657 research outputs found
Presence of 3d Quadrupole Moment in LaTiO3 Studied by 47,49Ti NMR
Ti NMR spectra of LaTiO3 are reexamined and the orbital state of this
compound is discussed. The NMR spectra of LaTiO3 taken at 1.5 K under zero
external field indicate a large nuclear quadrupole splitting. This splitting is
ascribed to the presence of the rather large quadrupole moment of 3d electrons
at Ti sites, suggesting that the orbital liquid model proposed for LaTiO3 is
inappropriate. The NMR spectra are well explained by the orbital ordering model
expressed approximately as originating from
a crystal field effect. It is also shown that most of the orbital moment is
quenched.Comment: 4 pages, 3 fugures; to appear in Phys. Rev. Let
A Search for X-Ray Evidence of a Compact Companion to the Unusual Wolf-Rayet Star HD 50896 (EZ CMa)
We analyze results of a approx.25 ksec ASCA X-ray observation of the unusual Wolf-Rayet star HD 50896 (= EZ CMa). This WN5 star shows optical and ultraviolet variability at a 3.766 day period, which has been interpreted as a possible signature of a compact companion. Our objective was to search for evidence of hard X-rays (greater than or equal to 5 keV) which could be present if the WN5 wind is accreting onto a compact object. The ASCA spectra are dominated by emission below 5 keV and show no significant emission in the harder 5-10 keV range. Weak emission lines are present, and the X-rays arise in an optically thin plasma which spans a range of temperatures from less than or equal to 0.4 keV up to at least approx. 2 keV. Excess X-ray absorption above the interstellar value is present, but the column density is no larger than N(sub H) approx. 10(exp 22)/sq cm. The absorption-corrected X-ray luminosity L(sub x)(0.5 - 10 keV) = 10(exp 32.85) erg/s gives L(sub x)/ L(sub bol) approx. 10(exp -6), a value that is typical of WN stars. No X-ray variability was detected. Our main conclusion is that the X-ray properties of HD 50896 are inconsistent with the behavior expected for wind accretion onto a neutron star or black hole companion. Alternative models based on wind shocks can explain most aspects of the X-ray behavior, and we argue that the hotter plasma near approx. 2 keV could be due to the WR wind shocking onto a normal (nondegenerate) companion
Influence of Road Surface Design on Vehicles\u27 Speed: Experiment Using Driving
Art and Design Research for the Future: Innovation and Art & Design ; September 26, 2017Conference: Tsukuba Global Science Week 2017Date: September 25-27, 2017Venue: Tsukuba International Congress CenterSponsored: University of Tsukub
Stellar Evolution Constraints on the Triple-Alpha Reaction Rate
We investigate the quantitative constraint on the triple-alpha reaction rate
based on stellar evolution theory, motivated by the recent significant revision
of the rate proposed by nuclear physics calculations. Targeted stellar models
were computed in order to investigate the impact of that rate in the mass range
of 0.8 < M / Msun < 25 and in the metallicity range between Z = 0 and Z = 0.02.
The revised rate has a significant impact on the evolution of low- and
intermediate-mass stars, while its influence on the evolution of massive stars
(M >~ 10 Msun) is minimal. We find that employing the revised rate suppresses
helium shell flashes on AGB phase for stars in the initial mass range 0.8 < M /
Msun < 6, which is contradictory to what is observed. The absence of helium
shell flashes is due to the weak temperature dependence of the revised
triple-alpha reaction cross section at the temperature involved. In our models,
it is suggested that the temperature dependence of the cross section should
have at least nu > 10 at T = 1 - 1.2 x 10^8 K where the cross section is
proportional to T^{nu}. We also derive the helium ignition curve to estimate
the maximum cross section to retain the low-mass first red giants. The
semi-analytically derived ignition curves suggest that the reaction rate should
be less than ~ 10^{-29} cm^6 s^{-1} mole^{-2} at ~ 10^{7.8} K, which
corresponds to about three orders of magnitude larger than that of the NACRE
compilation. In an effort to compromise with the revised rates, we calculate
and analyze models with enhanced CNO cycle reaction rates to increase the
maximum luminosity of the first giant branch. However, it is impossible to
reach the typical RGB tip luminosity even if all the reaction rates related to
CNO cycles are enhanced by more than ten orders of magnitude.Comment: 14 pages, 8 figures, accepted by the Ap
Mechanism of strong quenching of photosystem II chlorophyll fluorescence under drought stress in a lichen, Physciella melanchla, studied by subpicosecond fluorescence spectroscopy
AbstractThe mechanism of the severe quenching of chlorophyll (Chl) fluorescence under drought stress was studied in a lichen Physciella melanchla, which contains a photobiont green alga, Trebouxia sp., using a streak camera and a reflection-mode fluorescence up-conversion system. We detected a large 0.31 ps rise of fluorescence at 715 and 740 nm in the dry lichen suggesting the rapid energy influx to the 715–740 nm bands from the shorter-wavelength Chls with a small contribution from the internal conversion from Soret bands. The fluorescence, then, decayed with time constants of 23 and 112 ps, suggesting the rapid dissipation into heat through the quencher. The result confirms the accelerated 40 ps decay of fluorescence reported in another lichen (Veerman et al., 2007 [36]) and gives a direct evidence for the rapid energy transfer from bulk Chls to the longer-wavelength quencher. We simulated the entire PS II fluorescence kinetics by a global analysis and estimated the 20.2 ns−1 or 55.0 ns−1 energy transfer rate to the quencher that is connected either to the LHC II or to the PS II core antenna. The strong quenching with the 3–12 times higher rate compared to the reported NPQ rate, suggests the operation of a new type of quenching, such as the extreme case of Chl-aggregation in LHCII or a new type of quenching in PS II core antenna in dry lichens
Site-dependent Local Spin Susceptibility and Low-energy Excitation in a Weyl Semimetal WTe
Site-dependent local spin susceptibility is investigated with Te
nuclear magnetic resonance in a Weyl semimetal WTe. The nuclear
spin-lattice relaxation rate shows a dependence of the square of
temperature at high temperatures, followed by a constant behavior below 50
K. The temperature dependence features Weyl fermions appearing around the
linearly crossing bands. The Knight shift scales to the square root of
, corroborating a predominant spin contribution in low-lying
excitation. The observed dependence of and on the four Te sites
shows the site-dependent electron correlation and density of states. The
angular profile of the NMR spectrum gives the anisotropic hyperfine coupling
tensor, consistent with hole occupations on Te sites.Comment: 6 pages, 5 figure
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