Topological semimetals (TSMs) refer to electronic gapless phases that exhibit topological band crossings around the Fermi level and have intrigued enormous research interest in the past few decades. There have been many theoretical and experimental progresses regarding TSMs, and first-principles calculations have been proven to be an instrumental tool in finding candidate materials for TSMs. In this tutorial, we will focus on two representative types of TSMs—Weyl and Dirac semimetals and summarize the recent progress from the perspective of first-principles calculations. First of all, the basic concepts of TSMs, the generic topological invariants, and the frequently used techniques within first-principles calculations are briefly introduced. Second, taking typical materials as representative examples, we summarize the characteristic electronic properties, formation mechanisms, and general methodologies for Weyl and Dirac semimetals, respectively. In the last part, we present a short review of recent progresses on other types of TSMs.
With the aim to achieve air-stable polyradical species manifesting strong spin coupling, synthetic endeavors are made toward triradical molecules featuring a truxene-triyl skeleton. Commonly used steric-hindering side groups such as 2,4,6-trichlorophenyl and 9-anthracenyl are both found to be incompetent at stabilizing the targeted truxene triradical, which appears to be elusive from isolation and characterization. Nonetheless, singlecrystal structures of adducts formed by relevant radicals are obtained, which strongly suggests the transient existence of the designed triradicals. Finally, a truxene triradical comprising 1-anthracenyl along with two 9-anthracenyl substituents is successfully isolated and found to exhibit decent stability in air. We propose that because of the smaller dihedral angle assumed by 1-anthracenyl with respect to the plane of truxene-triyl, more effective pi-conjugation allows the spin density to be more widely delocalized and distributed to the anthracenyl side groups. Thus, higher stability is gained by the triradical molecule.
By harnessing a highly efficient metal-catalyzed tandem cycloaddition reaction as the key benzannulation step, a series of cyclopolyarene nanorings of varied sizes are obtained from poly(arylene-butadiynylene) macrocyclic precursors, which can be synthesized relatively conveniently. Interestingly, due to the nonparallel bond connectivity of the repeat unit, unique Mobius topology is manifested by the cyclopolyarene nanorings composed of an odd number of repeat units, whereas cylindrical tubular structures with radial conjugation are formed with those consisting of an even number of repeat units.
A neuroprosthesis is a type of precision medical device that is intended to manipulate the neuronal signals of the brain in a closed-loop fashion, while simultaneously receiving stimuli from the environment and controlling some part of a human brain or body. Incoming visual information can be processed by the brain in millisecond intervals. The retina computes visual scenes and sends its output to the cortex in the form of neuronal spikes for further computation. Thus, the neuronal signal of interest for a retinal neuroprosthesis is the neuronal spike. Closed-loop computation in a neuroprosthesis includes two stages: encoding a stimulus as a neuronal signal, and decoding it back into a stimulus. In this paper, we review some of the recent progress that has been achieved in visual computation models that use spikes to analyze natural scenes that include static images and dynamic videos. We hypothesize that in order to obtain a better understanding of the computational principles in the retina, a hypercircuit view of the retina is necessary, in which the different functional network motifs that have been revealed in the cortex neuronal network are taken into consideration when interacting with the retina. The different building blocks of the retina, which include a diversity of cell types and synaptic connections—both chemical synapses and electrical synapses (gap junctions)—make the retina an ideal neuronal network for adapting the computational techniques that have been developed in artificial intelligence to model the encoding and decoding of visual scenes. An overall systems approach to visual computation with neuronal spikes is necessary in order to advance the next generation of retinal neuroprosthesis as an artificial visual system.
We report a continuous record of surface ozone (O-3) in urban Beijing, China, from 2013 to 2019. A linear fit to the 7-year record shows that the annual MDA8-O-3 (the maximum daily average of 8-h O-3 concentration) and annual average O-3 increased by 2.30 and 1.91 ppbv yr(-1) (p < 0.05), respectively. Both the MDA8-O-3 level and the number of exceeding days are increased, demonstrating the surface O-3 pollution in Beijing is increasingly serious. An overall decrease in annual surface NO2 was observed at a rate of -1.21 ppbv yr(-1) (p < 0.01). The total oxidants (O-x, = NO2 + O-3) had an upward trend during 2013-2019 at a rate of 0.70 ppbv h(-1) (p = 0.168). The increasing O-3 and Or trends imply the atmospheric oxidation capacity is increasing in Beijing, even though the strict emission policies have been implemented. The periodical changes of surface O-3 in different time scales are studied. We found that the increases in O-3 are mainly at a high O-3 level with a threshold of 30 ppbv. The relative diurnal variability of surface O-3 is weakened, with a decrease in the diurnal amplitude variation. Both the extremely low and high 5% surface O-3 are increased, indicates an overall uplift of surface O-3. The weekday periodic trends showed an increment of weekend MDA8-O-3 (2.2 ppbv on average) and companies with a decrement of weekend NO2 (1.5 ppbv on average). The weekend effect provides a chance to look insights into reducing O-3 exceeding days during summertime and proposes the need for emission abatements of volatile organic compounds to the mitigation of ozone pollution in Beijing.
Perovskite solar cells have attracted great research interest as a promising candidate for silicon solar cells. Plenty of work has been reported to use perovskites to semitransparent windows and transparent photovoltaic (TPV) devices to obtain multifunctional systems. However, the narrow bandgap and sharp absorption edge of the typical perovskites prevent them from achieving the highest transparency to satisfy the requirements of aesthetic and integration, and the poor stability and toxic Pb compositions hinder their practical application. Herein, lead-free halide double perovskites with a wide bandgap and indirect bandgap characteristics is introduced to fabricate long-term stable transparent photovoltaic devices exhibiting high visible transmittance (73%) and considerable energy conversion efficiency (1.56%). Through further theoretical calculation and evaluation, a new strategy using indirect bandgap material on TPV devices is proposed to combine the enhancement of these two parameters. This approach will be a significant compliment to near-infrared-absorbing solar cells to selectively harvest light in the invisible region to obtain highly performing multi-junction smart windows on buildings, vehicles and mobile electronics, providing a new reasonable idea to realize TPVs with high efficiency and transparency simultaneously.