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Van Hove singularity (VHS) has been considered as a driving source for unconventional superconductivity. A VHS in two-dimensional (2D) materials consists of a saddle point connecting electron-like and hole-like bands. In a rare case, when a VHS appears at Fermi level, both electron-like and hole-like conduction can coexist, giving rise to an enhanced density of states as well as an attractive component of Coulomb interaction for unconventional electronic pairing. However, this van Hove scenario is often destroyed by an incorrect chemical potential or competing instabilities. Here, by using angle-resolved photoemission measurements, we report the observation of a VHS perfectly aligned with the Fermi level in a kagome superconductor CsV3-xTaxSb5 (x ~ 0.4), in which a record-high superconducting transition temperature is achieved among all the current variants of AV3Sb5 (A = Cs, Rb, K) at ambient pressure. Doping dependent measurements reveal the important role of van Hove scenario in boosting superconductivity, and spectroscopic-imaging scanning tunneling microscopy measurements indicate a distinct superconducting state in this system. The authors present ARPES and STM/STS measurements of the kagome superconductor CsV3-xTaxSb5. For the x = 0.4 Ta-doped sample, they report evidence for a van Hove singularity perfectly aligned with the Fermi level, and that the superconducting transition temperature is maximized at this doping. [ABSTRACT FROM AUTHOR]

Recent research has demonstrated that the CuFeTe2 compound has interesting physical properties. However, less research effort has been afforded to the study of the stable non-stoichiometric Cu1.1Fe1.2Te2 structure and electronic properties. In this paper, we report the synthesis and electronic properties of the non-stoichiometric (Cu, Fe) co-doped CuFeTe2 (Cu1.1Fe1.2Te2) with a stable single phase. The temperature-dependent resistance and magnetic susceptibility measurements showed an obvious phase transition from a semiconductor to metal when the temperature reached 360 K, revealing a spin-density-wave (SDW)-like state existing in the as-grown Cu1.1Fe1.2Te2, which has a higher SDW transition temperature than that of CuFeTe2. Interestingly, a spectral weight evolution of the angle-resolved photoemission spectroscopy in the study of the electronic structure of Cu1.1Fe1.2Te2 is observed at the same temperature of 360 K. Our work provides a scheme for exploration of novel physical phenomena related to SDW in two-dimensional materials. [ABSTRACT FROM AUTHOR]

We describe herein a stepwise approach to accomplish the covalent linkage of fullerenes C60 and C70 to few-layer Ti3C2Tx MXene, for the first time. The preparation process involves the functionalization of Ti3C2Tx with 4-benzaldehyde diazonium salt followed by 1,3-dipolar cycloaddition to the double bonds of C60 and C70 in the presence of aldehyde-decorated nanosheets and excess N-methylglycine. The synthesized nanohybrids, named C60-MXene and C70-MXene, respectively, were comprehensively characterized by crystallographic, spectroscopic, thermal, and electron microscopic means. UV-vis-NIR absorption measurements reveal a decrease in the electron density of the nanosheets upon the incorporation of fullerenes, indicating the occurrence of intercomponent electronic interactions. Z-scan studies demonstrate that C60-MXene and C70-MXene show an excellent nonlinear absorption performance, that is, an enhanced optical limiting (OL) effect compared to their individual components, under the irradiation of both nanosecond (ns) pulses at 532 nm and femtosecond (fs) pulses at 800 nm. The effective electron transfer behavior from Ti3C2Tx to the grafted fullerenes under photoexcitation should be responsible for the observed OL enhancement. Our findings showcase that C60-MXene and C70-MXene nanohybrids are promising OL candidates across spectral regions and temporal domains, and that the proposed strategy of interfacing fullerenes with Ti3C2Txvia covalent bonding provides a paradigm for engineering high-performance nonlinear optical (NLO) materials using covalently linked binary nanohybrids based on MXene nanosheets. [ABSTRACT FROM AUTHOR]

