Two-dimensional phase-change materials (PCMs) are pivotal for next-generation memory and quantum devices. Using first-principles calculations, we systematically explore the phase-change properties of 2D gallium-based chalcohalides in the hexagonal phase. By potential energy surface scanning, we identify GaOCl and GaOBr monolayers as prototypical PCMs hosting two stable crystalline polymorphs (O and T phases), distinguished by the halogen position. The energy barriers for the O-to-T or T-to-O structural phase transition range from 40.2 to 195.5 meV per formula unit, enabling reversible switching that can be further tuned by biaxial strain, electric fields, or intrinsic vacancies. The O-to-T transition triggers a cascade of functional changes: a drastic band gap narrowing (with GaOBr evolving into a topological insulator), reduced carrier effective masses, orbital reconstruction, and a shift of optical absorption into the infrared/visible spectrum. Additionally, the materials exhibit a switchable out-of-plane polarization and a phase-dependent piezoelectric response---strong in-plane for the O phase and out-of-plane for the T phase. These findings highlight 2D gallium-based chalcohalides as a versatile platform integrating phase-change functionality with topology, electronics, and mechanics.
Two-dimensional ferrovalley semiconductors offer a compelling platform for topologically nontrivial valleytronics, yet an explicit topological criterion governing the strain-driven quantum anomalous valley Hall effect (QAVHE) and its microscopic origin has remained largely unexplored. Here, combining first-principles calculations, a effective model, and crystal-field analysis for the FeCl2 monolayer, we resolve the complete five-stage phase sequence driven by biaxial tensile strain. Central to our analysis is an explicit Chern number criterion derived analytically from the two-band Hamiltonian: a nonzero Chern number requires the renormalized gaps of the two valleys to carry opposite signs. Crystal-field analysis further reveals why this condition is microscopically inevitable: in-plane strain couples selectively to the E-type orbitals via σ-bond weakening while leaving the A1-type orbital nearly unperturbed—a symmetry-selective modulation that guarantees orbital inversion and opposite-sign valley gaps over a finite strain window. These results establish a complete causal chain from strain to topology for QAVHE engineering in 2D magnetic materials.
Realizing tunable switching between unipolar magnetic semiconductor (UMS) and bipolar magnetic semiconductor (BMS) states in a single material is crucial for advanced multifunctional spintronic devices. In this study, we systematically investigated the strain-tunable transition between UMS and BMS in Mo-doped Janus 2H-VSeTe monolayer by using first-principles calculations. Our results show that Mo doping not only maintains the structural stability but also reconstructs its electronic structure. Upon doping, both the VBM and the CBM are occupied by spin-down electrons, exhibiting the characteristics of an indirect bandgap UMS, which is attributed to the modulation of electron occupancy in d orbitals by Mo doping and the splitting of Mo d orbitals in the trigonal prismatic crystal field. Furthermore, by applying biaxial strain, we achieved tunable transition between UMS and BMS. This transition mechanism originates from strain induced changes in interatomic bond lengths, which adjust orbital overlap and crystal field strength, thereby reshaping the electronic state distribution near the Fermi level. Our work proposes a synergistic strategy of transition metal doping and strain engineering to tailor the electronic structure of Janus 2H-VSeTe monolayer, realizing the tunable UMS-BMS transition in a single material and providing a theoretical basis for the design of high-performance multifunctional spintronic devices.
Two-dimensional transition metal dichalcogenides (TMDs) exhibit tunable electronic structures and unique thermal transport properties, making them attractive for nanoelectronic and thermoelectric applications. However, the mechanisms by which nanopore geometry governs electronic structure and thermal transport in nanoporous TMDs remain insufficiently understood. Here, a theoretical framework combining nanothermodynamics and atomic-bond-relaxation theory is developed to investigate the bandgap and thermal conductivity of nanoporous monolayer MoS2. Analytical relationships are established between these properties and key geometric parameters, including pore shape, characteristic length and periodic length. The results show that increasing nanopore size or decreasing periodic length enhances lattice relaxation at nanopore edges, leading to bandgap widening. Conversely, thermal conductivity decreases with increasing nanopore size and porosity but increases with periodic length, owing to competing effects of phonon-boundary scattering and phonon group velocity modulation. Among the nanopore geometries considered, hexagonal nanopores exhibit the largest bandgap modulation and the strongest suppression of thermal conductivity. These findings establish explicit structure property relationships between nanopore geometry and transport properties, providing quantitative physical insight into electronic and thermal transport modulation in nanoporous two-dimensional materials.
