223 research outputs found
Application of Nonlinear Conductivity Spectroscopy to Ion Transport in Solid Electrolytes
The field-dependent ion transport in thin samples of different glasses is
characterised by means of nonlinear conductivity spectroscopy. AC electric
fields with strengths up to 77 kV/cm are applied to the samples, and the
Fourier components of the current spectra are analysed. In the dc conductivity
regime and in the transition region to the dispersive conductivity, higher
harmonics in the current spectra are detected, which provide information about
higher--order conductivity coefficients. Our method ensures that these
higher--order conductivity coefficients are exclusively governed by
field--dependent ion transport and are not influenced by Joule heating effects.
We use the low-field dc conductivity and the higher--order dc
conductivity coefficient to calculate apparent jump distances
for the mobile ions, . Over a temperature range from 283 K to 353
K, we obtain values for between 39 \AA and 55 \AA . For all
glasses, we find a weak decrease of with increasing temperature.
Remarkably, the apparent jump distances calculated from our data are
considerably larger than typical values published in the literature for various
ion conducting glasses. These values were obtained by applying dc electric
fields. Our results provide clear evidence that the equation used in the
literature to calculate the apparent jump distances does not provide an
adequate physical description of field-dependent ion transport.Comment: 6 pages, 5 figure
Spectroscopic Study on the Beryllium Abundances of Red Giant Stars
An extensive spectroscopic study was carried out for the beryllium abundances
of 200 red giants (mostly of late G and early K type), which were determined
from the near-UV Be II 3131.066 line based on high-dispersion spectra obtained
by Subaru/HDS, with an aim of investigating the nature of surface Be contents
in these evolved giants; e.g., dependence upon stellar parameters, degree of
peculiarity along with its origin and build-up timing. We found that Be is
considerably deficient (to widely different degree from star to star) in the
photosphere of these evolved giants by ~1-3 dex (or more) compared to the
initial abundance. While the resulting Be abundances (A(Be)) appear to weakly
depend upon T_eff, log g, [Fe/H], M, age, and v_sin i, this may be attributed
to the metallicity dependence of A(Be) coupled with the mutual correlation
between these stellar parameters, since such tendencies almost disappear in the
metallicity-scaled Be abundance ([Be/Fe]). By comparing the Be abundances (as
well as their correlations with Li and C) to the recent theoretical predictions
based on sophisticated stellar evolution calculations, we concluded that such a
considerable extent/diversity of Be deficit is difficult to explain only by the
standard theory of first dredge-up in the envelope of red giants, and that some
extra mixing process (such as rotational or thermohaline mixing) must be
responsible, which presumably starts to operate already in the main-sequence
phase. This view is supported by the fact that appreciable Be depletion is seen
in less evolved intermediate-mass B-A type stars near to the main sequence.Comment: 29 pages, 10 figures, 3 tables, accepted for publication in Publ.
Astron. Soc. Japa
High-Dispersion Spectroscopic Study of Solar Twins: HIP 56948, HIP 79672, and HIP 100963
An intensive spectroscopic study was performed for three representative solar
twins (HIP 56948, HIP 79672, and HIP 100963) as well as for the Sun (Moon;
reference standard), with an intention of (1) quantitatively discussing the
relative-to-Sun similarities based on the precisely established differential
parameters and (2) investigating the reason causing the Li abundance
differences despite their similarities. It was concluded that HIP 56948 most
resembles the Sun in every respect including the Li abundance (though not
perfectly similar) among the three and deserves the name of "closest-ever solar
twin", while HIP 79672 and HIP 100963 have somewhat higher effective
temperature and appreciably higher surface Li composition. While there is an
indication of Li being rotation-dependent because the projected rotation in HIP
56948 (and the Sun) is slightly lower than the other two, the rotational
difference alone does not seem to be so large as to efficiently produce the
marked change in Li. Rather, this may be more likely to be attributed (at least
partly) to the slight difference in T_eff via some T_eff-sensitive
Li-controlling mechanism. Since the abundance of beryllium was found to be
essentially solar for all stars irrespective of Li, any physical process
causing the Li diversity should work only on Li without affecting Be.Comment: 11 pages, 6 figures, 5 tables, accepted for publication in PAS
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