Proposal for valleytronic materials:Ferrovalley metal and valley gapless semiconductor

Valleytronic materials can provide new degrees of freedom to future electronic devices.In this work,the concepts of the ferrovalley metal(FVM)and valley gapless semiconductor(VGS)are proposed,which can be achieved in valleytronic bilayer systems by electric field engineering.In valleytronic bilayer systems,the interaction between out-of-plane ferroelectricity and A-type antiferromagnetism can induce layer-polarized anomalous valley Hall(LP-AVH)effect.The K and−K valleys of FVM are both metallic,and electron and hole carriers simultaneously exist.In the extreme case,the FVM can become VGS by analogizing spin gapless semiconductor(SGS).Moreover,it is proposed that the valley splitting enhancement and valley polarization reversal can be achieved by electric field engineering in valleytronic bilayer systems.Taking the bilayer RuBr_(2)as an example,our proposal is confirmed by the first-principle calculations.The FVM and VGS can be achieved in bilayer RuBr_(2)by applying electric field.With appropriate electric field range,increasing electric field can enhance valley splitting,and the valley polarization can be reversed by flipping electric field direction.To effectively tune valley properties by electric field in bilayer systems,the parent monolayer should possess out-of-plane magnetization,and have large valley splitting.Our results shed light on the possible role of electric field in tuning valleytronic bilayer systems,and provide a way to design the ferrovalley-related material by electric field.

Symmetry-driven half-integer conductance quantization in Cobalt–fulvalene sandwich nanowire

Precise manipulation and monitoring spin transport in one-dimensional (1D) systems is a long-sought goal in the field of nano-spintronics. Based on first-principles calculations, we report the observation of half-integer conductance quantization in the Cobalt-fulvalene sandwich nanowire. Compared with a pure monatomic Cobalt wire, the introduction of fulvalene molecules leads to threeimportant features: Firstly, the strong coupling between the fulvalene and the Cobalt prevents the contamination of the ambientair, ensuring both chemical and physical stabilities;Secondly, the fulvalene symmetry-selectively filters out most of the d-typeorbitals of the Cobalt while leaving a single d-type orbital to form an open spin channel around the Fermi level, which offers amechanism to achieve the observed half-integer conductance;Thirdly, it maintains a superexchange coupling between adjacent Coatoms to achieve a high Curie temperature. Spin transport calculations show that this half-metallic nanowire can serve as a perfectspin filter or a spin valve device, thus revealing the potential of Cobalt-fulvalene sandwich nanowire as a promising building blockof high-performance spintronics technology.

Physics of electron emission and injection in two-dimensional materials: Theory and simulation

Electrically contacting two-dimensional(2D)materials is an inevitable process in the fabrication of devices for both the study of fundamental nanoscale charge transport physics and the design of high-performance novel electronic and optoelectronic devices.The physics of electrical contact formation and interfacial charge injection critically underlies the performance,energyefficiency and the functionality of 2D-material-based devices,thus representing one of the key factors in determining whether 2D materials can be successfully implemented as a new material basis for the development of nextgeneration beyond-silicon solid-state device technology.In this review,the recent developments in the theory and the computational simulation of electron emission,interfacial charge injection and electrical contact formation in 2D material interfaces,heterostructures,and devices are reviewed.Focusing on thermionic charge injection phenomena which are omnipresent in 2Dmaterials-based metal/semiconductor Schottky contacts,we summarize various transport models and scaling laws recently developed for 2D materials.Recent progress on the first-principle density functional theory simulation of 2D-material-based electrical contacts are also reviewed.This review aims to provide a crystalized summary on the physics of charge injection in the 2D Flatlands for bridging the theoretical and the experimental research communities of 2D material device physics and technology.

Nonlinear optical response of graphene in terahertz and near-infrared frequency regime

In this review, we discuss our recent theoretical work on the nonlinear optical response of graphene and its sister structure in terahertz （THz） and near-infrared frequency regime. Due to Dirac-like linear energymomentum dispersion, the third-order nonlinear current in graphene is much stronger than that in conventional semiconductors. The nonlinear current grows rapidly with increasing temperature and decreasing frequency. The third-order nonlinear current can be as strong as the linear current under moderate electric field strength of 104 V/cm. In bilayer graphene （BLG） with low energy trigonal warping effect, not only the optical response is strongly nonlinear, the optical nonlinearity is well-preserved at elevated temperature. In the presence ofa bandgap （such as semihydrogenated graphene （SHG））, there exists two well separated linear response and nonlinear response peaks. This suggests that SHG can have a unique potential as a two-color nonlinear material in the THz frequency regime where the relative intensity of the two colors can be tuned with the electric field. In a graphene superlattice structure of Kronig-Penney type periodic potential, the Dirac cone is elliptically deformed. We found that not only the optical nonlinearity is preserved in such a system, the total optical response is further enhanced by a factor proportional to the band anisotropy. This suggests that graphene superlattice is another potential candidate in THz device application.

经典电路网络中奇异束缚态的实验观察

Exceptional bound(EB)states represent a unique new class of robust bound states protected by the defectiveness of non-Hermitian exceptional points.Conceptually distinct from the more well-known topological states and non-Hermitian skin states,they were recently discovered as a novel source of negative entanglement entropy in the quantum entanglement context.Yet,EB states have been physically elusive,being originally interpreted as negative probability eigenstates of the propagator of nonHermitian Fermi gases.In this work,we show that EB states are in fact far more ubiquitous,also arising robustly in broad classes of systems whether classical or quantum.This hinges crucially on a newlydiscovered spectral flow that rigorously justifies the EB nature of small candidate lattice systems.As a highlight,we present their first experimental realization through an electrical circuit,where they manifest as prominent stable resonant voltage profiles.Our work brings a hitherto elusive but fundamentally distinctive quantum phenomenon into the realm of classical metamaterials,and provides a novel pathway for the engineering of robust modes in otherwise sensitive systems..