Flexoelectricity is the interplay between the strain gradient and the electric field or polarizations, which is ubiquitous in dielectrics and much more important in two-dimensional (2D) materials due to the high flexibility. Here, we employ first-principles calculations to study the flexural deformation of the single-layer CrI3 under a spatially oscillating electric field created by hydrogen fluoride (HF) molecular dipole arrays. It is found that the CrI3 single-layer spontaneously wrinkles and the bending curvature exhibits a nearly quadratic dependence on the maximal electric field with a robust bending direction. This behavior is associated with the inverse flexoelectric effect, which refers to the emergence of strain gradient induced by an external electric field, but the curvature under such space-varying electric field increases faster with the electric field strength than that under a homogeneous field. Our results provides new possibility for the design of electromechanical devices in 2D materials.

In this study, we predict a class of two-dimensional f-electron bipolar magnetic semiconductors, GdIMH ( M = S , Se), using first-principles calculations. Monolayer GdIMH has excellent dynamic and thermal stability, higher magnetic anisotropy, and in-plane easy magnetic plane. Notably, it is also a field-induced ferrovalley material. When magnetizing along the out-of-plane direction, the combination of ferromagnetic and spin-orbit coupling effects induces a valley polarization as high as 163 meV and the Berry curvature values at the K and K ' valleys exhibit opposite signs with different absolute values. Furthermore, under this out-of-plane magnetization, biaxial strain can further enhance the valley splitting and magnetic anisotropy energy, while charge doping can realize the transition from a bipolar magnetic semiconductor to a half-metal. These results indicate that the GdIMH monolayer is a potential multifunctional 2D f-electron ferromagnet.

Strong coupling (SC) between plasmonic nanocavities and excitons in two-dimensional transition-metal dichalcogenides (2D-TMDs) has promoted fundamental studies in quantum electrodynamics and applications in photonic quantum technologies. Although previous SC research with 2D-TMD predominantly characterized cavity polaritons through scattering spectroscopy, the observation of the complete anticrossing behavior in photoluminescence (PL) spectroscopy has been less frequently reported and is crucial for ascertaining the underlying physics. In this study, we robustly demonstrate an unambiguous SC between a single gold-nanorod cavity and monolayer WS2 excitons. This was achieved by observing complete upper and lower polariton branch emissions via both scattering and PL spectroscopy. The sharp tips of the plasmonic nanocavity of the nanorods give rise to a large single exciton coupling strength up to 14.9 meV. We estimate that the number of excitons in the strongly coupled entangled state range from 8.7 to 17.3. Correlated scattering and PL spectra measurements on a single coupled system confirm the presence of strong plasmon-exciton interactions. Further theoretical simulations using a coupled-oscillator model show excellent agreement with the measured scattering and PL spectral data, effectively capturing the energy separation and intensity ratio of the polaritonic peaks. The high yield of SC structures achieved presents an opportunity to explore their nonlinear, electrical, and quantum correlation properties, which may be sufficient for practical quantum optoelectronic devices.

The four-state method [{ Phys. Rev. B 84, 224,429 (2011)}] is a reliable approach for calculating parameters of spin Hamiltonian in magnetic materials. However, the conventional self-consistent implementations often suffer from convergence difficulty and high computational costs. Here, we propose a method to generate appropriate initial spin densities that significantly reduces the number of the required self-consistent iterations. Remarkably, we find that this initial distribution even enables the non-self-consistent calculations to yield reasonable results. To facilitate the application of this method, we have developed Spinss, an open-source code that creates the initial spin densities and other necessary files for both self-consistent and non-self-consistent density functional theory calculations. We provide a detailed description of the algorithm, input and output files, and application examples which demonstrate the effectiveness of Spinss for both bulk and low-dimensional systems.PROGRAM SUMMARYProgram title: SpinssCPC Library link to program files: https://doi.org/10.17632/9z8drdc7pm.1Licensing provisions: MITProgramming language: Fortran 90Nature of problem:The spin Hamiltonian is the foundation of computational studies of magnetic materials. While the conventional four-state method for extracting spin Hamiltonians is reliable, it often suffers from convergence difficulties and high computational costs. These challenges arise from constrained self-consistent ab initio calculations involving non-collinear spins and unnatural spin configurations. As a result, applying the four-state method to complex magnetic materials is difficult, hindering the theoretical design of novel magnetic materials and the study of magnetic phenomena.Solution method:The proposed solutions are: (1) using appropriate initial spin densities for self-consistent calculations in the four-state method to accelerate convergence and improve stability, and (2) performing non-self-consistent calculations based on these spin densities to completely avoid convergence problems. The Spinss code rotates local magnetic moments to target directions, generating initial spin densities for specific spin configurations. These densities serve as input for constrained self-consistent ab initio calculations, reducing the number of self-consistent iterations needed and improving computational efficiency. Spinss also enables non-self-consistent calculations using the generated spin densities, yielding reasonable magnetic exchange parameters without the need for computationally expensive constrained self-consistent cycles.

