Layer-Valley Hall Effect under Inversion and Time-Reversal Symmetries

Hall effects have been the central paradigms in modern physics,materials science and practical applications,and have led to many exciting breakthroughs,including the discovery of topological Chern invariants and the revolution of metrological resistance standard.To date,the Hall effects have mainly focused on a single degree of freedom(Do F),and most of them require the breaking of spatial-inversion and/or time-reversal symmetries.Here we demonstrate a new type of Hall effect,i.e.,layer-valley Hall effect,based on a combined layer-valley Do F characterized by the product of layer and valley indices.The layer-valley Hall effect has a quantum origin arising from the layer-valley contrasting Berry curvature,and can occur in nonmagnetic centrosymmetric crystals with both spatial-inversion and time-reversal symmetries,transcending the symmetry constraints of single Do F Hall effect based on the constituent layer or valley index.Moreover,the layer-valley Hall effect is highly tunable and shows a W-shaped pattern in response to the out-of-plane electric fields.Additionally,we discuss the potential detection approaches and material-specific design principles of layer-valley Hall effect.Our results demonstrate novel Hall physics and open up exotic paradigms for new research direction of layer-valleytronics that exploits the quantum nature of the coupled layer-valley DoF.

Recent progress in 2D group-V elemental monolayers:fabrications and properties

A large number of two-dimensional(2D)monoelemental materials with huge application potentials have been developed,since graphene was reported as a monoelemental material with unique properties.As cousins of graphene,2D group-V elemental monolayers have gained tremendous interest due to their electronic properties with significant fundamental bandgap.In this review,we extensively summarize the latest theoretical and experimental progress in group-V monoelemental materials,including the latest fabrication methods,the properties and potential applications of these 2D monoelementals.We also give a perspective of the challenges and opportunities of 2D monoelemental group-V monolayer materials and related functional nanodevices.

Pressure-mediated contact quality improvement between monolayer MoS_2 and graphite

Two-dimensional(2D) materials and their heterostructures have attracted a lot of attention due to their unique electronic and optical properties. MoS_2 as the most typical 2D semiconductors has great application potential in thin film transistors, photodetector, hydrogen evolution reaction, memory device, etc. However, the performance of MoS_2 devices is limited by the contact resistance and the improvement of its contact quality is important. In this work, we report the experimental investigation of pressure-enhanced contact quality between monolayer MoS_2 and graphite by conductive atom force microscope(C-AFM). It was found that at high pressure, the contact quality between graphite and MoS_2 is significantly improved. This pressure-mediated contact quality improvement between MoS_2 and graphite comes from the enhanced charge transfer between MoS_2 and graphite when MoS_2 is stretched. Our results provide a new way to enhance the contact quality between MoS_2 and graphite for further applications.

Progress on 2D topological insulators and potential applications in electronic devices

Two-dimensional topological insulators(2DTIs)have attracted increasing attention during the past few years.New 2DTIs with increasing larger spin-orbit coupling(SOC)gaps have been predicted by theoretical calculations and some of them have been synthesized experimentally.In this review,the 2DTIs,ranging from single element graphene-like materials to bi-elemental transition metal chalcogenides(TMDs)and to multi-elemental materials,with different thicknesses,structures,and phases,have been summarized and discussed.The topological properties(especially the quantum spin Hall effect and Dirac fermion feature)and potential applications have been summarized.This review also points out the challenge and opportunities for future 2DTI study,especially on the device applications based on the topological properties.

Predicted Pressure-Induced High-Energy-Density Iron Pentazolate Salts

Metal-pentazolate compounds as candidates for novel high-energy-density materials have attracted extensive attention in recent years.However,dehydrated pentazolate salts of transition metal iron are rarely reported.We predict two new iron pentazolate salts Fdd2-FeN10 and P1(No.1)-FeN10 using a constrained crystal search method based on first-principles calculations.We propose that the stable Fdd2-FeN_(10) crystal may be synthesized from FeN and N_(2) above 20 GPa,and its formation enthalpy is lower than the reported iron pentazolate salt(marked as P1(No.2)-FeN_(10)).Crystal P1(No.1)-FeN_(10) is composed of iron bispentazole molecules.Formation enthalpy,phonon spectrum and ab initio molecular dynamics calculations are performed to show their thermodynamic,mechanical and dynamic properties.Moreover,the high energy density(3.709 kJ/g,6.349 kJ/g)and good explosive performance indicate their potential applications as high-energy-density materials.

多带笼目超导体CsV_(3)Sb_(5)衍生体系的可调磁通涡旋束缚态

Vortices and bound states offer an effective means of comprehending the electronic properties of superconductors.Recently,surface-dependent vortex core states have been observed in the newly discovered kagome superconductors CsV_(3)Sb_(5).Although the spatial distribution of the sharp zero energy conductance peak appears similar to Majorana bound states arising from the superconducting Dirac surface states,its origin remains elusive.In this study,we present observations of tunable vortex bound states(VBSs)in two chemically-doped kagome superconductors Cs(V_(1-x)Tr_(x))_(3)Sb_(5)(Tr=Ta or Ti),using low-temperature scanning tunneling microscopy/spectroscopy.The CsV_(3)Sb_(5)-derived kagome superconductors exhibit full-gap-pairing superconductivity accompanied by the absence of long-range charge orders,in contrast to pristine CsV_(3)Sb_(5).Zero-energy conductance maps demonstrate a field-driven continuous reorientation transition of the vortex lattice,suggesting multiband superconductivity.The Ta-doped CsV_(3)Sb_(5)displays the conventional cross-shaped spatial evolution of Caroli-de Gennes-Matricon bound states,while the Tidoped CsV_(3)Sb_(5)exhibits a sharp,non-split zero-bias conductance peak(ZBCP)that persists over a long distance across the vortex.The spatial evolution of the non-split ZBCP is robust against surface effects and external magnetic field but is related to the doping concentrations.Our study reveals the tunable VBSs in multiband chemically-doped CsV_(3)Sb_(5)system and offers fresh insights into previously reported Y-shaped ZBCP in a non-quantum-limit condition at the surface of kagome superconductor.