We study the propagation of wave packets in monolayer graphene subjected to triangular electrostatic potentials with different geometries and polarities. Three potential configurations are considered: a triangular barrier, a triangular well, and a combined barrier–well structure. The results indicate that the transmission probability P strongly depends on both the potential geometry and the propagation direction of the wave packet. For the triangular barrier configuration, a pronounced directional dependence is observed, with non-monotonic transmission behavior for forward propagation and monotonic suppression for backward propagation as the potential magnitude increases. In contrast, the triangular potential well exhibits nearly perfect transmission for all considered energies and potential magnitudes, independent on wave-packet propagation direction. Remarkably, the combined barrier–well configuration gives rise to a qualitatively different transport regime that cannot be interpreted as a simple superposition of the individual barrier and well responses. In this mixed-polarity structure, the transmission can be strongly suppressed even for relatively small potentials, reaching values as low as P≈0.05, corresponding to only about 5% transmission, indicating very strong suppression of wave-packet transmission.These findings demonstrate that properly combined triangular electrostatic potentials provide an efficient mechanism for controlling wave-packet transport in graphene and may be useful for designing graphene-based electron-optics and directional transport devices.
Graphene nanoribbons (GNRs), as a family of one-dimensional (1D) materials, are promising candidates for carbon-based electronics due to their width-dependent properties. However, their open edges are prone to structural defects, which can significantly influence charge transport. Although first-principles calculations are widely used to investigate defect-induced electronic structures and transport phenomena, their high computational cost limits large-scale exploration. To address this challenge, we develop a supervised learning framework based on neural networks to predict the transmission spectra and current–voltage (I-V) characteristics of GNRs with various edge defect configurations. The trained model accurately reproduces both transmission spectra and I-V curves across a range of defect geometries. Furthermore, by optimizing the descriptors, the model exhibits strong generalizability to GNRs of varying widths. This machine learning framework provides a computationally efficient alternative to first-principles simulations for studying charge transport in GNR-based devices.
A systematic theoretical investigation is presented for the nonlinear optical properties of GaAs/Al0.3Ga0.7As asymmetric coupled cylindrical quantum well wires (CCQWWs) incorporating, for the first time in this geometry, the full position-dependent effective mass (PDM) through the BenDaniel–Duke kinetic-energy operator, combined with a systematic comparison against the widely used constant-mass approximation. A non-resonant intense laser field (ILF) is treated within the non-perturbative Floquet dressed-potential approach, and the three lowest-order nonlinear optical susceptibilities — optical rectification (NOR, χ(2)), second-harmonic generation (SHG, χ(2)), and third-harmonic generation (THG, χ(3)) — are computed via the compact density-matrix formalism. The ILF is found to reshape the double-well confinement potential dramatically: the THG peak amplitude is enhanced by up to 148-fold at α0=6 nm relative to the field-free case, arising from a two-photon intermediate resonance (ħω≈E31/2=33 meV at α0=6 nm) that drives the intermediate-state denominator D2 near zero. The NOR amplitude decreases overall with α0 (with a small local increase at α0=4 nm) due to ILF-induced suppression of the off-diagonal matrix element M13, while SHG reaches a 3-fold maximum at α0=4 nm. The PDM introduces corrections of up to 34% in the THG coefficient and up to 1.1 meV in the transition energies relative to the constant-mass approximation, with the sign and magnitude of the correction depending sensitively on α0.