Flexoelectricity, the modification of electric polarization induced by strain gradients, is ubiquitous in all materials and is typically a minor effect in bulk materials represented as a linear function of curvature. However, in 2D materials, the curvature can be dramatically large due to their flexibility, which naturally raises an intriguing question of whether flexoelectricity extends beyond the linear form. Here, we reveal a type of nonlinear flexoelectricity that depends on both the curvature and its gradients through first-principles calculations. This nonlinear flexoelectric effect only presents in certain materials such as monolayer CrI3, which is a consequence of the simultaneous breaking of the in-plane and out-of-plane mirror symmetries perpendicular to the curvature direction. In contrast, materials like graphene, h-BN, and 2H-WTe2 monolayers with no such symmetry breaking do not present such a nonlinear flexoelectric effect. Furthermore, a method to detect the nonlinear felexoelectricity is also proposed. Our research thus enriches the spectrum of flexophysics and has potential applications in flexoelectrics.

Single‐layer CrI3 has attracted considerable interest because of its 2D magnetism. However, the properties of defects in single‐layer CrI3 have not been thoroughly studied yet. In this study, a line defect in the CrI3 single layer, which is a grain boundary caused by the position flipping of iodine atoms, is theoretically investigated. Using first‐principles calculations, the structural and electronic properties of this grain boundary in both zigzag and armchair directions are explored. The grain boundary exhibits ferromagnetic exchange interaction between the two grains. The conduction band minimum and valence band maximum of the grain boundary states are in the energy gap of perfect CrI3 single layer and form a direct gap, indicating an enhancement in photoelectronic properties. Moreover, the partial charge density in real space reveals that the doped electrons or holes predominantly distribute at the grain boundary. This can support a purely spin‐polarized current along the grain boundary. Findings suggest that the grain boundary exhibits unique electronic and magnetic properties, potentially offering valuable insights for spintronics applications.

Carbon-framework-based nanostructures (CFBNs) with atomically precise well-tailored carbon-vacancy defect structures hold significant potentials for application in optoelectronic devices and catalysis materials. However, a major challenge for the practical application of these materials is their notable low conductivity. In this study, we conducted terahertz time-domain spectroscopy (THz-TDS) measurements and first-principles calculations to investigate the transport properties of CFBNs with different carbon-vacancy defect structures or porous configurations. Our THz-TDS measurements reveal that high carrier densities exist in CFBNs we studied and show that electron scattering at disordered lattices domains and defect or porous edges significantly reduces carrier mobility. Despite the observed high carrier densities, our first-principles calculations revealed a semiconducting band structure for pristine CFBNs and deep transition energy level for native defects. By combining these calculated results with the measured conductivity and carrier density, we propose that the high carrier density observed in CFBN samples is the result of thermally excited charge carriers in localized states, while the low conductivity is contributed by carrier hopping in localized states and extreme scattering in extended states. Both of these phenomena are induced by the disordered lattices in CFBNs. Our findings provide insights into the transport properties of CFBNs, which are essential for the advancement of high-performance carbon-based devices.

In this study, we comprehensively investigated the scaling law for elastic properties of three-dimensional honeycomb-like graphenes (3D graphenes) using hybrid neural network potential-based molecular dynamics simulations and theoretical analyses. The elastic constants were obtained as functions of honeycomb hole size, denoted by the graphene wall length L. All five independent elastic constants in the large-L limit are proportional to L-1. The associated coefficients are combinations of elastic constants of two-dimensional graphene. High-order terms including L-2 and L-3 emerge for finite L values. They have three origins, the distorted areas close to the joint lines of 3D graphenes, the variation in solid angles between graphene plates, and the bending distortion of graphene plates. Significantly, the chirality becomes essential with decreasing L because the joint line structures are different between the armchair and zigzag-type 3D graphenes. Our findings provide insights into the elastic properties of graphene-based superstructures and can be used for further studies on graphene-based materials.