Probing phase transition of band topology via radiation topology

Topological photonics has received extensive attention from researchers because it provides brand new physical principles to manipulate light.Band topology is characterized using the Berry phase defined by Bloch states.Until now,the scheme for experimentally probing the topological phase transition of band topology has always been relatively lacking in topological physics.Moreover,radiation topology can be aroused by the far-field polarization singularities of Bloch states,which is described by the Stokes phase.Although such two types of topologies are both related to Bloch states on the band structures,it is rather surprising that their development is almost independent.Here,in optical analogs of the quantum spin Hall effects(QSHEs)and Su-Schrieffer-Heeger model,we reveal the correlation between the phase transition of band topology and radiation topology and then demonstrate that the radiation topology can be employed to study the band topological transition.We experimentally demonstrate such an intriguing phenomenon in optical analogs of QSHEs.Our findings not only provide an insightful understanding of band topology and radiation topology,but also can serve as a route to manipulate light.

拓扑异质结(PbSe)_(5)(Bi_(2)Se_(3))_(6)高压诱导超导电性研究

最近,天然异质结(PbSe)_(5)(Bi_(2)Se_(3))_(6)在理论上预测并实验证实为拓扑绝缘体.本文通过高压原位量子调控在(PbSe)_(5)(Bi_(2)Se_(3))_(6)中成功引入超导电性.当压力增加到10 GPa时,超导电性突然出现,T_(c)约为4.6 K,随后T_(c)急剧下降.值得注意的是,当进一步施加压力到30 GPa以上时,出现了一种新的超导态,且T_(c)直到本实验最高压力仍未饱和.结合XRD和拉曼光谱,我们认为(PbSe)_(5)(Bi_(2)Se_(3))_(6)中两个超导态的出现与压力诱导的结构相变有关.拓扑异质结压力下出现新的量子态,为进一步研究拓扑和超导的关系提供了一个良好的平台.

Encyclopedia of emergent particles in three-dimensional crystals

The past decade has witnessed a surge of interest in exploring emergent particles in condensed matter systems.Novel particles,emerged as excitations around exotic band degeneracy points,continue to be reported in real materials and artificially engineered systems,but so far,we do not have a complete picture on all possible types of particles that can be achieved.Here,via systematic symmetry analysis and modeling,we accomplish a complete list of all possible particles in time-reversal-invariant systems.This includes both spinful particles such as electron quasiparticles in solids,and spinless particles such as phonons or even excitations in electric-circuit and mechanical networks.We establish detailed correspondence between the particle,the symmetry condition,the effective model,and the topological character.This obtained encyclopedia concludes the search for novel emergent particles and provides concrete guidance to achieve them in physical systems.

Weakened interlayer coupling in two-dimensional MoSe2 flakes with screw dislocations

The screw dislocations are intriguing defects that are often observed in natural and artificial materials. The dislocation spirals break the reflection and inversion symmetries of the lattices and modify the interlayer coupling in layer-structured materials, inducing additional complexity in layer stacking and thus novel properties in materials. Here, we report on the interlayer coupling of two-dimensional (2D) MoSe2 flakes with screw dislocations by atomic force microscopy (AFM), Raman spectra and photoluminescence (PL) spectra. By controlling the supersaturation conditions, 2D MoSe2 flakes with screw dislocations are grown on amorphous SiO2 substrates by chemical vapor deposition (CVD). AFM measurements reveal that the interlayer spacing in such 2D MoSe2 flakes with screw dislocation is slightly widened with respect to the normal AA- or AB-stacked ones due to the presence of the screw dislocations. Raman and PL spectra show that the interlayer coupling is weaker and thus the band gap is wider than that in the normal AA- or AB-stacked ones. Our work demonstrates that the interlayer coupling of 2D transition metal dichalcogenides (TMDCs) flakes can be tuned by the induction of screw dislocations, which is very helpful for developing novel catalysts and electronic devices.

Sign-reversible valley-dependent Berry phase effects in 2D valley-half-semiconductors

Manipulating valley-dependent Berry phase effects provides remarkable opportunities for both fundamental research and practical applications.Here,by referring to effective model analysis,we propose a general scheme for realizing topological magneto-valley phase transitions.More importantly,by using valley-half-semiconducting VSi2N4 as an outstanding example,we investigate sign change of valley-dependent Berry phase effects which drive the change-in-sign valley anomalous transport characteristics via external means such as biaxial strain,electric field,and correlation effects.As a result,this gives rise to quantized versions of valley anomalous transport phenomena.Our findings not only uncover a general framework to control valley degree of freedom,but also motivate further research in the direction of multifunctional quantum devices in valleytronics and spintronics.

Type-II topological metals

Topological metals(TMs)are a kind of special metallic materials,which feature nontrivial band Cross-ings near the Fermi energy,giving rise to peculiar quasiparticle excitations.TMs can be classified based on the characteristics of these band crossings.For example,according to the dimensionality of the crossing,TMs can be classifed into nodal-point,nodal-line,and nodal-surface metals.Another important property is the type of dispersion.According to degree of the tilt of the local dispersion around the crossing,we have typeI and type-II dispersions.This leads to significant distinctions in the physical properties of the materials,owing to their contrasting Fermi surface topologies.In this article,we briefly review the recent advances in this research direction,focusing on the concepts,the physical properties,and the material realizations of the type-Il nodal-point and nodal-line TMs.