Search

Chiral quasiparticles, such as magnetic skyrmions, are fundamental phenomena in the field of condensed‐matter physics. So far, several chiral quasiparticles with nontrivial topology have been theoretically predicted and experimentally confirmed, most characterized by carrying a nonzero topological charge. Here, through model analysis, a mechanism of realizing chiral magnetic quasiparticles with zero topological charge in the 2D lattice is proposed. The physics is related to the marriage of layer‐dependent staggered Dzyaloshinskii‐Moriya interaction and interlayer exchange interaction, and the nontrivial topology is protected indirectly. By means of first‐principles calculations and atomic spin model simulations, this mechanism in a real material of bilayer Tl2NO2 is further demonstrated. The realized chiral quasiparticles with zero topological charge can be considered as the superposition of vortex and anti‐vortex. Even though the topological charge is zero, their topologies are nontrivial, indirectly protected by the correlated magnetic bimerons. The finding opens up a new direction for the research on topological spin structures in 2D lattice. [ABSTRACT FROM AUTHOR]

2D magnetic semiconductors exhibit great potential for next‐generation spintronics, but realizing their full capabilities has been hindered by the low Curie temperatures (Tc) below 50 K observed in current materials. Here, a new mechanism to substantially enhance the Tc of 2D semiconducting materials through incorporating both in‐plane and out‐of‐plane superexchange interactions enabled by structural design is demonstrated. Specifically, monolayer Cr2Se3 is synthesized with a five‐layer Se–Cr–Se–Cr–Se atomic structure using molecular beam epitaxy (MBE). This unique structure not only possesses optimized in‐plane superexchange interaction but also incorporates out‐of‐plane Cr–Se–Cr couplings. Scanning tunneling spectroscopy (STS) and angular‐resolved photoemission spectroscopy (ARPES) confirm its semiconducting nature. Remarkably, the ferromagnetic phase transition observed by ARPES and Magnetic Force Microscopy (MFM) indicated that its Tc is up to 230 K. This not only establishes a new record for Tc in 2D ferromagnetic semiconductor materials but also introduces a novel approach to modulating materials' properties by manipulating the vertical dimension in 2D materials. [ABSTRACT FROM AUTHOR]

In recent years, 2D valley physics has attracted tremendous attention in both fundamental research and device applications. The search for a coupling valley with exotic physical properties can enable the exploration of novel quantum devices and is therefore of significant interest. Here, using model analysis, a novel mechanism is proposed that realizes strong coupling between strain and valley physics in a 2D antiferromagnetic honeycomb lattice in a controllable fashion. This idea is to modulate the exchange interactions through strain, thereby mediating the inversion and time‐reversal symmetries. As valley physics are connected with the symmetries, this enables a strong strain‐valley coupling. On the basis of first‐principles calculations, the validity of this mechanism is further demonstrated in a series of single‐layer systems, i.e., AuCl3, CrI3, and VPTe3, which are predicted to host valley polarization and anomalous valley Hall effect. The change in bonding angle is found to play a crucial role in mediating the exchange interaction and thus the strain‐valley coupling process. The microscopic origin is also discussed. These explored phenomena and insights greatly enrich the valley physics and 2D materials research. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions are topologically protected chiral spin textures that hold great promise for information storage and processing. The current research efforts on magnetic skyrmions are exclusively based on d‐orbital magnetism, which restricts their existence in a narrow window of temperature‐external magnetic field (T‐B) phase diagram. Herein, employing first‐principles and Monte‐Carlo simulations, the work reports the identification of d0 magnetic skyrmions in 2D lattice of Tl2NO2. Arising from inversion asymmetry and strong spin‐orbit coupling compensated by ligand of heavy element, large Dzyaloshinskii–Moriya interaction is obtained in monolayer Tl2NO2. This, competed with p‐orbital exchange interaction, leads to the d0 skyrmion physics under external magnetic field. Importantly, different from d‐orbital topological magnetism, the d0 magnetic skyrmions can be stabilized in a wide window of T‐B phase diagram. The underlying physics is related to the small magnetic moment and delocalization character of d0 magnetism. Furthermore, the work also demonstrates that the d0 magnetic skyrmions are strongly coupled with ferroelectricity. These findings open a new direction for magnetic skyrmion research. [ABSTRACT FROM AUTHOR]

