141 research outputs found
Optical pulse induced ultrafast antiferrodistortive transition in SrTiO3
The ultrafast dynamics of the antiferrodistortive (AFD) phase transition in
perovskite SrTiO3 is monitored via time-domain Brillouin scattering. Using
femtosecond optical pulses, we induce a thermally driven tetragonal-to-cubic
structural transformation and detect notable changes in the frequency of
Brillouin oscillations (BO) induced by propagating acoustic phonons. First, we
establish a fingerprint frequency of different regions across the temperature
phase diagram of the AFD transition characterized by tetragonal and cubic
phases in the low and high temperature sides, respectively. Then, we
demonstrate that in a sample nominally kept in tetragonal phase, deposition of
sufficient thermal energy induces an instantaneous transformation of the
heat-affected region to the cubic phase. Coupling the measured depth-resolved
BO frequency with a time and depth-resolved heat diffusion model, we detect a
reverse cubic-to-tetragonal phase transformation occurring on a time scale of
hundreds of picoseconds. We attribute this ultrafast phase transformation in
the perovskite to a structural resemblance between atomic displacements of the
R-point soft optic mode of the cubic phase and the tetragonal phase, both
characterized by anti-phase rotation of oxygen octahedra. Evidence of such a
fast structural transition in perovskites can open up new avenues in the field
of information processing and energy storage.Comment: 15 Pages, 4 Figure
First-principles determination of the phonon-point defect scattering and thermal transport due to fission products in ThO2
This work presents the first principles calculations of the lattice thermal
conductivity degradation due to point defects in thorium dioxide using an
alternative solution of the Pierels-Boltzmann transport equation. We have used
the non-perturbative Green's function methodology to compute the phonon point
defect scattering rates that consider the local distortion around the point
defect, including the mass difference changes, interatomic force constants and
structural relaxation near the point defects. The point defects considered in
the work include the vacancy of thorium and oxygen, substitution of helium,
krypton, zirconium, iodine, xenon, in the thorium site, and the three different
configuration of the Schottky defects. The results of the phonon-defect
scattering rate reveals that among the considered intrinsic defects, the
thorium vacancy and helium substitution in the thorium site scatter the phonon
most due to substantial changes in the force constant and structural
distortions. The scattering of phonons due to the substitutional defects
unveils that the zirconium atom scatters phonons the least, followed by xenon,
iodine, krypton, and helium. This is contrary to the intuition that the
scattering strength follows HeTh > KrTh > ZrTh > ITh > XeTh based on the mass
difference. This striking difference in the zirconium phonon scattering is due
to the local chemical environment changes. Zirconium is an electropositive
element with valency similar to thorium and, therefore, can bond with the
oxygen atoms, thus creating less force constant variance compared to iodine, an
electronegative element, noble gas helium, xenon, and krypton. These results
can serve as the benchmark for the analytical models and help the
engineering-scale modeling effort for nuclear design.Comment: 10 page
Phonon thermal transport in UO via self-consistent perturbation theory
Computing thermal transport from first-principles in UO is complicated
due to the challenges associated with Mott physics. Here we use irreducible
derivative approaches to compute the cubic and quartic phonon interactions in
UO from first-principles, and we perform enhanced thermal transport
computations by evaluating the phonon Green's function via self-consistent
diagrammatic perturbation theory. Our predicted phonon lifetimes at K
agree well with our inelastic neutron scattering measurements across the entire
Brillouin zone, and our thermal conductivity predictions agree well with
previous measurements. Both the changes due to thermal expansion and
self-consistent contributions are nontrivial at high temperatures, though the
effects tend to cancel, and interband transitions yield a substantial
contribution
Optical studies of structural phase transition in the vanadium-based kagome metal ScV6Sn6
In condensed matter physics, materials with kagome lattice exhibit exotic
emergent quantum states, including charge density wave (CDW), superconductivity
and magnetism. Very recently, hexagonal kagome metal ScV6Sn6 was found to
undergo fascinating first-order structural phase transition at around 92 K and
a 3x3x3 CDW modulation. The bulk electronic band properties are enlightened for
comprehending the origin of the structural phase transition. Here, we perform a
optical spectroscopy study on the monocrystalline compound across the
transition temperature. The structural transition gives rise to the abrupt
changes of optical spectra without observing gap development behavior. The
optical measurements revealed a sudden reconstruction of the band structure
after transition. We emphasize that the phase transition is of the first order
and distinctly different from the conventional density-wave type condensation.