Competitive optical limiters that can protect sensitive instruments and eyes from intense ultrafast laser irradiation across a wide spectral range are key to the expanding use of ultrafast pulsed lasers. Herein, for the first time, we report a facile and covalent diazonium approach to the functionalization of tin disulfide (SnS2) nanosheets with organic porphyrins, in the pursuit of excellent optical limiting performance. The SnS2–porphyrin nanohybrid (SnS2–Por) was assembled by reacting SnS2 nanosheets with a porphyrin diazonium salt that had been chemically activated with the reducing agent potassium iodide. In the femtosecond (fs) regime, the SnS2–Por nanohybrid shows significantly enhanced reverse saturable absorption and effective nonlinear optical absorption coefficients, which are 2.8- to 5.8-fold higher than those of pristine SnS2 nanosheets under identical laser irradiation; the optical limiting thresholds of the SnS2–Por nanohybrid are 2736 μJ cm−2 at 800 nm and 1650 μJ cm−2 at 515 nm, which outstrip most reported fs-active NLO materials (e.g., WS2, InSe, carbon nanodots, and graphene oxide), suggesting promising applications in ultrafast optical limiting. A dynamic energy model was proposed to demonstrate the relationship between charge transfer in the excited states of the SnS2–Por nanohybrid and the nonlinear optical performance. This work not only offers a paradigm for facile covalent functionalization of SnS2, but it also sheds new light on the construction of nanohybrids with excellent optical limiting performance. [ABSTRACT FROM AUTHOR]

The initial delamination defects may arise during the preparation of carbon fiber‐reinforced polymer (CFRP) composite laminates, so its effects need to be considered in the evolution of mechanical properties. In this article, the behaviors of composite laminates with initial delamination defects under impact and compression after impact (CAI) are investigated by numerical method. The simulation framework composed of the micromechanics of failure (MMF) theory and the mixed mode exponential cohesive zone model (ECZM) is constructed. The numerical analysis compares the effects of the interaction between four different initial delamination defects and impact loading on impact response and CAI strength at two impact velocity levels (1.336 and 2.314 m/s). Furthermore, the failure mechanisms of laminates with initial delamination defect are disclosed under CAI loading. The findings indicate that the difference in impact response is minor. The CAI strength is reduced from 250.5/173.5 to 191.5 MPa/139.1 MPa. The sufficient condition for the further significant reduction of CAI strength is that the initial delamination defects merge with the impact‐induced delamination cracks in the width direction of laminates. Moreover, the failure mechanism for CAI condition is mostly matrix damage and interlaminar crack propagation. This work provides a valuable reference for engineering failure analysis of composite laminates. [ABSTRACT FROM AUTHOR]

Covalent functionalization of two-dimensional transition-metal dichalcogenides (TMDs) enables the manipulation of optical and electronic properties in hybrid structures and enhances their versatility for various potential applications. Herein, exfoliated few-layer TiS2 is covalently functionalized by tetraphenylporphyrin (TPP) diazonium salts for the first time. The newly synthesized TPP–TiS2 nanohybrid is fully characterized by crystallographic, spectroscopic, microscopic, and thermal methods. Its excited-state behavior is investigated by steady-state and time-resolved photoluminescence measurements, indicating the formation of a donor–acceptor (D–A) system, in which an efficient energy transfer from the photoexcited TPP to TiS2 layer is expected to occur. Z-scan studies show a significantly enhanced saturation absorption (SA) of the TPP–TiS2 nanohybrid in comparison with that of pure TiS2 at 532 nm under nanosecond (ns) pulses, though TPP exhibits typical reverse saturation absorption (RSA). The reinforced band-filling effect due to intrahybrid energy transfer may play an important role in determining the nonlinear absorption performance of TPP–TiS2. Our results shed new light on the preparation of a TiS2-derived nanohybrid by constructing a suitable D–A binary system, which is important in developing photonic devices such as mode-locked or Q-switched lasers. [ABSTRACT FROM AUTHOR]