Valley manipulation by sliding-induced tuning of the magnetic proximity effect in heterostructures.

Spontaneous valley polarization, resulting from the magnetic proximity effect, holds tremendous potential for information processing and storage. This effect is highly sensitive to the interfacial electronic properties, encompassing both charge transitions and spin configurations. In this study, we propose the manipulation of valley splitting by leveraging the tunable magnetic proximity effect through sliding an inversion-symmetric antiferromagnetic (AFM-I) monolayer within a TMD/AFM-I/TMD heterostructure. The presence of the antiferromagnetic monolayer enhances the robustness of the magnetic order during interlayer sliding. Notably, we demonstrate that the polarized stacking of the heterostructure enables the generation of intrinsic out-of-plane and in-plane electric polarization. Intriguingly, interlayer sliding not only reverses the out-of-plane and in-plane electric polarization but also alters the layer-resolved valley splitting, thereby contributing to the emergence of the anomalous valley Hall effect and the layer Hall effect. In addition, the manipulation of valleys remains consistent with both the valley optical selection rules and the intra/interlayer emission energy, which are contingent upon the interlayer sliding. The findings of this work hold promise for potential applications in the field of valleytronics.

Frequency-Stable Robust Wireless Power Transfer Based on High-Order Pseudo-Hermitian Physics.

Nonradiative wireless power transfer (WPT) technology has made considerable progress with the application of the parity-time (PT) symmetry concept. In this Letter, we extend the standard second-order PT-symmetric Hamiltonian to a high-order symmetric tridiagonal pseudo-Hermitian Hamiltonian, relaxing the limitation of multisource/multiload systems based on non-Hermitian physics. We propose a three-mode pseudo-Hermitian dual-transmitter-single-receiver circuit and demonstrate that robust efficiency and stable frequency WPT can be attained despite the absence of PT symmetry. In addition, no active tuning is required when the coupling coefficient between the intermediate transmitter and the receiver is changed. The application of pseudo-Hermitian theory to classical circuit systems opens up an avenue for expanding the application of coupled multicoil systems.

Rational Design of Black Phosphorus-Based Direct Z-Scheme Photocatalysts for Overall Water Splitting: The Role of Defects.

Black phosphorus (BP) has received increasing interest as a promising photocatalyst for water splitting. Nevertheless, exploring the underlying hydrogen evolution reaction (HER) mechanism and improving the water oxidizing ability remains an urgent task. Here, using first-principles calculations, we uncover the role of point defects in improving the HER activity of BP photocatalysts. We demonstrate that the defective phosphorene can be effectively activated by the photoinduced electrons under solar light, exhibiting high HER catalytic activity in a broad pH range (0-10). Besides, we propose that the direct Z-scheme in the defective BP/SnSe2 heterobilayer is quite feasible for photocatalytic overall water splitting. This mechanism could be further verified based on the excited state dynamics method. The role of point defects in the photocatalytic mechanism provides useful insights for the development of BP photocatalysts.

Tunable topological electronic states in the honeycomb-kagome lattices of nitrogen/oxygen-doped graphene nanomeshes.

The rich and exotic electronic properties of graphene nanomeshes (GNMs) have been attracting interest due to their superiority to pristine graphene. Using first-principles calculations, we considered three graphene meshes doped with nitrogen and oxygen atoms (C10N3, C9N4 and C10O3). The electronic band structures of these GNMs in terms of the proximity of the Fermi level featured a two-dimensional (2D) honeycomb-kagome lattice with concurrent kagome and Dirac bands. The position of the Fermi level can be regulated by the doping ratio, resulting in different topological quantum states, namely topological Dirac semimetals and Dirac nodal line (DNL) semimetals. More interestingly, the adsorption of rhenium (Re) atoms in the voids of the C10N3 (Re@ C10N3) GNMs induced quantum anomalous Hall (QAH) states, as verified by the nonzero Chern numbers and chiral edge states. These GNMs offer a promising platform superior to pristine graphene for regulating multiple topological states.

