2,646 research outputs found
Intrinsic Luminescence from Self-Trapped Excitons in Bi4Ge3O12 and Bi12GeO20: Decay Kinetics and Multiplication of Electronic Excitations
The intrinsic luminescence appearing at 500 nm in Bi4Ge3O12 (e-BGO) and that at 450 nm in Bi12GeO20 (s-BGO) have been studied over a wide range of temperature T = 5–300 K by using a Nd:YAG laser and synchrotron radiation as excitation light sources. Luminescence decay curves in e-BGO depend dramatically on the laser power; they are composed of three decay components under high-density excitation, while they show a single exponential decay at low-density excitation. From temperature dependences of the decay time and emission intensity, it is clarified that the triplet state of a self-trapped exciton (STE) responsible for the e-BGO luminescence consists of a pair of closely spaced sublevels with separation energy of 5.7 meV. The decay curves of s-BGO luminescence are essentially nonexponential, irrespective of the excitation power. Time-resolved luminescence measurements of s-BGO suggest the existence of a singlet state lying higher than the triplet STE state. The excitation spectra for the intrinsic luminescence bands have been measured up to 35 eV (35 nm) at 5 K. From the obtained spectra, it is obvious that the multiplication of electronic excitations takes place efficiently in both BGOs. The production processes of multiple excitons are discussed by referring to a recent study on the electronic structures. ©2010 The Physical Society of JapanArticleJournal of Physical Society of Japan. 79:074717 (2010)journal articl
New records for the liverwort and hornwort flora of Vietnam, 1
After the examination of the Cryptogam collection in the Herbarium of the University of Science, Vietnam National University Ho Chi Minh City (PHH), 25 species proved to be new to Vietnam, including one hornwort and 24 liverworts. Among them, four genera: Denotarisia Grolle, Gongylanthus Nees, Leiomitra Lindb. and Lepicolea Dumort. are new records for the country. Diagnostic characters and illustrations are given for some taxa, as well as locality notes and habitat descriptions are provided for each collecting area
Suppression of geometrical barrier in crystals by Josephson vortex stacks
Differential magneto-optics are used to study the effect of dc in-plane
magnetic field on hysteretic behavior due to geometrical barriers in
crystals. In absence of in-plane field a vortex
dome is visualized in the sample center surrounded by barrier-dominated
flux-free regions. With in-plane field, stacks of Josephson vortices form
vortex chains which are surprisingly found to protrude out of the dome into the
vortex-free regions. The chains are imaged to extend up to the sample edges,
thus providing easy channels for vortex entry and for drain of the dome through
geometrical barrier, suppressing the magnetic hysteresis. Reduction of the
vortex energy due to crossing with Josephson vortices is evaluated to be about
two orders of magnitude too small to account for the formation of the
protruding chains. We present a model and numerical calculations that
qualitatively describe the observed phenomena by taking into account the
demagnetization effects in which flux expulsion from the pristine regions
results in vortex focusing and in the chain protrusion. Comparative
measurements on a sample with narrow etched grooves provide further support to
the proposed model.Comment: 12 figures (low res.) Higher resolution figures are available at the
Phys Rev B version. Typos correcte
Infrared/optical - X-ray simultaneous observations of X-ray flares in GRB 071112C and GRB 080506
We investigate the origin of short X-ray flares which are occasionally
observed in early stages of afterglows of gamma-ray bursts (GRBs). We observed
two events, GRB 071112C and GRB 080506, before the start of X-ray flares in the
optical and near-infrared (NIR) bands with the 1.5-m Kanata telescope. In
conjunction with published X-ray and optical data, we analyzed densely sampled
light curves of the early afterglows and spectral energy distributions (SEDs)
in the NIR-X-ray ranges. We found that the SEDs had a break between the optical
and X-ray bands in the normal decay phases of both GRBs regardless of the model
for the correction of the interstellar extinction in host galaxies of GRBs. In
the X-ray flares, X-ray flux increased by 3 and 15 times in the case of GRB
071112C and 080506, respectively, and the X-ray spectra became harder than
those in the normal decay phases. No significant variation in the optical-NIR
range was detected together with the X-ray flares. These results suggest that
the X-ray flares were associated with either late internal shocks or external
shocks from two-component jets.Comment: 10 pages, 5 figures, accepted to Astronomy and Astrophysic
Comparative study of excitonic structures and luminescence properties of Bi4Ge3O12 and Bi12GeO20
Published in physica status solidi (b), Vol. 245, No. 12, pp2733-2736, 2008.Bi12GeO20 crystals have been performed, in addition to relativistic molecular orbital calculations. Both materials consist of the same elements but form different crystal structures. Based on the obtained results, the excitonic reflection structures and luminescence properties of Bi4Ge3O12 and Bi12GeO20 are discussed in comparison with each other.Bookphysica status solidi (b). 245(12):2733-2736 (2008)journal articl
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