ABSTRACT
High time resolution and accuracy are of critical importance in the studies of timing analysis and time delay localization of gamma-ray bursts (GRBs), soft gamma-ray repeaters (SGRs) and ...pulsars. The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) consisting of two micro-satellites, GECAM-A and GECAM-B, launched on 2020 December 10, is aimed at monitoring and locating X-ray and GRBs all over the sky. To achieve its scientific goals, GECAM is designed to have the highest time resolution (0.1 $\mu {\rm s}$) among all GRB detectors ever flown. Here, we make a comprehensive time calibration campaign including both on-ground and on-orbit tests to derive not only the relative time accuracy of GECAM satellites and detectors, but also the absolute time accuracy of GECAM-B. Using the on-ground calibration with a $\rm ^{22}Na$ radioactive source, we find that the relative time accuracy between GECAM-A and GECAM-B is about 0.15 $\mu {\rm s}$ (1σ). To measure the relative time accuracy between all detectors of a single GECAM satellite, cosmic-ray events detected on orbit are utilized since they could produce many secondary particles simultaneously record by multiple detectors. We find that the relative time accuracy among all detectors onboard GECAM-B is about 0.12 $\mu {\rm s}$ (1σ). Finally, we use the novel Li-CCF method to perform the absolute time calibration with Crab pulsar and SGR J1935+2154, both of which were jointly observed by GECAM-B and Fermi/GBM, and obtain that the time difference between GECAM-B and Fermi/GBM is 3.06 ± 6.04 $\mu {\rm s}$ (1σ).
The Tibetan Plateau is the highest and one of the most demanding environments ever inhabited by humans. We investigated the timing and mechanisms of its initial colonization at the Nwya Devu site, ...located nearly 4600 meters above sea level. This site, dating from 40,000 to 30,000 years ago, is the highest Paleolithic archaeological site yet identified globally. Nwya Devu has yielded an abundant blade tool assemblage, indicating hitherto-unknown capacities for the survival of modern humans who camped in this environment. This site deepens the history of the peopling of the "roof of the world" and the antiquity of human high-altitude occupations more generally.
PrBi, a sister member of the rare-earth monopnictide family, is an excellent candidate for studying extreme magnetoresistance and nontrivial topological electronic states. In this study, we perform ...angular magnetoresistance measurements as well as bulk and surface band structure calculations on this compound. PrBi's magnetoresistance is revealed to be significantly angle-dependent and shows a fourfold symmetry as always observed in the nonmagnetic isostructural counterparts, including LaSb, LaBi, and LuBi. Its angular magnetoresistance can be reproduced well using the semiclassical two-band model. The deduced parameters suggest that PrBi hosts an elongated electron pocket with a mobility anisotropy of 3.13 and is slightly uncompensated in its carrier concentration. Our bulk and surface band structure calculations confirm the anisotropic electronic features. Moreover, we reveal that a nodal-line-shaped surface state appears at the
X&cmb.macr;
point, and is associated with the quadratic dispersion along the
&z.Ggrm;
X&cmb.macr;
direction, and the linear type-I Dirac dispersion along the
X&cmb.macr;
M&cmb.macr;
direction. Owing to the type-I Dirac dispersion feature, PrBi could serve as a promising material platform for studying many unexpected physical properties, such as the highly anisotropic transport and valley polarization of electrons.
PrBi shows extreme and anisotropic magnetoresistance as well as nontrivial electronic band structures with a nodal-line-shaped surface state at the
X&cmb.macr;
point.
•MnCo2O4 nanowire array is prepared by a fast and facile hydrothermal method.•MnCo2O4 nanowire array exhibits noticeable pseudocapacitive properties.•The as-prepared nanowire array is also a ...promising material for Li-ion batteries.
One-dimension MnCo2O4 nanowire arrays are synthesized on nickel foam by a facile hydrothermal method. The MnCo2O4 nanowires are highly crystalline with an average diameter of 70nm and exhibit excellent properties for electrochemical energy storage. Impressively, the MnCo2O4 nanowire array exhibits noticeable pseudocapacitive performance with a high capacitance of 349.8 F g−1 at 1 A g−1 and 328.9 F g−1 at 20 A g−1 as well as excellent cycling stability. As an anode material for Li-ion batteries, the MnCo2O4 nanowire array delivers an initial specific discharge capacity of 1288.6 mAh g−1 at 100mAg−1, with reversible capacity retention of 92.7% after 50 cycles. The outstanding electrochemical performances are mainly attributed to its nanowire array architecture which provides large reaction surface area, fast ion and electron transfer and good structure stability.