Field-controlled quantum nanostructures offer a powerful platform for tailoring bound-state spectra and localization, yet the combined roles of magnetic confinement, Aharonov–Bohm phase control, and screened impurity attraction remain insufficiently understood in pseudodot systems. In this work, we investigate an electron confined in a two-dimensional quantum pseudodot under the simultaneous action of a perpendicular magnetic field, an Aharonov–Bohm flux, and a central screened Coulomb impurity. Within the effective-mass approximation, the system is described by a pseudoharmonic confinement that incorporates both harmonic trapping and singular-core repulsion, together with a Yukawa-type impurity potential. The resulting dimensionless radial Schrödinger equation is solved using a stable self-adjoint numerical eigensolver, enabling systematic exploration of the multidimensional control space. The results reveal a clear and physically rich confinement–screening competition. Increasing the screening parameter suppresses the impurity attraction, shifts the spectrum upward, and drives the system from impurity-assisted localization toward dot-dominated confinement. In contrast, the magnetic field strengthens the effective confinement and enhances impurity influence by compressing the wavefunction toward the core region. The Aharonov–Bohm flux lifts the symmetry and produces a pronounced, externally tunable angular-momentum splitting. Spatial observables, including the mean radius and core probability, confirm the real-space redistribution of the bound states and expose a critical screening boundary in the magnetic-field–screening plane. To accelerate and interpret this multidimensional analysis, we develop machine-learning surrogates based on Random Forest, XGBoost, and multilayer perceptron models for multi-output prediction of the scaled energy, adjacent-channel splitting, and mean radius, together with localization-regime classification. Rather than proposing an entirely new confinement model, the present work provides a unified spectral–localization and ML-assisted mapping framework for identifying confinement–screening crossover behavior in field-controlled low-dimensional quantum nanostructures.
In the field of two-dimensional materials, bond strength exerts a significantly influence on carrier transport properties. Modulating interatomic interaction can be achieved by doping elements with differing electronegativity or by altering bond types, such as transitioning among the three primary chemical bonds or utilizing the recently proposed metavalent bonding. This study focuses on two stable tetra-pentagonal TlS monolayers (α- and β-phases) composed of elements with a large electronegativity difference and stereochemically active lone pair electrons. In the α-TlS structure, sulfur forms anionic S-S covalent dimers, whereas atomic rearrangement in the β-phase leads to the emergence of cationic metallic Tl-Tl dimers. Through a comparative analysis of two phases with identical elemental compositions but distinct dimer configurations, we reveal that their differing transport properties stem from the specific roles dimers play in electron and phonon transport, despite the presence of similar weak Tl-S ionic bonds in both systems. Specifically, the combination of a larger p-type Seebeck coefficient, resulting from its flat valence band dispersion, and the ultralow lattice thermal conductivity arising from strong anharmonicity leads to a ZT value of 2.35 in α-TlS. On the other hand, the metallic Tl-Tl dimers in β-TlS yield a light hole effective mass and thus superior p-type electrical conductivity, leading to a substantial ZT of 2.23 while still maintaining low lattice thermal conductivity via hierarchical chemical bonding characteristics. This work not only elucidates the subtle role of chemical bond configurations in modulating electron and phonon transport within these structures, but also provides a strategy for searching for high performance low-dimensional thermoelectrics.
This paper proposes a four-band high-sensitivity terahertz absorber based on AlCuFe quasicrystal, with an absorption rate higher than 93% at four characteristic frequencies of 4.04 THz, 5.08 THz, 6.42 THz, and 7.47 THz. The absorption rate at the 6.42 THz frequency point exceeds 99%. The excellent absorbing performance of the device stems from the synergistic coupling of cavity resonance, guided mode resonance, and localized surface plasmon resonance. It can be applied in high-sensitivity terahertz sensing and detection. When detecting the biological suspension system within the refractive index range of 1.00-1.10, a significant resonance frequency shift can be observed.The refractive index sensitivities of M1, M2, M3, and M4 are 800 GHz·RIU−1, 1090 GHz·RIU−1, 400 GHz·RIU−1, and 3120 GHz·RIU−1 respectively. The overall maximum refractive index sensitivity can reach 3120 GHz·RIU−1, the highest quality factor Q is 108.08, the maximum normalized detection efficiency FOM is 28.65, and it has excellent detection resolution capability. Among them, the M2 resonant peak corresponding to 5.08 THz has the best comprehensive sensing performance, possesses excellent polarization-independent characteristics and robustness to incident angle, and the absorption spectra of TE and TM polarization modes are basically overlapping. Within the incident angle range of 0°to 60°, the resonant frequency drift is only 0.05 THz, and the absorption amplitude fluctuation is weak, which is suitable for refractive index detection in biological suspension systems. At the same time, by adjusting the Fermi level of AlCuFe material and the geometric size of the microstructure, the resonant response of the device can be flexibly tuned, and the practical application scope can be expanded. This absorber has the advantages of multi-band efficient absorption, polarization-insensitive, stable resonant response at large incident angles, and multi-channel refractive index sensing. It has broad application potential in biomedical sensing and integrated terahertz functional devices.
