta}
High-transition-temperature (high-Tc) superconductivity develops near
antiferromagnetic phases, and it is possible that magnetic excitations
contribute to the superconducting pairing mechanism. To assess the role of
antiferromagnetism, it is essential to understand the doping and temperature
dependence of the two-dimensional antiferromagnetic spin correlations. The
phase diagram is asymmetric with respect to electron and hole doping, and for
the comparatively less-studied electron-doped materials, the antiferromagnetic
phase extends much further with doping [1, 2] and appears to overlap with the
superconducting phase. The archetypical electron-doped compound
Nd{2-x}Ce{x}CuO{4\pm\delta} (NCCO) shows bulk superconductivity above x \approx
0.13 [3, 4], while evidence for antiferromagnetic order has been found up to x
\approx 0.17 [2, 5, 6]. Here we report inelastic magnetic neutron-scattering
measurements that point to the distinct possibility that genuine long-range
antiferromagnetism and superconductivity do not coexist. The data reveal a
magnetic quantum critical point where superconductivity first appears,
consistent with an exotic quantum phase transition between the two phases [7].
We also demonstrate that the pseudogap phenomenon in the electron-doped
materials, which is associated with pronounced charge anomalies [8-11], arises
from a build-up of spin correlations, in agreement with recent theoretical
proposals [12, 13].Comment: 5 pages, 4 figure
Microwave and millimeter wave spectroscopy in the slightly hole-doped ladders of SrCuO
We have measured the temperature- and frequency dependence of the microwave
and millimeter wave conductivity along both the ladder
(c-axis) and the leg (a-axis) directions in SrCuO. Below a
temperature (170 K), we observed a stronger frequency dependence in
than that in , forming a small
resonance peak developed between 30 GHz and 100 GHz. We also observed nonlinear
dc conduction along the c-axis at rather low electric fields below . These
results suggest some collective excitation contributes to the c-axis charge
dynamics of the slightly hole-doped ladders of SrCuO below
.Comment: 7 pages, 4 figure, to be published in Europhysics Letter
Superconductivity of the spin ladder system: Are the superconducting pairing and the spin-gap formation of the same origin?
Pressure-induced superconductivity in a spin-ladder cuprate
SrCaCuO has not been studied on a microscopic level so
far although the superconductivity was already discovered in 1996. We have
improved high-pressure technique with using a large high-quality crystal, and
succeeded in studying the superconductivity using Cu nuclear magnetic
resonance (NMR). We found that anomalous metallic state reflecting the
spin-ladder structure is realized and the superconductivity possesses a
s-wavelike character in the meaning that a finite gap exists in the
quasi-particle excitation: At pressure of 3.5GPa we observed two excitation
modes in the normal state from the relaxation rate . One gives rise
to an activation-type component in , and the other -linear
component linking directly with the superconductivity. This gapless mode likely
arises from free motion of holon-spinon bound states appearing by hole doping,
and the pairing of them likely causes the superconductivity.Comment: to be published in Phys. Rev. Let
Suppression of the charge-density-wave state in Sr_14Cu_24O_41 by calcium doping
The charge response in the spin chain/ladder compound Sr_14-xCa_xCu_24O_41 is
characterized by DC resistivity, low-frequency dielectric spectroscopy and
optical spectroscopy. We identify a phase transition below which a
charge-density wave (CDW) develops in the ladder arrays. Calcium doping
suppresses this phase with the transition temperature decreasing from 210 K for
x=0 to 10 K for x=9, and the CDW gap from 130 meV down to 3 meV, respectively.
This suppression is due to the worsened nesting originating from the increase
of the inter-ladder tight-binding hopping integrals, as well as from disorder
introduced at the Sr sites. These results altogether speak in favor of
two-dimensional superconductivity under pressure.Comment: 4 pages, 4 figures, accepted for publication in PR
Three-Dimensional Multiband d-p Model of Superconductivity in Spin-Chain Ladder Cuprate
We study the superconductivity in the three-dimensional multiband d-p model,
in which a CuO-ladder layer and a CuO-chain layer are alternately
stacked, as a model of the superconducting spin-chain ladder cuprate.
-Wave-like triplet superconductivity is found to be the most stable, and
its dependence on interlayer coupling can explain the superconducting
transition temperature dependence on pressure in real superconducting
spin-chain ladder cuprates. The superconductivity may be enhanced if hole
transfer from the chain layer to the ladder layer can be promoted beyond the
typical transfer rate.Comment: 16 pages, 8 figure
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