Prediction of 2D IV-VI semiconductors: auxetic materials with direct bandgap and strong optical absorption.

Auxetic materials are highly desirable for advanced applications because of their negative Poisson's ratios, which are rather scarce in two-dimensional materials. Motivated by the elemental mutation method, we predict a new class of monolayer IV-VI semiconductors, namely, δ-IV-VI monolayers (GeS, GeSe, SiS and SiSe). Distinctly different from the previously predicted IV-VI monolayers, the newly predicted δ-MX (X = Ge and Si; M = S and Se) monolayers exhibit a puckered unit cell with a space group of Pca21. Their stabilities were confirmed by first-principles lattice dynamics and molecular dynamics calculations. In particular, all these MX monolayers possess a large bandgap in the range of 2.08-2.65 eV and pronounced anisotropic mechanical properties, which are demonstrated by direction-dependent in-plane Young's moduli and Poisson's ratios. Furthermore, all these 2D MX monolayers possess negative Poisson's ratios (even up to about -0.3 for SiSe). Strong optical absorption is observed in these δ-IV-VI monolayers. These interesting physical properties will stimulate the development of 2D flexible devices based on IV-VI semiconductor monolayers.

Highly-anisotropic plasmons in two-dimensional hyperbolic copper borides.

Hyperbolic materials have wide application prospects, such as all-angle negative refraction, sub-diffraction imaging and nano-sensing, owning to the unusual electromagnetic response characteristics. Compared with artificial hyperbolic metamaterials, natural hyperbolic materials have many advantages. Anisotropic two-dimensional (2D) materials show great potential in the field of optoelectronics due to the intrinsic in-plane anisotropy. Here, the electronic and optical properties of two hyperbolic 2D materials, monolayer CuB6 and CuB3, are investigated using first-principles calculations. They are predicted to have multiple broadband hyperbolic windows with low loss and highly-anisotropic plasmon excitation from infrared to ultraviolet regions. Remarkably, plasmon propagation along the x-direction is almost forbidden in CuB3 monolayer. The hyperbolic windows and plasmonic properties of these 2D copper borides can be effectively regulated by electron (or hole) doping, which offers a promising strategy for tuning the optical properties of the materials.

Revealing topology with transformation optics.

Symmetry deepens our insight into a physical system and its interplay with topology enables the discovery of topological phases. Symmetry analysis is conventionally performed either in the physical space of interest, or in the corresponding reciprocal space. Here we borrow the concept of virtual space from transformation optics to demonstrate how a certain class of symmetries can be visualised in a transformed, spectrally related coordinate space, illuminating the underlying topological transitions. By projecting a plasmonic system in a higher-dimensional virtual space onto a lower-dimensional system in real space, we show how transformation optics allows us to construct a topologically non-trivial system by inspecting its modes in the virtual space. Interestingly, we find that the topological invariant can be controlled via the singularities in the conformal mapping, enabling the intuitive engineering of edge states. The confluence of transformation optics and topology here can be generalized to other wave realms beyond photonics.

Multiple Dirac cones and Lifshitz transition in a two-dimensional Cairo lattice as a Hawking evaporation analogue.

The linear energy-momentum dispersion of Dirac cones offers a unique platform for mimicking the fantastical phenomena in high energy physics, such as Dirac fermions and black hole (BH) horizons. Three types of Dirac cones (I, III, and II) with different tilts have been proposed individually in specific materials but lack of integral lattice model. Here, we demonstrated the three types of Dirac cones inherited in aπ-conjugated Cairo lattice of double-degeneratedπandpzorbitals by means of tight-binding (TB) approach, which paves a way for the design of two-dimensional (2D) Dirac materials. From first-principles calculations, we predicted a candidate material,penta-NiSb2monolayer, to achieve these multiple Dirac cones and the Lifshitz transition between different Dirac cones driven by external biaxial strain. The coexistence of the three types of Dirac cones renderspenta-NiSb2monolayer a promising 2D fermionic analogue to simulate the event-horizon evaporation with a high Hawking temperature.

