Theoretically long gamma-ray bursts (GRBs) are expected to happen in low-metallicity environments, because in a single massive star scenario, low iron abundance prevents loss of angular momentum ...through stellar wind, resulting in ultra-relativistic jets and the burst. In this sense, not just a simple metallicity measurement but also low iron abundance (Fe/H −1.0) is essentially important. Observationally, however, oxygen abundance has been measured more often due to stronger emission. In terms of oxygen abundance, some GRBs have been reported to be hosted by high-metallicity star-forming galaxies, in tension with theoretical predictions. Here we compare iron and oxygen abundances for the first time for GRB host galaxies (GRB 980425 and 080517) based on the emission-line diagnostics. The estimated total iron abundances, including iron in both gas and dust, are well below the solar value. The total iron abundances can be explained by the typical value of theoretical predictions (Fe/H −1.0), despite high oxygen abundance in one of them. According to our iron abundance measurements, the single massive star scenario still survives even if the oxygen abundance of the host is very high, such as the solar value. Relying only on oxygen abundance could mislead us on the origin of the GRBs. The measured oxygen-to-iron ratios, O/Fe, can be comparable to the highest values among the iron-measured galaxies in the Sloan Digital Sky Survey. Possible theoretical explanations of such high O/Fe include the young age of the hosts, top-heavy initial mass function, and fallback mechanism of the iron element in supernova explosions.
Abstract
Recently, theoretical studies show that layered HfTe
5
is at the boundary of weak & strong topological insulator (TI) and might crossover to a Dirac semimetal state by changing lattice ...parameters. The topological properties of 3D stacked HfTe
5
are expected hence to be sensitive to pressures tuning. Here, we report pressure induced phase evolution in both electronic & crystal structures for HfTe
5
with a culmination of pressure induced superconductivity. Our experiments indicated that the temperature for anomaly resistance peak (Tp) due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI. The HfTe
5
crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum. The highest superconducting transition temperature (Tc) around 5 K was achieved at 20 GPa. Crystal structure studies indicate that HfTe
5
transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure. Based on transport, structure studies a comprehensive phase diagram of HfTe
5
is constructed as function of pressure. The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.
The two-dimensional semiconductor 1T-HfSe2 is found to have highly tunable transport properties under pressure including metallization and pressure-driven superconductivity. The temperature-dependent ...resistivity of the sample suggests that a charge-density wave (CDW) state exists at low pressures in HfSe2, but it is suppressed below 20 GPa. It is further found that metallization takes place at ∼24 GPa followed by the appearance of a superconducting state at 26 GPa with a Tc of 6.7 K. Upon further compression to 33 GPa, the Tc increases monotonically to 7.5 K. Raman spectra, x-ray diffraction, transport measurements, and density-functional theory calculations suggest that the occurrence of the pressure-induced metallization and superconducting transition are intimately linked to a structural phase transition from the trigonal (P 3‾ m1) to a hexagonal structure (P63/mmc). Because of the phase transition, a massive structural reconstruction, and substantial band-structure changes around the Fermi level take place, which are due to the modification of weak van der Waals forces. The pressure-induced manipulation of the transport properties of 1T-HfSe2 could provide crucial information towards its practical applications.
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•1T-HfSe2 is found to have highly tunable transport properties under pressure including metallization and superconductivity.•A charge-density wave state is found to exist at low pressures in HfSe2 below 20 GPa.•The metallization takes place at ∼24 GPa followed by the bulk superconductivity with a Tc of 6.7 K at 26 GPa.•The metallization and superconducting transition are linked to a phase transition from the trigonal to a hexagonal structure.
Magnesium silicates are the dominant minerals in the earth's mantle. Their preferred orientation is important for understanding the rheology and seismic anisotropy in the deep earth. Here we report ...results of radial synchrotron diffraction diamond anvil cell (DAC) experiments on San Carlos olivine, axially compressed to 50 GPa. Experiments were performed at room temperature, except for brief laser heating to induce phase transformations. High stresses and development of preferred orientation were observed in diffraction images. Quantitative texture information was obtained by analyzing the images with the Rietveld method. With increasing pressure (between 9 and 43 GPa) olivine develops a texture with 001 axes perpendicular to the compression direction that is compatible with {hk0}001 pencil glide. Ringwoodite (between 20 and 50 GPa) develops weak preferred orientation with {011} lattice planes perpendicular to the compression direction. After the phase transformation, perovskite and magnesiowüstite display transformation textures that are then modified by continuing deformation. Magnesiowüstite has a weak 〈111〉 maximum parallel to the compression direction that changes towards 〈001〉 with increasing deformation. Perovskite, transforming from olivine, has a pronounced (100) transformation texture and with increasing deformation a {012} maximum develops. The transformation texture is probably produced by mechanical {110} twinning and nucleation in orientations that minimize elastic strain energy. The deformation texture of perovskite is due to slip.