We study the process e^{+}e^{-}→Λ_{c}^{+}Λover ¯_{c}^{-} at twelve center-of-mass energies from 4.6119 to 4.9509 GeV using data samples collected by the BESIII detector at the BEPCII collider. The ...Born cross sections and effective form factors (|G_{eff}|) are determined with unprecedented precision after combining the single and double-tag methods based on the decay process Λ_{c}^{+}→pK^{-}π^{+}. Flat cross sections around 4.63 GeV are obtained and no indication of the resonant structure Y(4630), as reported by Belle, is found. In addition, no oscillatory behavior is discerned in the |G_{eff}| energy dependence of Λ_{c}^{+}, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the Λ_{c}^{+} baryon at each energy point, the moduli of electric and magnetic form factors (|G_{E}| and |G_{M}|) are extracted and separated. For the first time, the energy dependence of the form factor ratio |G_{E}/G_{M}| is observed, which can be well described by an oscillatory function.
Mesoporous n-type hematite bunched nanowires (α-Fe2O3 BNWs) and Zn-, In- and Sn-doped α-Fe2O3 BNWs were synthesized by nanocasting method, and then the influence of the different-valence ...metals-doping on the microstructures, components and gas-sensing properties is discussed in detail. All samples present mesoporous-structure and are composed of the uniform α-Fe2O3 nanowires. The Zn, Fe, Sn and In-doped concentration decreased with the increasing radius of their hydrated ions. The ethanol gas-sensing results indicate that the different-valence metals-doping greatly affects the response of α-Fe2O3 BNWs sensors to ethanol gas. The sensitivity to 100 ppm ethanol gas increased from 10.046 for Fe1.896Zn0.104O2.948 BNWs sensor up to 45.556 for Fe1.938Sn0.062O3.031 BNWs sensor with the increasing metal valence. The high-valence Sn-doping not only decreases the ground state resistance, but also reduces barrier and work function of α-Fe2O3 BNWs, which results in the improved the sensitivity of α-Fe2O3 BNWs.
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•All samples present mesoporous-structure with the similar microstructures.•The doped concentration decreased with the increasing radius of their hydrated ions.•Metal-doping greatly affects the resistance and the gas-sensing properties of all sensors.•The high-valence doping reduces barrier and work function of α-Fe2O3 BNWs.•The Sn-doping supplies more inner electron and adsorbs more O− at surface in air.
Flow over aligned and staggered cube arrays is a classic model problem for rough-wall turbulent boundary layers. Earlier studies of this model problem mainly looked at rough surfaces with a moderate ...coverage density, i.e.
$\unicodeSTIX{x1D706}_{p}>O(3\,\%)$
, where
$\unicodeSTIX{x1D706}_{p}$
is the surface coverage density and is defined to be the ratio between the area occupied by the roughness and the total ground area. At lower surface coverage densities, i.e.
$\unicodeSTIX{x1D706}_{p}<O(3\,\%)$
, it is conventionally thought that cubical roughness acts like isolated roughness elements; and that the single-cube drag coefficient, i.e.
$C_{d}\equiv f/(\unicodeSTIX{x1D70C}U_{h}^{2}h^{2})$
, equals
$C_{R}$
. Here,
$f$
is the drag force on one cubical roughness element,
$\unicodeSTIX{x1D70C}=\text{const.}$
is the fluid density,
$h$
is the height of the cube,
$U_{h}$
is the spatially and temporally averaged wind speed at the cube height, and
$C_{R}$
is the drag coefficient of an isolated cube. In this work, we conduct large-eddy simulations and direct numerical simulations of flow over wall-mounted cubes with very low surface coverage densities, i.e.
$0.08\,\%<\unicodeSTIX{x1D706}_{p}<4.4\,\%$
. The large-eddy simulations are at nominally infinite Reynolds numbers. The results challenge the conventional thinking, and we show that, at very low surface coverage densities, the single-cube drag coefficient may increase as a function of
$\unicodeSTIX{x1D706}_{p}$
. Our analysis suggests that this behaviour may be attributed to secondary turbulent flows. Secondary turbulent flows are often found above spanwise-heterogeneous roughness. Although the roughness considered in this work is nominally homogeneous, the secondary flows in our simulations are very similar to those observed above spanwise-heterogeneous surface roughness. These secondary vortices redistribute the fluid momentum in the outer layer, leading to high-momentum pathways above the wall-mounted cubes and low-momentum pathways at the two sides of the wall-mounted cubes. As a result, the spatially and temporally averaged wind speed at the cube height, i.e.
$U_{h}$
, is an underestimate of the incoming flow to the cubes, which in turn leads to a large drag coefficient
$C_{d}$
.
Based on electron-positron collision data collected with the BESIII detector operating at the Beijing Electron-Positron Collider II storage rings, the value of ...R≡σ(e^{+}e^{-}→hadrons)/σ(e^{+}e^{-}→μ^{+}μ^{-}) is measured at 14 center-of-mass energies from 2.2324 to 3.6710 GeV. The resulting uncertainties are less than 3.0% and are dominated by systematic uncertainties.
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