Floquet-Dirac fermions in monolayer graphene by Wannier functions.

Wannier functions have been widely applied in the study of topological properties and Floquet-Bloch bands of materials. Usually, the real-space Wannier functions are linked to thek-space Hamiltonian by two types of Fourier transform (FT), namely lattice-gauge FT (LGFT) and atomic-gauge FT (AGFT), but the differences between these two FTs on Floquet-Bloch bands have rarely been addressed. Taking monolayer graphene as an example, we demonstrate that LGFT gives different topological descriptions on the Floquet-Bloch bands for the structurally equivalent directions which are obviously unphysical, while AGFT is immune to this dilemma. We introduce the atomic-laser periodic effect to explain the different Floquet-Bloch bands between the LGFT and AGFT. Using AGFT, we showed that linearly polarized laser could effectively manipulate the properties of the Dirac fermions in graphene, such as the location, generation and annihilation of Dirac points. This proposal offers not only deeper understanding on the role of Wannier functions in solving the Floquet systems, but also a promising platform to study the interaction between the time-periodic laser field and materials.

New Spiral Form of Carbon Nitride with Ultrasoftness and Tunable Electronic Structures.

The structural diversity and multifunctionality of carbon nitride materials distinct from pure carbon materials are drawing increasing interest. Using first-principles calculations, we proposed a stable spiral structure of carbon nitride, namely spiral-C3N, which is composed of sp2-hybridized carbon and pyridine nitrogen with a 60° helical symmetry along the z-direction. The stability was verified from the cohesive energy, phonon spectrum, and elastic constants. Despite the strong covalent bonds of the spiral framework, the spiral-C3N exhibits a hardness lower than 12.00 GPa, in sharp contrast to the superhardness of cubic carbon nitrides reported in previous literature, which can be attributed to the unique porous configuration. The softness of the spiral-C3N was also confirmed by the small ideal strengths, which are, respectively, 33.00 GPa at a tensile strain of 0.22 along the [1̅21̅0] direction and 18.00 GPa at a shear strain of 0.52 in the (0001)[1̅21̅0] direction. Electronic band structure of spiral-C3N exhibits metallic features. A metal-semiconductor transition can be triggered by hydrogenation of the pyridine nitrogen atoms of spiral-C3N. Such a new three-dimensional spiral framework of sp2-hyperdized carbon and nitrogen atoms not only enriches the family of carbon nitride materials but also finds application in energy conversion and storage.

Giant negative Poisson's ratio in two-dimensional V-shaped materials.

Two-dimensional (2D) auxetic materials with exceptional negative Poisson's ratios (NPR) are drawing increasing interest due to their potential use in medicine, fasteners, tougher composites and many other applications. Improving the auxetic performance of 2D materials is currently crucial. Here, using first-principles calculations, we demonstrated giant in-plane NPRs in MX monolayers (M = Al, Ga, In, Zn, Cd; X = P, As, Sb, S, Se, Te) with a unique V-shaped configuration. Our calculations showed that GaP, GaAs, GaSb, ZnS and ZnTe monolayers exhibit exceptional all-angle in-plane NPRs. Remarkably, the AlP monolayer possesses a giant NPR of -1.779, by far the largest NPR in 2D materials. The NPRs of these MX monolayers are correlated to the highly anisotropic features of the V-shaped geometry. The exotic mechanical properties of the V-shaped MX monolayers provide a new family of 2D auxetic materials, as well as a useful guidance for tuning the NPR of 2D materials.

Ferroelectricity and multiferroicity in two-dimensional Sc2P2Se6 and ScCrP2Se6 monolayers.