Our results provide insight into the origin of the structural phase transition
in the new kagome metal compound.Comment: 7 pages, 4 figure
Pigments of aminophenoxazinones and viridomycins produced by termite-associated Streptomyces tanashiensis BYF-112
Termite-associated Streptomyces tanashiensis BYF-112 was found as a potential source for yellow and green pigments, which were stable under the tested temperature, light and metal ions. Eight metabolites (1–8), including four new natural yellow pigments aminophenoxazinones (1–4), and two rarely iron dependent green pigments viridomycin A and F (9–10) were isolated from BYF-112 cultured in YMS and YMS treated with FeSO4, respectively. The metabolites 2–4 displayed a significant safety performance on the normal liver cell line L-02, while the metabolite 1 showed weak cytotoxicity against the L-02 and several cancer cells. Especially, in the filter paper disc tests, the compound 1 possessed strong antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) with the zone of inhibition (ZOI) of 15.3 mm, which was equal to that of referenced levofloxacin (ZOI = 15.2 mm). And the metabolite 1 also showed moderate antibacterial activities against Micrococcus teragenus and S. aureus, with the ZOI values of 15.3 and 17.2 mm. In addition, by the minimum inhibitory concentration (MIC) assay, the compound 1 displayed potential antibacterial activities against M. teragenus, S. aureus and MRSA, with the MIC values of 12.5, 12.5, and 25.0 μg/ml, respectively. The present results indicate that BYF-112 may be a promising source for safe and bioactive pigments, which can be used for further development and industrial applications
Strong nonlinear optical response and transient symmetry switching in Type-II Weyl semimetal -WP2
The topological Weyl semimetals with peculiar band structure exhibit novel
nonlinear optical enhancement phenomena even for light at optical wavelengths.
While many intriguing nonlinear optical effects were constantly uncovered in
type-I semimetals, few experimental works focused on basic nonlinear optical
properties in type-II Weyl semimetals. Here we perform a fundamental static and
time-resolved second harmonic generation (SHG) on the three dimensional Type-II
Weyl semimetal candidate -WP. Although -WP exhibits
extremely high conductivity and an extraordinarily large mean free path, the
second harmonic generation is unscreened by conduction electrons, we observed
rather strong SHG response compared to non-topological polar metals and
archetypal ferroelectric insulators. Additionally, our time-resolved SHG
experiment traces ultrafast symmetry switching and reveals that polar metal
-WP tends to form inversion symmetric metastable state after
photo-excitation. Intense femtosecond laser pulse could optically drive
symmetry switching and tune nonlinear optical response on ultrafast timescales
although the interlayer coupling of -WP is very strong. Our work is
illuminating for the polar metal nonlinear optics and potential ultrafast
topological optoelectronic applications.Comment: 8 pages, 5 figure
Potential of Core-Collapse Supernova Neutrino Detection at JUNO
JUNO is an underground neutrino observatory under construction in Jiangmen, China. It uses 20kton liquid scintillator as target, which enables it to detect supernova burst neutrinos of a large statistics for the next galactic core-collapse supernova (CCSN) and also pre-supernova neutrinos from the nearby CCSN progenitors. All flavors of supernova burst neutrinos can be detected by JUNO via several interaction channels, including inverse beta decay, elastic scattering on electron and proton, interactions on C12 nuclei, etc. This retains the possibility for JUNO to reconstruct the energy spectra of supernova burst neutrinos of all flavors. The real time monitoring systems based on FPGA and DAQ are under development in JUNO, which allow prompt alert and trigger-less data acquisition of CCSN events. The alert performances of both monitoring systems have been thoroughly studied using simulations. Moreover, once a CCSN is tagged, the system can give fast characterizations, such as directionality and light curve
Detection of the Diffuse Supernova Neutrino Background with JUNO
As an underground multi-purpose neutrino detector with 20 kton liquid scintillator, Jiangmen Underground Neutrino Observatory (JUNO) is competitive with and complementary to the water-Cherenkov detectors on the search for the diffuse supernova neutrino background (DSNB). Typical supernova models predict 2-4 events per year within the optimal observation window in the JUNO detector. The dominant background is from the neutral-current (NC) interaction of atmospheric neutrinos with 12C nuclei, which surpasses the DSNB by more than one order of magnitude. We evaluated the systematic uncertainty of NC background from the spread of a variety of data-driven models and further developed a method to determine NC background within 15\% with {\it{in}} {\it{situ}} measurements after ten years of running. Besides, the NC-like backgrounds can be effectively suppressed by the intrinsic pulse-shape discrimination (PSD) capabilities of liquid scintillators. In this talk, I will present in detail the improvements on NC background uncertainty evaluation, PSD discriminator development, and finally, the potential of DSNB sensitivity in JUNO
Real-time Monitoring for the Next Core-Collapse Supernova in JUNO
Core-collapse supernova (CCSN) is one of the most energetic astrophysical
events in the Universe. The early and prompt detection of neutrinos before
(pre-SN) and during the SN burst is a unique opportunity to realize the
multi-messenger observation of the CCSN events. In this work, we describe the
monitoring concept and present the sensitivity of the system to the pre-SN and
SN neutrinos at the Jiangmen Underground Neutrino Observatory (JUNO), which is
a 20 kton liquid scintillator detector under construction in South China. The
real-time monitoring system is designed with both the prompt monitors on the
electronic board and online monitors at the data acquisition stage, in order to
ensure both the alert speed and alert coverage of progenitor stars. By assuming
a false alert rate of 1 per year, this monitoring system can be sensitive to
the pre-SN neutrinos up to the distance of about 1.6 (0.9) kpc and SN neutrinos
up to about 370 (360) kpc for a progenitor mass of 30 for the case
of normal (inverted) mass ordering. The pointing ability of the CCSN is
evaluated by using the accumulated event anisotropy of the inverse beta decay
interactions from pre-SN or SN neutrinos, which, along with the early alert,
can play important roles for the followup multi-messenger observations of the
next Galactic or nearby extragalactic CCSN.Comment: 24 pages, 9 figure
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