Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies 1 , is ubiquitous in modern photonic systems.However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material, and so far, dynamical control of optical nonlinearities remains confined to research labs as a spectroscopic tool 2 . Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe2, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-

In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I ¼ 2 for excitons inside the light cone, that is, bright excitons. Under moderate strain, the I ¼ 2 Dirac cone splits into two degenerate I ¼ 1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton's optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin.

Metallic glasses attract considerable interest due to their unique combination of superb properties and processability. Predicting their formation from known alloy parameters remains the major hindrance to the discovery of new systems. Here, we propose a descriptor based on the heuristics that structural and energetic ‘confusion' obstructs crystalline growth, and demonstrate its validity by experiments on two well-known glass-forming alloy systems. We then develop a robust model for predicting glass formation ability based on the geometrical and energetic features of crystalline phases calculated ab initio in the AFLOW framework. Our findings indicate that the formation of metallic glass phases could be much more common than currently thought, with more than 17% of binary alloy systems potential glass formers. Our approach pinpoints favourable compositions and demonstrates that smart descriptors, based solely on alloy properties available in online repositories, offer the sought-after key for accelerated discovery of metallic glasses.

electricity. Electrochemical water splitting includes oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Compared with HER, OER is more sluggish owing to the multiple electrons transfer and is thus mainly responsible for the high overpotential of electrochemical water splitting. [1b] To reduce the overpotential, electrocatalysts are required to accelerate the OER.Among the developed non-noble-metalbased electrocatalysts for OER. [2] Layered double hydroxides (LDH) are attractive due to their high electrocatalytic activity. [1b,3] Compared with powdery LDH, self-supported LDH is more desirable due to the simplified electrode preparation and the intimate contact with current collectors. [4] Hydrothermal, [5] electrodeposition, [6] and corrosion engineering [7] methods have been adopted to prepare self-supported LDH. Among these methods, corrosion engineering is more promising due to the absence of intentional energy input, mild reaction conditions, simple instruments, and high scalability. [8] By adjusting the substrate and corrosion solution, various self-supported LDH can be obtained via corrosion engineering. Self-supported NiCo-LDH, [9] NiCoRu-LDH, [9b] and NiFe-LDH [10] were prepared by the corrosion of nickel foam in CoCl 2 , CoCl 2 &RuCl 3 , and FeCl 2 or Fe(NO 3 ) 3 solution, respectively. Other than the nickel foam, iron foam and nickel-iron foam were also used to prepare different self-supported LDH among which the NiFe-LDH on iron foam (NiFe-LDH@IF) exhibited the best electrocatalytic performance toward OER. [11] For example, Liu reported that the NiFe-LDH@IF required only 340 mV to reach 1000 mA cm -2 where it remained stable for more than 6000 h. [7a] This is the best reported electrocatalytic stability so far. [8] However, the further optimization of the growth and electrocatalytic performance of the NiFe-LDH@IF is hindered by some fundamental issues. Based on previous reports, the growth mechanism of the NiFe-LDH@IF remains controvers ial, [7a,8,11c,12] and the growth process is still unclear. Moreover, the effects of corrosion conditions on the growth and electrocatalytic performance of the NiFe-LDH@IF are not fully understood.To address these issues, we systematically characterized the NiFe-LDH@IF prepared under different corrosion conditions. Based on these results, we elucidate the growth mechanism For sustainable hydrogen production, electrochemical water splitting is a promising method whose efficiency is limited by its anodic reaction, i.e., the oxygen evolution reaction (OER). One of the best electrocatalysts for the OER is the self-supported nickel-iron layered double hydroxides on iron foam (NiFe-LDH@IF) prepared by corrosion engineering. However, the further development of NiFe-LDH@IF is hampered by a lack of understanding regarding the growth mechanism and the effects of corrosion conditions on the electrocatalytic activity. Herein, the growth mechanism is studied, revealing for the first time that NiFe-LDH@IF is formed by the preferential precipitation of Ni...

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A series of noble metal high entropy alloys with up to six constituent elements has been produced by casting. PtPdRhIrCuNi forms single-phase face-centered cubic solid solution, and its stability is confirmed by annealing experiments. This alloy deforms homogeneously to ~30% to a high ultimate compression strength of 1839 MPa. We discuss rules for the formation of single-phase solid solution.

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