Fast radio bursts (FRBs) are millisecond-duration radio transients of unknown physical origin observed at extragalactic distances
. It has long been speculated that magnetars are the engine powering ...repeating bursts from FRB sources
, but no convincing evidence has been collected so far
. Recently, the Galactic magnetar SRG 1935+2154 entered an active phase by emitting intense soft γ-ray bursts
. One FRB-like event with two peaks (FRB 200428) and a luminosity slightly lower than the faintest extragalactic FRBs was detected from the source, in association with a soft γ-ray/hard-X-ray flare
. Here we report an eight-hour targeted radio observational campaign comprising four sessions and assisted by multi-wavelength (optical and hard-X-ray) data. During the third session, 29 soft-γ-ray repeater (SGR) bursts were detected in γ-ray energies. Throughout the observing period, we detected no single dispersed pulsed emission coincident with the arrivals of SGR bursts, but unfortunately we were not observing when the FRB was detected. The non-detection places a fluence upper limit that is eight orders of magnitude lower than the fluence of FRB 200428. Our results suggest that FRB-SGR burst associations are rare. FRBs may be highly relativistic and geometrically beamed, or FRB-like events associated with SGR bursts may have narrow spectra and characteristic frequencies outside the observed band. It is also possible that the physical conditions required to achieve coherent radiation in SGR bursts are difficult to satisfy, and that only under extreme conditions could an FRB be associated with an SGR burst.
Mg-based materials are promising candidates for high capacity hydrogen storage. However, their poor hydrogenation/dehydrogenation kinetics and high desorption temperature are the main obstacles to ...their applications. This paper reports a method for in situ formation of cycle stable CeH2.73-MgH2-Ni nanocomposites, from the hydrogenation of as-melt Mg80Ce18Ni2 alloy, with excellent hydrogen storage performance. The nanocomposites demonstrate reversible hydrogen storage capacity of more than 4.0 wt %, at a low desorption temperature with fast kinetics and long cycle life. The temperature for the full hydrogenation/dehydrogenation cycle of the composites is significantly decreased to 505 K, which is about 100 K lower than that for pure Mg. The hydrogen desorption activation energy is 63 ± 3 kJ/mol H2 for the composites, which is significantly lower than those of Mg3Ce alloy and pure Mg (104 ± 7 and 158 ± 2 kJ/mol H2, respectively). X-ray diffraction and transmission electron microscopy have been used to reveal the mechanism that delivers this excellent cycle stability and fast hydriding/dehydriding kinetics. It is found that the hydriding/dehydriding process is catalyzed by the combination of in situ formed extremely fine CeH2/CeH2.73 and Ni to Mg/MgH2. In addition, this nanocomposite structure can effectively suppress Mg/MgH2 grain growth and enable the material to maintain its high performance for more than 500 hydrogenation dehydrogenation cycles.
Large-scale, highly integrated and low-power-consuming hardware is becoming progressively more important for realizing optical neural networks (ONNs) capable of advanced optical computing. ...Traditional experimental implementations need N
units such as Mach-Zehnder interferometers (MZIs) for an input dimension N to realize typical computing operations (convolutions and matrix multiplication), resulting in limited scalability and consuming excessive power. Here, we propose the integrated diffractive optical network for implementing parallel Fourier transforms, convolution operations and application-specific optical computing using two ultracompact diffractive cells (Fourier transform operation) and only N MZIs. The footprint and energy consumption scales linearly with the input data dimension, instead of the quadratic scaling in the traditional ONN framework. A ~10-fold reduction in both footprint and energy consumption, as well as equal high accuracy with previous MZI-based ONNs was experimentally achieved for computations performed on the MNIST and Fashion-MNIST datasets. The integrated diffractive optical network (IDNN) chip demonstrates a promising avenue towards scalable and low-power-consumption optical computational chips for optical-artificial-intelligence.
Single chip integrated spectrometers are critical to bring chemical and biological sensing, spectroscopy, and spectral imaging into robust, compact and cost-effective devices. Existing on-chip ...spectrometer approaches fail to realize both high resolution and broad band. Here we demonstrate a microring resonator-assisted Fourier-transform (RAFT) spectrometer, which is realized using a tunable Mach-Zehnder interferometer (MZI) cascaded with a tunable microring resonator (MRR) to enhance the resolution, integrated with a photodetector onto a single chip. The MRR boosts the resolution to 0.47 nm, far beyond the Rayleigh criterion of the tunable MZI-based Fourier-transform spectrometer. A single channel achieves large bandwidth of ~ 90 nm with low power consumption (35 mW for MRR and 1.8 W for MZI) at the expense of degraded signal-to-noise ratio due to time-multiplexing. Integrating a RAFT element array is envisaged to dramatically extend the bandwidth for spectral analytical applications such as chemical and biological sensing, spectroscopy, image spectrometry, etc.
