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A new type of fermion, corresponding to a three-fold degeneracy in the electronic band structure of crystalline molybdenum phosphide, is observed, which lies conceptually between Dirac and Weyl fermions. Triple-point fermions Quantum field theory predicts three types of fermion—Dirac, Weyl and Majorana—but so far only the first type has been detected experimentally as an elementary particle in high-energy physics. However, in recent years quasiparticle analogues of all three types have been observed in crystalline materials with non-trivial topological energy structure. These topological systems could potentially also host new types of fermionic quasiparticle that go beyond the standard description from quantum field theory. Hong Ding and colleagues use spectroscopic measurements to study the electronic band structure of the topological semimetal molybdenum phosphide and observe 'triple points'—states with three-fold degeneracy. These states lie conceptually between Dirac points (four-fold degeneracy) and Weyl points (two-fold degeneracy) and are associated with a new type of three-component fermionic quasiparticle. The authors also observe Weyl points in the system, which suggests that it could be used to study the interplay between different types of fermion. In quantum field theory, Lorentz invariance leads to three types of fermion—Dirac, Weyl and Majorana. Although the existence of Weyl and Majorana fermions as elementary particles in high-energy physics is debated, all three types of fermion have been proposed to exist as low-energy, long-wavelength quasiparticle excitations in condensed-matter systems 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 . The existence of Dirac and Weyl fermions in condensed-matter systems has been confirmed experimentally 13 , 14 , 15 , 16 , 17 , 18 , and that of Majorana fermions is supported by various experiments 19 , 20 . However, in condensed-matter systems, fermions in crystals are constrained by the symmetries of the 230 crystal space groups rather than by Lorentz invariance, giving rise to the possibility of finding other types of fermionic excitation that have no counterparts in high-energy physics 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 . Here we use angle-resolved photoemission spectroscopy to demonstrate the existence of a triply degenerate point in the electronic structure of crystalline molybdenum phosphide. Quasiparticle excitations near a triply degenerate point are three-component fermions, beyond the conventional Dirac–Weyl–Majorana classification, which attributes Dirac and Weyl fermions to four- and two-fold degenerate points, respectively. We also observe pairs of Weyl points in the bulk electronic structure of the crystal that coexist with the three-component fermions. This material thus represents a platform for studying the interplay between different types of fermions. Our experimental discovery opens up a way of exploring the new physics of unconventional fermions in condensed-matter systems.

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr 3 crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties. Neutron and X-ray scattering measurements provide further insight into the anharmonic behaviour of lead halide perovskites, revealing that rotations of PbBr 6 octahedra in CsPbBr 3 crystals occur in a correlated fashion along two-dimensional planes.

The International Society for Rock Mechanics (ISRM) has suggested a notched semi-circular bend technique in split Hopkinson pressure bar (SHPB) testing to determine the dynamic mode I fracture toughness of rock. Due to the transient nature of dynamic loading and limited experimental techniques, the dynamic fracture process associated with energy partitions remains far from being fully understood. In this study, the dynamic fracturing of the notched semi-circular bend rock specimen in SHPB testing is numerically simulated for the first time by the discrete element method (DEM) and evaluated in both microlevel and energy points of view. The results confirm the validity of this DEM model to reproduce the dynamic fracturing and the feasibility to simultaneously measure key dynamic rock fracture parameters, including initiation fracture toughness, fracture energy, and propagation fracture toughness. In particular, the force equilibrium of the specimen can be effectively achieved by virtue of a ramped incident pulse, and the fracture onset in the vicinity of the crack tip is found to synchronize with the peak force, both of which guarantee the quasistatic data reduction method employed to determine the dynamic fracture toughness. Moreover, the energy partition analysis indicates that simplifications, including friction energy neglect, can cause an overestimation of the propagation fracture toughness, especially under a higher loading rate.

A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs. Weyl semimetals exhibit exotic properties owing to the presence of Weyl fermions. Here, Xu et al . show that tantalum phosphide is an ideal platform for studying the transport properties of these particles because its low-energy properties are dominated by a single type of Weyl fermion.

Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co₃Sn₂S₂ and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co₃Sn₂S₂ as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.

Extensive insecticide use has led to the resistance of mosquitoes to these insecticides, posing a major barrier to mosquito control. Previous Solexa high-throughput sequencing of Culex pipiens pallens in the laboratory has revealed that the abundance of a novel microRNA (miRNA), miR-13664, was higher in a deltamethrin-sensitive (DS) strain than a deltamethrin-resistant (DR) strain. Real-time quantitative PCR revealed that the miR-13664 transcript level was lower in the DR strain than in the DS strain. MiR-13664 oversupply in the DR strain increased the susceptibility of these mosquitoes to deltamethrin, whereas inhibition of miR-13664 made the DS strain more resistant to deltamethrin. Results of bioinformatic analysis, quantitative reverse-transcriptase polymerase chain reaction, luciferase assay and miR mimic/inhibitor microinjection revealed CpCYP314A1 to be a target of miR-13664. In addition, downregulation of CpCYP314A1 expression in the DR strain reduced the resistance of mosquitoes to deltamethrin. Taken together, our results indicate that miR-13664 could regulate deltamethrin resistance by interacting with CpCYP314A1, providing new insights into mosquito resistance mechanisms.

Practical, efficient synthesis of metal oxide nanocrystals with good crystallinity and high specific surface area by a modified polymer-network gel method is demonstrated, taking ZnO nanocrystals as an example. A novel stepwise heat treatment yields significant improvement in crystal quality. Such nanophase materials can effectively degrade common organic dyes under solar radiation and can perform very well in photo-assisted detection of NO 2 gas. Other typical metal oxide nanocrystals with good crystallinity and high specific surface area were also synthesized successfully under similar conditions. This work provides a general strategy for the synthesis of metal oxide nanocrystals, balancing the crystallinity and specific surface area.

The heavily human-perturbed coastal oceans are hotspots of nitrous oxide (N 2 O) emission to the atmosphere. The processes underpinning the N 2 O flux, however, remain poorly understood, leading to large uncertainties in assessing global N 2 O budgets. Using a suite of nitrogen isotope labeling experiments, we show that multiple processes contribute to N 2 O production throughout the estuarine-coastal gradient, sustaining intensive N 2 O flux to the atmosphere. Unexpectedly, denitrification, rather than ammonia oxidation as previously assumed, constitutes the major source of N 2 O in well-oxygenated coastal waters. Size-fractionated manipulation experiments with gene analysis further reveal niche partitioning of ammonia oxidizers and denitrifiers across the particle size spectrum; denitrification dominated on large particles and ammonia oxidizers on small particles. Total N 2 O production rate increases with substrate and particle concentrations, suggesting a crucial interplay between nutrients and particles in controlling N 2 O production. The controlling factors identified here may help understand climate feedback mechanisms between human activity and coastal oceans. Incomplete denitrification associated with the micro-niche of marine particles, instead of nitrification as previously assumed, is a major source of N 2 O in the turbid coastal waters, even though the water is well-oxygenated.

Trillions of microbes inhabiting the intestine form a complex ecological community, affecting the normal physiology and pathological susceptibility through their collective metabolic activities and interactions with the host. Increased numbers of diseases have been found to be associated with disturbances in this ecosystem. There is evidence that intestinal microflora undergoes alterations in patients with irritable bowel syndrome (IBS). IBS is a frequent functional gut disease with negative impact on the patient’s quality of life. Although the etiology and pathology of IBS remain largely unknown, it is generally accepted that the interaction between the microbiota and the host is associated with IBS. However, there are no specific or effective therapies for the treatment of IBS at present. In recent years, researchers have shown a growing interest in seeking safer and more effective alternatives for the treatment of IBS by focusing their attention on the potential role of probiotics and prebiotics. In this review, we will discuss alterations in intestinal microbiota and how these alterations may exacerbate IBS, and introduce several new IBS treatment options aiming at re-establishing a healthy and beneficial ecosystem.