Two-dimensional van der Waals heterostructures (vdWHs) with tunable band alignment can be very useful for developing minimized multifunctional and controllable devices, but so far they are scarcely reported. Here, using first-principles calculations, we systematically investigate the electronic properties of Tl2O/WTe2 vdWH. Our results indicate that it is a direct bandgap semiconductor harboring a straddling type-I band alignment, with the conduction band minimum (CBM) and valence band maximum (VBM) both from two-dimensional WTe2. Interestingly, upon introducing feasible external strain or electric field, its band alignment can be easily transformed into staggered type-II, with CBM and VBM separated in different layers, achieving the long-sought tunable multiple band alignments. Along with this, the intriguing direct-to-indirect bandgap transition is also achieved in Tl2O/WTe2 vdWH. Our work thus provides a promising candidate in the field of two-dimensional multifunctional and controllable electronics. [ABSTRACT FROM AUTHOR]

Quantitative in situ elemental analyses of the impure deposited layer on the plasma-facing components (PFCs) of magnetically confined fusion devices such as EAST are crucial and challenging. Laser-induced breakdown spectroscopy (LIBS) is one of the most promising methods. In this study, we present an improved quantitative analytic algorithm for the deposited layers (in vacuum) of both major and minor elements via one-point calibration laser-induced breakdown spectroscopy (OPC-LIBS). OPC-LIBS is a variation of the calibration-free LIBS approach involving one matrix-matched standard sample with a known composition that effectively corrects the errors, caused by the uncertainty of Einstein's coefficient and inaccurate efficiency of the spectrum acquisition system of the CF-LIBS method. We found that the OPC-LIBS method is more accurate for the determination of composition of a series of Al–W–Mo coatings (EAST-like deposits) compared to the conventional CF-LIBS approach at 10−5 mbar to simulate the pressure condition of a fusion vessel. In the case of OPC-LIBS measurement, the relative error of major elements such as Al (85.65–95.51 at%) was below 2%, and the relative error of trace elements W (2.32–9.56 at%) was within 35% and Mo (2.17–11.66 at%) was within 20%. Finally, our results verify the ability of quantitative analysis by using the OPC method in a high-vacuum environment and film samples. [ABSTRACT FROM AUTHOR]

Valley-related multiple Hall effect in a two-dimensional lattice is of notable interest both for its fundamental physics and for its potential applications. In this work, by means of a low energy k·p model analysis, a mechanism of producing valley-related multiple Hall effects in the hexagonal lattice via strain engineering is proposed, and a general picture of valley-contrasted band inversion is developed. Through first-principles calculations, this mechanism is further established in a ferromagnetic hexagonal lattice of single-layer ScI2. Single-layer ScI2 prefers in-plane magnetization and exhibits neither anomalous valley Hall effects nor valley-polarized quantum anomalous Hall effects in nature. Remarkably, these two Hall effects emerge simultaneously in this system under 4.705% tensile strain and disappear simultaneously when further increasing strain, suggesting the exotic valley-related multiple Hall effect. The underlying physical mechanism is revealed using a model analysis and is generally applicable. Our work greatly enriches the valley-related physics. [ABSTRACT FROM AUTHOR]