Two-dimensional (2D) multiferroic materials with coexistence of ferroelectricity and ferromagnetism have attracted extensive research interest due to novel physical properties and potential applications, such as in non-volatile storage nanodevices. Here, using first-principles calculations, we predicted two types of 2D materials, Sc2P2Se6 and ScCrP2Se6 monolayers with ferroelectric (FE) and multiferroic properties, respectively. The Sc2P2Se6 monolayer has out-of-plane FE polarization originating from the asymmetrical arrangement of P atoms. The FE phase is separated from the antiferroelectric (AFE) phase by an energy barrier of 0.13 eV, ensuring the stability of the FE state at room temperature. The ScCrP2Se6 monolayer formed by substituting half of the Sc atoms of Sc2P2Se6 with Cr exhibits multiferroic properties. The magnetic ground state of the ScCrP2Se6 monolayer is tunable, due to the disparity of an indirect exchange interaction between the FE and AFE phases. A reversible electrical switching between the ferromagnetic and antiferromagnetic states can be achieved in a multiferroic ScCrP2Se6 monolayer. Our theoretical results offer a new platform for the further study of 2D multiferroicity and nonvolatile magnetoelectric nanodevices.

Highly-efficient overall water splitting in 2D Janus group-III chalcogenide multilayers: the roles of intrinsic electric filed and vacancy defects.

Two-dimensional (2D) van der Waals materials have been widely adopted as photocatalysts for water splitting, but the energy conversion efficiency remains low. On the basis of first-principles calculations, we demonstrate that the 2D Janus group-III chalcogenide multilayers: InGaXY, M2XY and InGaX2 (M = In/Ga; X, Y = S/Se/Te), are promising photocatalysts for highly-efficient overall water splitting. The intrinsic electric field enhances the spatial separations of photogenerated carriers and alters the band alignment, which is more pronounced compared with the Janus monolayers. High solar-to-hydrogen (STH) efficiency with the upper limit of 38.5% was predicted in the Janus multilayers. More excitingly, the Ga vacancy of InGaSSe bilayer effectively lowers the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to the levels provided solely by the photogenerated carriers. Our theoretical results suggest that the 2D Janus group-III chalcogenide multilayers could be utilized as highly efficient photocatalysts for overall water splitting without the needs of sacrificial reagents.

Valley polarization and ferroelectricity in a two-dimensional GaAsC6 monolayer.

Valley polarization and ferroelectricity are the two basic concepts in electronic device applications. However, the coexistence of these two scenarios in one material has not been reported. Here, using first-principles calculations, we demonstrated that the two-dimensional GaAsC6 monolayer which is a hybrid structure of GaAs and graphene has a pair of inequivalent valleys with opposite Berry curvatures and an intrinsic out-of-plane spontaneous electric polarization. It also has a direct band gap of about 1.937 eV and a high carrier mobility of about 1.80 × 105 cm2 V-1 s-1, which are promising for electronic device applications. The integration of valley polarization and ferroelectricity in a single material offers a promising platform for the design of electronic devices.

Valley-selective circular dichroism and high carrier mobility of graphene-like BC6N.

The two-dimensional (2D) hybrid structures of boron nitride (BN) and graphene with properties superior to the individuals are long desired. In this work, we demonstrate theoretically that this goal can be reached in a new graphene-like borocarbonitride (g-BC6N) whose domain has been synthesized in recent experiments. It has a direct band gap of 1.833 eV and a high carrier mobility comparable to that of black phosphorene. The inversion symmetry breaking in g-BC6N leads to a pair of inequivalent valleys with opposite Berry curvatures in the vicinities of the vertices (K and K') of the Brillouin zone. The coexistence of valley-selective circular dichroism and high carrier mobility in g-BNC6 is beneficial to realize the valley Hall effect. We also propose a tight-binding (TB) model to describe the intrinsic features of this type of lattice, revealing a new class of 2D valleytronic materials.