New non-equiatomic Ti(25+x)-Zr25-Nb25-Ta(25-x) (x = 0, 5, 10, 15, 20, in at%) medium entropy alloys (MEAs) have been designed using the atomic mismatch approach and fabricated through a conventional ...arc-melting process. These novel MEAs were derived from a recently developed equiatomic Ti-Zr-Nb-Ta MEA by gradually replacing its Ta content with Ti. Each non-equiatomic MEA solidified as a single solid-solution phase, which was characterised in detail and compared with Pandat™ simulation and empirical rules. Systematic tensile mechanical property data revealed the existence of a brittle-to-ductile transition for Ti-Zr-Nb-Ta MEAs, i.e., when 15 at% of Ta in the equiatomic Ti25-Zr25-Nb25-Ta25 MEA was replaced by Ti to become a Ti40-Zr25-Nb25-Ta10 MEA. The transition occurs corresponding to a small reduction in atomic mismatch from 4.72% to 4.65% but a signficant drop in nanoindentation hardness from 4.2 GPa to 3.5 GPa. In particular, both the as-cast Ti40-Zr25-Nb25-Ta10 and Ti45-Zr25-Nb25-Ta5 MEAs exhibited excellent tensile strain to fracture (>18%) and tensile strength (>900 MPa) with much reduced density compared to the brittle Ti25-Zr25-Nb25-Ta25 MEA. They are both among a very small number of strong and ductile (tensile strain >15%) HEAs reported to date. Their tensile mechanical properties can be further tuned by adjusting the atomic mismatch of the resulting single solid-solution phase in conjunction with the improved understanding of the microstructures of these MEAs.
Generalized Dicke models can be implemented in hybrid quantum systems built from ensembles of nitrogen-vacancy (NV) centers in diamond coupled to superconducting microwave cavities. By engineering ...cavity assisted Raman transitions between two spin states of the NV defect, a fully tunable model for collective light-matter interactions in the ultrastrong coupling limit can be obtained. Our analysis of the resulting nonequilibrium phases for a single cavity and for coupled cavity arrays shows that different superradiant phase transitions can be observed using existing experimental technologies, even in the presence of large inhomogeneous broadening of the spin ensemble. The phase diagram of the Dicke lattice model displays distinct features induced by dissipation, which can serve as a genuine experimental signature for phase transitions in driven open quantum systems.
Nickel-based superalloys are used in high strength, high-value applications, such as gas turbine discs in aero engines. In these applications the integrity of the disc is critical and therefore ...understanding crack initiation mechanisms is of high importance. With an increasing trend towards powder metallurgy routes for discs, sometimes unwanted non-metallic inclusions are introduced during manufacture. These inclusions vary in size from ∼10 μm to 200 μm which is comparable to the grain size of the nickel-based superalloys. Cracks often initiate near these inclusions, and the precise size, shape, location and path of these cracks are microstructurally sensitive. In this study, we focus on crack initiation at the microstructural length scale using a controlled three-point bend test, with the inclusion deliberately located within the tensile fibre of the beam. Electron backscatter diffraction (EBSD) is combined with high spatial resolution digital image correlation (HR-DIC) to explore full field plastic strain distributions, together with finite element modelling, to understand the micro-crack nucleation mechanisms. This full field information and controlled sample geometry enable us to systematically test crack nucleation criteria. We find that a combined stored energy and dislocation density provide promising results. These findings potentially facilitate more reliable and accurate lifing prediction tools to be developed and applied to engineering components.
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•High resolution digital image correlation.•High resolution electron backscatter diffraction.•Crack nucleation.•Non-metallic inclusion.
Selective laser melting is an additive manufacturing technology that produces metal parts; 316 L stainless steel is one of the most widely used material in the selective laser melting process. The ...tribology performance of selective laser melting processed parts is critically important for industrial applications but related studies are rare. In this study, friction and wear behaviors of 316 L stainless steel processed both by selective laser melting and traditional methods have been investigated using a ring-on-disc rig under lubricated conditions. The selective laser melting processed sample had molten pools, pores and fine grains in the microstructure, unlike the traditionally processed sample. The selective laser melting processed sample had slightly lower friction and wear than the traditionally processed sample when in contact with brass. When the selective laser melting and traditionally processed samples came in contact with a hard material, the differences in friction and wear were noticeable. The wear mechanism of the selective laser melting processed sample under the given contact condition has been proposed. Pores and refined grains in the selective laser melting processed 316 L stainless steel affected the friction and wear differently; consequently, both factors needs to be considered together.
The deglacial history of CO
release from the deep North Pacific remains unresolved. This is due to conflicting indications about subarctic Pacific ventilation changes based on various marine proxies, ...especially for Heinrich Stadial 1 (HS-1) when a rapid atmospheric CO
rise occurs. Here, we use a complex Earth System Model to investigate the deglacial North Pacific overturning and its control on ocean stratification. Our results show an enhanced intermediate-to-deep ocean stratification coeval with intensified North Pacific Intermediate Water (NPIW) formation during HS-1, compared to the Last Glacial Maximum. The stronger NPIW formation causes lower salinities and higher temperatures at intermediate depths. By lowering NPIW densities, this enlarges vertical density gradient and thus enhances intermediate-to-deep ocean stratification during HS-1. Physically, this process prevents the North Pacific deep waters from a better communication with the upper oceans, thus prolongs the existing isolation of glacial Pacific abyssal carbons during HS-1.
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