The manipulation over diverse topological matters has become a critical demand for advancing quantum devices and topological spintronics. However, such experimental demonstrations remain scarce. Here, based on a novel breathing kagome magnetic Weyl semimetal LaCrGe3, we realize a spin-rotation driven Weyl state evolution under the control of an external magnetic field. While the breathing of kagome lattice is revealed to boost the desired topologic state, the predicted Weyl points are observed around the Fermi level via angle-resolved photoemission spectroscopy, and are further corroborated by transport effects of chiral-anomaly-related negative magnetoresistance and large anomalous Hall conductivity. By rotating the external magnetic field, we demonstrated that the reorientation of magnetic moments can drive the motion of Weyl points in momentum space, which is characterized by a highly tunable angle-dependent Hall response. Our study presents a modulation of both topology and transport via spin orientation that offers fundamental insights for developing next-generation spin-based functional devices based on topological physics.

The van der Waals Fe5-xGeTe2 is a 3d ferromagnetic metal with a high Curie temperature of 275 K. We report herein the observation of an exceptional weak antilocalization (WAL) effect that can persist up to 120 K in an Fe5-xGeTe2 nanoflake, indicating the dual nature with both itinerant and localized magnetism of 3d electrons. The WAL behavior is characterized by the magnetoconductance peak around zero magnetic field and is supported by the calculated localized nondispersive flat band around the Fermi level. The peak to dip crossover starting around 60 K in magnetoconductance is visible, which could be ascribed to temperature-induced changes in Fe magnetic moments and the coupled electronic band structure as revealed by angle-resolved photoemission spectroscopy and first-principles calculations. Our findings would be instructive for understanding the magnetic exchanges in transition metal magnets as well as for the design of next-generation room-temperature spintronic devices.

The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the preformed exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, which is the signatures of an exciton gas phase prior to its condensation into the final CDW state. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.

Anisotropic 2D materials have interesting properties and could find some unique applications in next generation electronics. In this chapter, we will review the research progress on some representative anisotropic 2D materials. We will show that anisotropic charge carrier transport, optoelectronic response, and ferroelectric behaviour have been studied and anisotropic devices have been demonstrated. Some remaining challenges will be discussed for future studies.

Significance The large linear positive magnetoresistance (LPMR) is widely observed in emergent topological materials. Various explanations, including linear band structure, impurity scattering, guiding center motion, or internode scattering, have been proposed. However, experimentally, this phenomenon is still lacking applicable quantitative description. In this work, we report a magnetic Weyl semimetal CoS2, with the largest LPMR among known magnetic topological materials. To understand the LPMR behavior, we establish an intrinsic model that the slope of the linear MR is determined by the average of the Berry curvature near the Fermi surface and prove this model with experimental data by introducing temperature effect.

The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high-temperature BEC transition. The material realization of EI has been elusive, which is further challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the pre-condensation exciton gas phase is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in epitaxially grown monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band- and energy-dependent folding behavior in a two-step process, evidenced by an exciton gas phase prior to its condensation into the final CDW state. The excellent agreement between experiments and theoretical predictions on the recovery of the pristine band structure by carrier-density-dependent suppression of the CDW state further corroborates the monolayer 1T-ZrTe2 as an EI. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.

Heterostructure devices based on two-dimensional materials have been under intensive study due to their intriguing electrical and optical properties. One key factor in understanding these devices is their nanometer-scale band profiles, which is challenging to obtain in devices. Here, we use a technique named contact-mode scanning tunneling spectroscopy to directly visualize the band profiles of MoS2/WSe2 heterostructure devices at different gate voltages with nanometer resolution. The long-held view of a conventional p-n junction in the MoS2/WSe2 heterostructure is reexamined. Due to strong inter- and intralayer charge transfer, the MoS2 layer in contact with WSe2 is found to convert from n-type to p-type, and a series of gate-tunable p-n and p-p+ junctions are developed in the devices. Highly conductive edges are also discovered which could strongly affect the device properties.

Twisted van der Waals systems have been receiving recent attention due to their potential for moiré-induced band modulation and corresponding exotic correlated phases. Here, we present a Raman spectroscopic study of artificial trilayer graphene (3LG), represented by monolayer graphene (1LG) on top of Bernal-stacked bilayer graphene (2LG), as a function of the twist angle (θt) with respect to each other. The artificially twisted 3LG with θt >5° shows a distinctive 2D peak, which is literally composed of the typical 2D peak of 1LG and that of 2LG, without signatures of strong coupling between the 1LG and the 2LG. The overall trends of the relative Raman shift and the full width at half maxima of the 2D peak are also provided as a function of θt ranging from 0° to 30°. In particular, non-twisted 3LG shows 2D peak characteristics very similar to those of natural Bernal-stacked 3LG, revealing that the top 1LG and the bottom 2LG are translationally rearranged to be the most thermodynamically stable state. We also realized slightly twisted 3LG with a finite θt <1°, which presents the signature of coexisting Bernal-stacked (ABA) and rhombohedral (ABC) 3LG domains.

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China Key Laboratory of Magnetic Molecules and Magnetic Information Materials of Ministry of Education, Research Institute of Materials Science of Shanxi Normal University Linfen 041004, China School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China

The ternary van der Waals material Nb2Six-1Te4 demonstrates many interesting properties as the content of Si is changed, ranging from metallic Nb3SiTe6 (x = 5/3) to narrow-gap semiconductor Nb2SiTe4 (x = 2) and with the emergence of one-dimensional Dirac fermion excitations in between. An in-depth understanding of their properties with different stoichiometry is important. Here we use scanning tunneling microscopy and spectroscopy to reveal that Nb2Six-1Te4 is a system with spontaneously developed and self-aligned one-dimensional metallic chains embedded in a two-dimensional semiconductor. Electron quasiparticles form one- and two-dimensional standing waves side by side. This special microscopic structure results in strong transport anisotropy. Along the chain direction the material behaves like a metal, while perpendicular to the chain direction, it behaves like a semiconductor. These findings provide an important basis for further investigation of this intriguing system.