Crop production is a large source of atmospheric ammonia (NH
), which poses risks to air quality, human health and ecosystems
. However, estimating global NH
emissions from croplands is subject to ...uncertainties because of data limitations, thereby limiting the accurate identification of mitigation options and efficacy
. Here we develop a machine learning model for generating crop-specific and spatially explicit NH
emission factors globally (5-arcmin resolution) based on a compiled dataset of field observations. We show that global NH
emissions from rice, wheat and maize fields in 2018 were 4.3 ± 1.0 Tg N yr
, lower than previous estimates that did not fully consider fertilizer management practices
. Furthermore, spatially optimizing fertilizer management, as guided by the machine learning model, has the potential to reduce the NH
emissions by about 38% (1.6 ± 0.4 Tg N yr
) without altering total fertilizer nitrogen inputs. Specifically, we estimate potential NH
emissions reductions of 47% (44-56%) for rice, 27% (24-28%) for maize and 26% (20-28%) for wheat cultivation, respectively. Under future climate change scenarios, we estimate that NH
emissions could increase by 4.0 ± 2.7% under SSP1-2.6 and 5.5 ± 5.7% under SSP5-8.5 by 2030-2060. However, targeted fertilizer management has the potential to mitigate these increases.
Superconductivity in Boron Eremets, Mikhail I.; Struzhkin, Viktor V.; Mao, Ho-kwang ...
Science (American Association for the Advancement of Science),
07/2001, Letnik:
293, Številka:
5528
Journal Article
Recenzirano
Metals formed from light elements are predicted to exhibit intriguing states of electronic order. Of these materials, those containing boron are of considerable current interest because of their ...relatively high superconducting temperatures. We have investigated elemental boron to very high pressure using diamond anvil cell electrical conductivity techniques. We find that boron transforms from a nonmetal to a superconductor at about 160 gigapascals (GPa). The critical temperature of the transition increases from 6 kelvin (K) at 175 GPa to 11.2 K at 250 GPa, giving a positive pressure derivative of 0.05 K/GPa. Although the observed metallization pressure is compatible with the predictions of first-principles calculations, superconductivity in boron remains to be explored theoretically. The present results constitute a record pressure for both electrical conductivity studies and investigations of superconductivity in dense matter.
Hydrogen Storage in Molecular Compounds Mao, Wendy L.; Mao, Ho-kwang
Proceedings of the National Academy of Sciences,
01/2004, Letnik:
101, Številka:
3
Journal Article
Recenzirano
Odprti dostop
At low temperature (T) and high pressure (P), gas molecules can be held in ice cages to form crystalline molecular compounds that may have application for energy storage. We synthesized a hydrogen ...clathrate hydrate, H2(H2O)2, that holds 50 g/liter hydrogen by volume or 5.3 wt %. The clathrate, synthesized at 200-300 MPa and 240-249 K, can be preserved to ambient P at 77 K. The stored hydrogen is released when the clathrate is warmed to 140 K at ambient P. Low T also stabilizes other molecular compounds containing large amounts of molecular hydrogen, although not to ambient P, e.g., the stability field for H2(H2O) filled ice (11.2 wt % molecular hydrogen) is extended from 2,300 MPa at 300 K to 600 MPa at 190 K, and that for (H2)4CH
4(33.4 wt % molecular hydrogen) is extended from 5,000 MPa at 300 K to 200 MPa at 77 K. These unique characteristics show the potential of developing low-T molecular crystalline compounds as a new means for hydrogen storage.
Abstract
Purpose
To estimate the direct and indirect costs of end-stage renal disease (ESRD) patients in the first and second years of initiating peritoneal dialysis (PD), hospital-based ...haemodialysis (HD) and nocturnal home HD.
Methods
A cost analysis was performed to estimate the annual costs of PD, hospital-based HD and nocturnal home HD for ESRD patients from both the health service provider’s and societal perspectives. Empirical data on healthcare resource use, patients’ out-of-pocket costs, time spent on transportation and dialysis by ESRD patients and time spent by caregivers were analysed. All costs were expressed in Hong Kong year 2017 dollars.
Results
Analysis was based on 402 ESRD patients on maintenance dialysis (PD: 189; hospital-based HD: 170; and nocturnal home HD: 43). From the perspective of the healthcare provider, hospital-based HD had the highest total annual direct medical costs in the initial year (mean ± SD) (hospital-based HD = $400 057 ± 62 822; PD = $118 467 ± 15 559; nocturnal home HD = $223 358 ± 18 055; P < 0.001) and second year (hospital-based HD = $360 924 ± 63 014; PD = $80 796 ± 15 820; nocturnal home HD = $87 028 ± 9059; P < 0.001). From the societal perspective, hospital-based HD had the highest total annual costs in the initial year (hospital-based HD = $452 151 ± 73 327; PD = $189 191 ± 61 735; nocturnal home HD = $242 038 ± 28 281; P < 0.001) and second year (hospital-based HD = $413 017 ± 73 501; PD = $151 520 ± 60 353; nocturnal home HD = $105 708 ± 23 853; P < 0.001).
Conclusions
This study quantified the economic burden of ESRD patients, and assessed the annual healthcare and societal costs in the initial and second years of PD, hospital-based HD and nocturnal home HD in Hong Kong. From both perspectives, PD is cost-saving relative to hospital-based HD and nocturnal home HD, except that nocturnal home HD has the lowest cost in the second year of treatment from the societal perspective. Results from this cost analysis facilitate economic evaluation in Hong Kong for health services and management targeted at ESRD patients.
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