As an in situ and remote diagnostic technique, laser-induced breakdown spectroscopy (LIBS) has been well proven to detect the elemental distribution on plasma-facing materials (PFMs) in the magnetic confinement nuclear fusion device of Experimental Advanced Superconducting Tokamak (EAST). However, due to the limited optical windows and irregular shape of the PFM surface, such as the monoblock structure of the divertor, the laser ablation angles (LAAs) of LIBS always vary with the laser spot scanning on the different regions of the PFMs. In this work, a LIBS system with a coaxial collection configuration based on the linear array fiber bundle is developed to investigate the features of laser-induced tungsten (W) plasma spectra at variable LAAs under vacuum conditions. The results show that the LIBS signal intensity, signal to background ratio, and signal to noise ratio of the W II line have different trends compared with W I under different LAAs. The intensities of the W I line decrease faster than those of W II with the increase of LAAs. Moreover, it is confirmed that the emission of the continuum background propagates mainly along the direction of the incident laser, while the plume of W I and W II propagates along the normal direction of the target. For the coaxial LIBS system, the results show that the plasma spectral collection area with a diameter of no less than 7 mm can minimize the spectral signal loss rate under different LAAs, no matter for W I or W II. This result will further improve the understanding of the spatial evolution of LIBS plasma at variable LAAs, to optimize and upgrade the current collection system of the in situ LIBS in the nuclear fusion device of EAST. [ABSTRACT FROM AUTHOR]

Stable valley-layer coupling, which can be against spin–orbit coupling (SOC), is of both fundamental and technological importance as it offers a design principle for 2D valleytronics; however, a reliable mechanism to achieve the goal is missing. In this Letter, a general rule to design such valley-layer coupling is mapped out from symmetry analysis. The degenerate valleys with valley-contrasted layer physics and protected valley-layer coupling can be present in bilayer lattice, when special symmetry operations between layers depending on the inversion center are satisfied. Such valley-layer coupling and its stability against SOC are further revealed in a real material of MnF4 based on first-principles. The distinctive properties, such as optical selection of valley and electric polarization of interlayer excitons, are observed in such a unique system. Our results not only provide a feasible principle to design materials with stable valley-layer coupling but also greatly enrich the physics and broaden the scientific impact of 2D valleytronics. [ABSTRACT FROM AUTHOR]

The analytical capabilities of laser-induced breakdown spectroscopy (LIBS) for in situ elemental analysis of nuclear materials at elevated temperatures were estimated. A certified molybdenum–tungsten alloy (Mo 70 wt% and W 30 wt%) was chosen as a replacement for the high Z impurities deposited on plasma facing components (PFCs) in tokamak. The influence of sample temperature (Ts) on the optical emission of the laser-induced molybdenum–tungsten plasma was investigated with a nanosecond infrared (1064 nm) laser pulse. LIBS experiments were carried out in a wide temperature range from 20 to 410 °C. Surface morphology and depth profiling measurements of the ablated crater for the different temperature conditions were performed. A noticeable dependence of signal emission intensity on Ts has been found. An increase in Ts leads to an increase in the spectral line emissions. In the laser ablation process, the ablation rate, plasma temperature and electron density increase significantly with Ts. In addition, the reflectivity of the alloy surface drops dramatically once oxidation occurs. The decrease of surface reflectivity leads to a greater laser energy coupled to the target and a higher LIBS signal. These results will help improve the LIBS quantitative analysis of high Z impurities in the tokamak device. [ABSTRACT FROM AUTHOR]

We have investigated a new double pulse laser-induced breakdown spectroscopy (DP-LIBS) method using the combination of circular and annular nanosecond laser pulses. The aim of this study is to enhance the LIBS sensitivity through a better coupling of laser energy to the target and ablated matter, leading to more efficient production of trace analyte atoms in an excited state. Using the annular laser pulse and circular pulse delayed by 10 μs, without superposition of the laser-ablation zones, we achieved nearly 4 times signal enhancement and nearly 3 times LOD improvement of the trace elements (Li and Mg) in aluminum alloy as compared to the case of simultaneous irradiation of annular and circular pulses. Time resolved studies show that the annular–circular laser ablation can lead to a hot and dense plasma after the second pulse irradiation, and therefore stronger spectral emission from the plasma was obtained. This could help us to develop a more effective in situ and minimally destructive LIBS system for the trace impurity deposition on the first wall in the fusion device. [ABSTRACT FROM AUTHOR]

