Recent progress on the planar Hall effect in quantum materials

The planar Hall effect(PHE),which originates from anisotropic magnetoresistance,presents a qualitative and simple approach to characterize electronic structures of quantum materials by applying an in-plane rotating magnetic field to induce identical oscillations in both longitudinal and transverse resistances.In this review,we focus on the recent research on the PHE in various quantum materials,including ferromagnetic materials,topological insulators,Weyl semimetals,and orbital anisotropic matters.Firstly,we briefly introduce the family of Hall effect and give a basic deduction of PHE formula with the second-order resistance tensor,showing the mechanism of the characteristicπ-period oscillation in trigonometric function form with aπ/4 phase delay between the longitudinal and transverse resistances.Then,we will introduce the four main mechanisms to realize PHE in quantum materials.After that,the origin of the anomalous planar Hall effect(APHE)results,of which the curve shapes deviate from that of PHE,will be reviewed and discussed.Finally,the challenges and prospects for this field of study are discussed.

Honeycomb silicon: a review of silicene

Silicene, a new allotrope of silicon in a twodimensional honeycomb structure, has attracted intensive research interest due to its novel physical and chemical properties. Unlike carbon atoms in graphene, silicon atoms prefer to adopt sp2/sp3-hybridized state in silicene,enhancing chemical activity on the surface and allowing tunable electronic states by chemical functionalization. The silicene monolayers epitaxially grown on Ag(111) surfaces demonstrate various reconstructions with different electronic structures. In this article, the structure, phonon modes, electronic properties, and chemical properties of silicene are reviewed based on theoretical and experimental works in recent years.

The role of oxygen vacancies in the high cycling endurance and quantum conductance in BiVO4-based resistive switching memory

Resistive random access memory(RRAM)has emerged as a new discipline promoting the development of new materials and devices toward a broad range of electronic and energy applications.Here,we realized a memristive device with weak dependence on the top electrodes and demonstrated the quantized conductance(QC)nature in BiVO4 matrix.The electronic properties have been investigated by the measurements of I-V curves,where the resistive switching(RS)phenomenon with stable switching ratio and excellent longterm retention capabilities are identified.Two more inert materials,TiN and Pd,are applied as the top electrodes to exclude the influence of electrodes on the RS states and QC behavior.The X-ray photoelectron spectroscopy results and transport measurements reveal that the conductive filament(CF)is composed by elemental bismuth.The naturally existed oxygen vacancies in BiVO4 matrix plays as the role of catalyst in the formation and dissolution of CF in BiVO4-based RRAM device,which is the primary cause for the observed weak dependence of switching performance in this device on the type of top electrodes.Our results clearly illustrate that BiVO4 could be a new idea platform to realize the high scalability,high cycling endurance,and multilevel storage RRAM devices.

Theoretical insights into nitrogen oxide activation on halogen defect-rich{001}facets of bismuth oxyhalide

Surface vacancies,serving as the activation centers for surface-adsorbed species,have been widely applied in catalysts to improve their activity and selectivity.In the case of ternary compound semiconductors,there is some controversy about exposed atoms and surface defects.Two-dimensional layered BiOCl is an important photocatalyst,which has had numerous studies focused on its oxygen vacancy(O_V)and bismuth vacancy(Bi_V).It has been realized that its(001)surface can consist of exposed halogen atoms rather than oxygen atoms,which thus needs a new explanation for its surface defect engineering mechanism.Using first-principles calculations,the activation behavior of NO_X(NO_(2),NO,N_(2)O)at a chlorine vacancy(Cl_V)on the BiOCl(001)surface is systematically studied.It is found that after introducing Cl_V on BiOCl(001)surfaces,NO_X molecules all show excellent activities with longer chemical bonds by capturing electrons from the catalyst.Our work furnishes fundamental insight into the activation of small molecules on defect-rich surfaces of ternary compound catalysts.

Advances in bismuth-based topological quantum materials by scanning tunneling microscopy

In recent years,topological quantum materials(TQMs)have attracted intensive attention in the area of condensed matter physics due to their novel topologies and their promising applications in quantum computing,spin electronics and next-generation integrated circuits.Scanning tunneling microscopy/spectroscopy(STM/STS)is regarded as a powerful technique to characterize the local density of states with atomic resolution,which is ideally suited to the measurement of the bulk-boundary correspondence of TQMs.In this review,using STM/STS,we focus on recent research on bismuth-based TQMs,including quantum-spin Hall insulators,3D weak topological insulators(TIs),high-order TIs,topological Dirac semi-metals and dual TIs.Efficient methods for the modulation of the topological properties of the TQMs are introduced,such as interlayer interaction,thickness variation and local electric field perturbation.Finally,the challenges and prospects for this field of study are discussed.