Since the first successful fabrication in 2004[1],graphene has received tremendous attention due to its extremely simple atomic structure and alluring physical properties.For example,its mass-less low energy excitations have a linear dispersion and thus its transport property is governed by Dirac equation instead of Schrödinger equation.These special electronic structures suppress the intra-valley and inter-valley backscatterings,leading to the half-integer and fractional quantum Hall effect[2]under magnetic field and the relativistic quantum tunneling described by the Klein paradox[3].

Recently, two-dimensional (2D) bilayer magnetic systems have been widely studied. Their interlayer magnetic interactions play a vital role in the magnetic properties. In this paper, we theoretically studied the interlayer magnetic interactions, magnetic states, and ferroelectricity of π/3-twisted CrX2 (X = Se, Te) bilayers (π/3-CrX2). Our study reveals that the lateral shift could switch the magnetic state of the π/3-CrSe2 between interlayer ferromagnetic and antiferromagnetic, while just tuning the strength of the interlayer antiferromagnetic interactions in π/3-CrTe2. Furthermore, the lateral shift can alter the off-plane electric polarization in both π/3-CrSe2 and π/3-CrTe2. These results show that stacking is an effective way to tune both the magnetic and ferroelectric properties of 1T-CrX2 bilayers, making the 1T-CrX2 bilayers hold promise for 2D spintronic devices.

In the present study, we proposed a new type of three-dimensional (3D) graphene structures, which are orthorhombic and become tetragonal for the higher symmetry cases, by using the crystal structure prediction method and classic molecular dynamics (MD) simulations. The elastic properties have been systematically investigated, which show that the elastic constants can be expressed as functions of the graphene wall length L, i.e. the hole size. Most of the elastic constants are composed of L−1 and L−2 terms, where L−1 term is the main part and the less important L−2 term comes from the joint areas between graphene walls. Our predicted orthorhombic 3D graphene structure and the scaling laws in the elastic properties provide theoretical references for future experimental studies on the superstructures of graphene.

Recently, CrSe2, a new ferromagnetic van der Waals two-dimensional material, was discovered to be highly stable under ambient conditions, making it an attractive candidate for fundamental research and potential device applications. Here, we study the interlayer interactions of bilayer CrSe2using first-principles calculations. We demonstrate that the interlayer interaction depends on the stacking structure. The AA and AB stackings exhibit antiferromagnetic (AFM) interlayer interactions, while the AC stacking exhibits ferromagnetic (FM) interlayer interaction. Furthermore, the interlayer interaction can be further tuned by tensile strain and charge doping. Specifically, under large tensile strain, most stacking structures exhibit FM interlayer interactions. Conversely, under heavy electron doping, all stacking structures exhibit AFM interlayer interactions. These findings are useful for designing spintronic devices based on CrSe2.

High-performance active terahertz modulators as the indispensable core components are of great importance for the next generation communication technology. However, they currently suffer from the tradeoff between modulation depth and speed. Here, we introduce two-dimensional (2D) tellurium (Te) nanofilms with the unique structure as a new class of optically controlled terahertz modulators and demonstrate their integrated heterojunctions can successfully improve the device performances to the optimal and applicable levels among the existing all-2D broadband modulators. Further photoresponse measurements confirm the significant impact of the stacking order. We first clarify the direction of the substrate-induced electric field through first-principles calculations and uncover the unusual interaction mechanism in the photoexcited carrier dynamics associated with the charge transfer and interlayer exciton recombination. This advances the fundamental and applicative research of Te nanomaterials in high-performance terahertz optoelectronics.

Plutonium polyhydrides are fascinating systems not only because plutonium is an important element in nuclear industry with complicated 5f electron behaviors, but also because metal polyhydrides are potential superconducting materials. Here, we have performed first-principles calculations, which adopts the projectedaugmented wave (PAW) approach with the Perdew-Burke-Ernzerhof (PBE)-type functional including an onsite Hubbard U correction, to investigate the structural, electronic, and superconducting properties of hydrogen-rich PuHx (6 <= x <= 10) in clathrate structure under high pressures up to 350 GPa. All systems are found to be metallic with occupied states near the Fermi level contributed mainly by Pu-5f electrons. The highest superconducting transition temperature T-c of PuHx is only 26.4 K (PuH10 under 300 GPa), and other predicted Tc are less than 1 K, showing that the f electrons in plutonium are harmful to the superconductivity in plutonium polyhydrides.