Topological magnetism in low-dimensional systems is of fundamental and practical importance in condensed-matter physics and material science. Here, using first-principles and Monte Carlo simulations, we present that multiple topological magnetism (i.e., skyrmion and bimeron) can survive in van der Waals heterostructure MnTe 2 /ZrS 2 . Arising from interlayer coupling, MnTe 2 /ZrS 2 can harbor a large Dzyaloshinskii-Moriya interaction. This, combined with exchange interaction, yields an intriguing skyrmion phase under a tiny magnetic field of 75 mT. Meanwhile, upon harnessing a small electric field, magnetic bimeron can be observed in MnTe 2 /ZrS 2 , suggesting the existence of multiple topological magnetism. Through interlayer sliding, both topological magnetisms can be switched on-off. In addition, the impacts of d ∥ and K eff on these spin textures are revealed, and a dimensionless parameter κ is utilized to describe their joint effect. These explored phenomena and insights not only are useful for fundamental research in topological magnetism but also enable novel applications in nanodevices.

An antiferromagnetic skyrmion crystal (AF-SkX), a regular array of antiferromagnetic skyrmions, is a fundamental phenomenon in the field of condensed-matter physics. So far, very few proposals have been made to realize the AF-SkX, and most have been based on three-dimensional (3D) materials. Herein, using first-principles calculations and Monte Carlo simulations, we report the identification of AF-SkX in a two-dimensional lattice of the Janus monolayer CrSi 2 N 2 As 2 . Arising from the broken inversion symmetry and strong spin-orbit coupling, a large Dzyaloshinskii-Moriya interaction is obtained in the Janus monolayer CrSi 2 N 2 As 2 . This, combined with the geometric frustration of its triangular lattice, gives rise to the skyrmion physics and long-sought AF-SkX in the presence of an external magnetic field. More intriguingly, this system presents two different antiferromagnetic skyrmion phases, and such a phenomenon is distinct from those reported in 3D systems. Furthermore, by contacting with Sc 2 CO 2 , the creation and annihilation of AF-SkX in Janus monolayer CrSi 2 N 2 As 2 can be achieved through ferroelectricity. These findings greatly enrich the research on antiferromagnetic skyrmions.

The realization of magnetic skyrmions in nanostructures holds great promise for both fundamental research and device applications. Despite recent progress, intrinsic magnetic skyrmions in two-dimensional lattice are still rarely explored. Here, using first-principles calculations and Monte Carlo simulations, we report the identification of spontaneous magnetic skyrmions in single-layer CrInX 3 (X = Te, Se). Because of the joint effect of broken inversion symmetry and strong spin-orbit coupling, inherent large Dzyaloshinskii-Moriya interaction occurs in both systems, endowing the intriguing Néel-type skyrmions. By further imposing moderate magnetic field, the skyrmion phase can be obtained and is stable within a wide temperature range. Particularly for single-layer CrInTe 3 , the size of skyrmions is sub-10 nm and the skyrmion phase can be maintained at an elevated temperature of ∼180 K. In addition, the phase diagrams of their topological spin textures under the variation of magnetic parameters of D , J , and K are mapped out.

Two-dimensional auxetic materials have attracted considerable attention due to their potential applications in medicine, tougher composites, defense, and so on. However, they are scare especially at low dimension, as auxetic materials are mainly realized in engineered materials and structures. Here, using first-principles calculations, we identify a compelling two-dimensional auxetic material, single-layer Ag 2 S, which possesses large negative Poisson's ratios in both in-plane and out-of-plane directions, but anisotropic ultralow Young's modulus. Such a coexistence of simultaneous negative Poisson's ratios in two directions is extremely rare, which is mainly originated from its particular zigzag-shaped buckling structure. In addition, contrary to the previously known metal-shrouded single-layer M 2 X (M = metal, X = nonmetal), single-layer Ag 2 S is the first nonmetal-shrouded M 2 X. Electronic calculations show that it is an indirect-gap semiconductor with gap value of 2.83 eV, and it can be turned to be direct with strain. These intriguing properties make single-layer Ag 2 S a promising auxetic material in electronics and mechanics.