Stacking order has a strong influence on the coupling between the two layers of twisted bilayer graphene (BLG), which in turn determines its physical properties. Here, we report the investigation of the interlayer coupling of the epitaxially grown single-crystal 30°-twisted BLG on Cu(111) at the atomic scale. The stacking order and morphology of BLG is controlled by a rationally designed two-step growth process, that is, the thermodynamically controlled nucleation and kinetically controlled growth. The crystal structure of the 30°-twisted bilayer graphene (30°-tBLG) is determined to have quasicrystal-like symmetry. The electronic properties and interlayer coupling of the 30°-tBLG are investigated using scanning tunneling microscopy and spectroscopy. The energy-dependent local density of states with in situ electrostatic doping shows that the electronic states in two graphene layers are decoupled near the Dirac point. A linear dispersion originated from the constituent graphene monolayers is discovered with doubled degeneracy. This study contributes to controlled growth of twist-angle-defined BLG and provides insights on the electronic properties and interlayer coupling in this intriguing system.

Black phosphorus has attracted great research interest due to its numerous applications in electronic devices, optoelectronic devices, energy storage, and so on. Compared with the majority of two-dimensional materials, black phosphorus possesses a unique property-its strong in-plane anisotropy. All the properties reported so far, including its effective mass, electron mobility, light absorption, thermal conductivity, and so on, have shown great anisotropy in the basal plane. This property renders black phosphorus applications not achievable with other two-dimensional materials. In this work, however, we discover a remarkable isotropic behavior in the strongly anisotropic black phosphorus-its electrostatic screening of point charges. We use the tip-induced band bending of a scanning tunneling microscope to map out the Coulomb field of ionized potassium adatoms on black phosphorus and reveal its isotropic charge screening. This discovery is important for understanding electron scattering and transport in black phosphorus.

Plasma treatment is a powerful tool to tune the properties of two-dimensional materials. Previous studies have utilized various plasma treatments on two-dimensional materials. We find a new effect of plasma treatment. After controlled oxygen-plasma treatment on field-effect transistors based on two-dimensional SnSe 2 , the capacitive coupling between the silicon back gate and the channel through the 300nm SiO 2 dielectric can be dramatically enhanced by about two orders of magnitude (from 11 nF/cm 2 to 880 nF/cm 2 ), reaching good efficiency of ion-liquid gating. At the same time, plasma treated devices show large hysteresis in the gate sweep demonstrating memory behavior. We reveal that this spontaneous ion gating and hysteresis are achieved with the assistance of a thin layer of water film automatically formed on the sample surface with water molecules from the ambient air, due to the change in hydrophilicity of the plasma treated samples. The water film acts as the ion liquid to couple the back gate and the channel. Thanks to the rich carrier dynamics in plasma-treated two-dimensional transistors, synaptic functions are realized to demonstrate short- and long-term memories in a single device. This work provides a new perspective on the effects of plasma treatment and a facile route for realizing neuromorphic devices.

This research aims to investigate the cell-nanomaterial interaction between epithelial-mesenchymal transition of A549 cell and electrospinning nanofibers composed of polyvinyl alcohol (PVA)/silk sericin (SS). The electrospinning of regenerated nanofiber was performed with water as a spinning solvent and glutaraldehyde as a chemical cross-linker. Solution concentration, applied voltage and spin distances as well as other parameters were optimized to generate fine nanofibers with smooth surface in good homogeneity. From the scanning electron microscopy (SEM) analysis, the nanofibers had an average diameter of 200nm. Epithelial-mesenchymal transition (EMT) is a process by which epithelial cells lose their cell polarity to become mesenchymal stem cells. This transition is affected by multiple biochemical and physical factors in cell metabolism cascade. Herein, we investigate the biophysical effect on A549 EMT by culturing cells on nanofibrous mats with different topography and composition. The cell viability was evaluated by biochemical assay and its morphology was observed with SEM. The results demonstrate that cells appropriately attached to the surface of the nanofibrous mats with extended morphology by their filopodia. Gene expression analysis was conducted by real-time PCR using multiple markers for detecting EMT: N-cadherin (NCad), Vimentin (Vim), Fibronectin (Fib) and Matrix metallopeptidase (MMP9). An increasing expression pattern was observed on NCad, Vim, Fib, with respect to a negative control as cell cultured on polystyrene dish. This result indicates the 200nm PVA/SS nanofibers may induce A549 cells to process epithelial-mesenchymal transition during the culturing.

By developing a nonperturbative quantum-field scattering theory, we treat the Kapitza-Dirac(K-D) diffraction effect observed by Batelaan et al. our theory shows that in the case of strictly antiparallel (non-focusing) standing-wave laser light, K-D effect occurs only when the incident angle and outgoing angle of the electron beam subject to the Bragg’s law. Our theory also shows that with a focusing standing-wave laser light, a perpendicularly injected electron beam can produce the diffraction pattern. With increasing laser intensity,the envelope of the diffraction pattern shows a crescent line shape which indicates the middle case passing to the angular splitting observed by Bucsbaum et al. from patterns observed by Batatelaan et al.