Hydrogen‐rich materials synthesized at megabar pressures have revolutionized the field of high‐temperature superconductivity. Among hydrogen‐rich materials, clathrate hydrides which show near‐room‐temperature superconductivity have been discovered in recent years. Herein, clathrate actinide hydrides are constructed, and their electronic properties and electron–phonon interaction as well as superconductivity under high pressures are theoretically investigated with density functional theory tools. The calculations adopt projected‐augmented‐wave (PAW) approach. The Perdew–Burke–Ernzerhof (PBE)‐type generalized gradient approximation (GGA) is used for the exchange–correlation potentials. Specifically, Th–H and Pu–H compounds are presented, of which the stability is demonstrated via calculated phonon dispersions, and the superconducting transition temperatures T c under 100, 200, and 400 GPa are estimated. ThH 10 among Th–H compounds has the highest transition temperature of superconductivity, and T c of ThH 10 can reach 151.8 K under 200 GPa. PuH 10 is also better than other investigated Pu–H compounds in superconductivity, but the highest T c of PuH 10 is only 31.4 K under 400 GPa, much lower than that of ThH 10 .

Polar metals have generated significant interest since the ferroelectric-like structural transition in metallic LiOsO was discovered. Herein, we report on a strain-modulated polar metal in the ferroelectric/metal superlattice of 1 : 1 KNbO/CaNbO. Using first-principles calculations, we have investigated the structural distortions, including polar distortions and octahedral rotations, and layer-by-layer electronic structures in the KNbO/CaNbO superlattice under different epitaxial strains. Along the stacking direction, the superlattice has almost parallel polar displacements under compressive strain, whereas both in-plane and out-of-plane antiferroelectric-like polar displacements are robust under intermediate strain, which is connected to the octahedral tilting pattern and interlayer electron transfer. In addition, the in-plane polar distortions are enhanced by tensile strains and have a sudden increase at 4% tensile strain. The metallicity is mainly contributed by d electrons from Nb atoms. And orbital-resolved electron distributions in each layer show that d-orbital splitting is related not only to the epitaxial strain but also to the direction of polar displacements. Our results suggest an efficient way to tune polar distortions as well as local metallicity epitaxial strains in the superlattice.

GdI2 monolayer is a promising material for spintronics applications due to its robust room-temperature ferromagnetism and sizable valley polarization. In two-dimensional van der Waals magnets, interlayer magnetic coupling plays a crucial role in device applications. The performance of these devices can be effectively tuned by adjusting the stacking order, charge doping, and strain. By performing first-principles calculations, we have demonstrated that the interlayer magnetic coupling in bilayer GdI2 is highly dependent on the stacking order, which can be tuned between ferromagnetic (FM) and antiferromagnetic orders through lateral shifting. Furthermore, the interlayer magnetic coupling can also be tuned by charge doping and strain, where both electron and hole doping can enhance the FM coupling interaction between layers, and the interlayer FM coupling can be strengthened with increasing biaxial tensile strain. These results show that bilayer GdI2 has rich tunable interlayer magnetic interactions, which can be used in designing interesting spin tunnel field-effect transistor devices.

Polar metals have generated significant interest since the ferroelectric-like structural transition in metallic LiOsO3 was discovered. Herein, we report on a strain-modulated polar metal in the ferroelectric/metal superlattice of 1 : 1 KNbO3/CaNbO3. Using first-principles calculations, we have investigated the structural distortions, including polar distortions and octahedral rotations, and layer-by-layer electronic structures in the KNbO3/CaNbO3 superlattice under different epitaxial strains. Along the stacking direction, the superlattice has almost parallel polar displacements under compressive strain, whereas both in-plane and out-of-plane antiferroelectric-like polar displacements are robust under intermediate strain, which is connected to the octahedral tilting pattern and interlayer electron transfer. In addition, the in-plane polar distortions are enhanced by tensile strains and have a sudden increase at 4% tensile strain. The metallicity is mainly contributed by d electrons from Nb atoms. And orbital-resolved electron distributions in each layer show that d-orbital splitting is related not only to the epitaxial strain but also to the direction of polar displacements. Our results suggest an efficient way to tune polar distortions as well as local metallicity via epitaxial strains in the superlattice.