8,236 research outputs found
Reexamining the temperature and neutron density conditions for r-process nucleosynthesis with augmented nuclear mass models
We explore the effects of nuclear masses on the temperature and neutron
density conditions required for r-process nucleosynthesis using four nuclear
mass models augmented by the latest atomic mass evaluation. For each model we
derive the conditions for producing the observed abundance peaks at mass
numbers A ~ 80, 130, and 195 under the waiting-point approximation and further
determine the sets of conditions that can best reproduce the r-process
abundance patterns (r-patterns) inferred for the solar system and observed in
metal-poor stars of the Milky Way halo. In broad agreement with previous
studies, we find that (1) the conditions for producing abundance peaks at A ~
80 and 195 tend to be very different, which suggests that, at least for some
nuclear mass models, these two peaks are not produced simultaneously; (2) the
typical conditions required by the critical waiting-point (CWP) nuclei with the
N = 126 closed neutron shell overlap significantly with those required by the
N=82 CWP nuclei, which enables coproduction of abundance peaks at A ~ 130 and
195 in accordance with observations of many metal-poor stars; and (3) the
typical conditions required by the N = 82 CWP nuclei can reproduce the
r-pattern observed in the metal-poor star HD 122563, which differs greatly from
the solar r-pattern. We also examine how nuclear mass uncertainties affect the
conditions required for the r-process and identify some key nuclei
including76Ni to 78Ni, 82Zn, 131Cd, and 132Cd for precise mass measurements at
rare-isotope beam facilities.Comment: 28 pages,9 figures,1 tabl
Isolation and purification of alkaline keratinase from Bacillus sp. 50-3
A new alkaline keratinase extracted from Bacillus sp. 50-3 was isolated and purified in this study. Solid ammonium sulfate was selected to precipitate the enzyme. Its proper adding mass was also determined.Through solid ammonium sulfate precipitation and liquid chromatography via the DEAE-Sephadex-A50 column using with azokeratin as a substrate, 17.7-fold purification with a yield of 46.5% was obtained. The purification effect was determined through SDS-PAGE, and the molecular weight of the enzyme was found at 27 423 Da by the MALDI-TOF-MS. Its simple purification step and high yield using a cheap medium attest to the great biotechnological potential of keratinase, especially in environmentprotection and in recycling valuable materials from wastes
Temperature dependence of electron-spin relaxation in a single InAs quantum dot at zero applied magnetic field
The temperature-dependent electron spin relaxation of positively charged
excitons in a single InAs quantum dot (QD) was measured by time-resolved
photoluminescence spectroscopy at zero applied magnetic fields. The
experimental results show that the electron-spin relaxation is clearly divided
into two different temperature regimes: (i) T < 50 K, spin relaxation depends
on the dynamical nuclear spin polarization (DNSP) and is approximately
temperature-independent, as predicted by Merkulov et al. (ii) T > about 50 K,
spin relaxation speeds up with increasing temperature. A model of two LO phonon
scattering process coupled with hyperfine interaction is proposed to account
for the accelerated electron spin relaxation at higher temperatures.Comment: 10 pages, 4 figure
A Quantum Tweezer for Atoms
We propose a quantum tweezer for extracting a desired number of neutral atoms
from a reservoir. A trapped Bose-Einstein condensate (BEC) is used as the
reservoir, taking advantage of its coherent nature, which can guarantee a
constant outcome. The tweezer is an attractive quantum dot, which may be
generated by red-detuned laser light. By moving with certain speeds, the dot
can extract a desired number of atoms from the BEC through Landau-Zener
tunneling. The feasibility of our quantum tweezer is demonstrated through
realistic and extensive model calculations.Comment: 4 pages, 6 figures Revised versio
Weak Field Phase Diagram for an Integer Quantum Hall Liquid
We study the localization properties in the transition from a two-dimensional
electron gas at zero magnetic field into an integer quantum Hall (QH) liquid.
By carrying out a direct calculation of the localization length for a finite
size sample using a transfer matrix technique, we systematically investigate
the field and disorder dependences of the metal-insulator transition in the
weak field QH regime. We obtain a different phase diagram from the one
conjectured in previous theoretical studies. In particular, we find that: (1)
the extended state energy for each Landau level (LL) is {\it always}
linear in magnetic field; (2) for a given Landau level and disorder
configuration there exists a critical magnetic field below which the
extended state disappears; (3) the lower LLs are more robust to the
metal-insulator transition with smaller . We attribute the above results
to strong LL coupling effect. Experimental implications of our work are
discussed.Comment: 4 pages, ReVTeX 3.0, 4 figures (available upon request
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