This paper systematically investigates the structural properties, electronic structure, phonon dispersion, electron-phonon coupling (EPC) constants, and superconductivity of H-functionalized M2N (M = Sc, Zr, Nb, Hf, and Ta) using first-principles calculations. Hydrogen functionalization induces superconductivity or enhances superconducting behavior in nearly all studied M2N (M = Sc, Zr, Nb, Hf, and Ta). The Tc values of M2NH2 (M = Zr, Nb, Hf, and Ta) reach a maximum of 9.6 K. The increases in λ and Tc of M2NH2 (M = Zr, Nb, Hf, and Ta) mainly originate from the contribution of low-frequency phonon modes dominated by the transition metal atoms. More importantly, phonon softening is also observed in specific low-frequency regions, which further contributes to the enhancement of EPC. Beyond the dominant low-frequency contribution, the remaining EPC is mainly distributed between intermediate-frequency phonons dominated by N atoms and a high-frequency region associated with H atoms. These findings will help researchers fill the gap in understanding the superconductivity of MXenes and advance current knowledge in this field.
Geometric effects affect the electronic properties of charged particles in nanostructured semiconductor systems. It is well known that regions of high curvature can induce additional electronic confinement. Motivated by these curvature-induced position-dependent mass effects, we investigate the bound states in a bilayer-graphene toroidal nanostructure with a position-dependent effective mass. Using the effective-mass approximation within a single-band approach, we solve the stationary Schrödinger-like equation in toroidal coordinates, including the geometry-induced da Costa potential and a curvature- and position-dependent effective mass. The mass profile is constructed from a scalar invariant formed by the mean and Gaussian curvatures of the surface, inspired by the local curvature density entering Willmore-type elastic energies. This allows us to reduce the problem to a one-dimensional Hermitian Schrödinger-like equation with a mass-dependent effective potential. Our results show that the curvature-mass coupling controls both the depth and topology of the effective confinement, that negative values of the coupling can split the inner-equator geometric well into two lateral wells separated by a repulsive barrier, and that finite azimuthal angular momentum can shift the dominant localization region toward the outer equator of the torus.
We investigate the influence of geometry on the electronic states of an electron confined to a graphene surface that is flat on average but exhibits a Gaussian deformation. Using the method developed by da Costa, we derive a Schrödinger-type equation that effectively describes the dynamics of an electron with position-dependent mass M(r). We show that M(r) depends explicitly on the geometric parameters of the deformation and is closely related to the Gaussian curvature associated with the surface metric. Bound states are found in the absence of orbital angular momentum for several geometric configurations, and their number is shown to depend sensitively on the curvature of the deformation. No bound states are observed for nonzero orbital angular momentum. We also analyze the effects of the geometric deformation on electronic transitions in the terahertz regime. Under appropriate conditions, the resulting two-level system may provide a platform for qubit encoding.
This study explored the adsorption properties of black phosphorene (BP) and its defects, as well as metal (Al, Cu, Fe) doping systems, for CO, H2S, NH3, and NO gases using density functional theory (DFT). The results reveal that physisorption governs all four gas/BP systems. A single phosphorus vacancy enhances gas adsorption, with NO/BPV shifting to chemisorption. Doping Al, Cu, and Fe into the vacancy further increases the adsorption energies. Among the investigated systems, BPV-Cu exhibits the best balance between electrical response and recovery behavior, making it a promising reusable gas-sensing material, whereas BPV-Al and BPV-Fe provide stronger adsorption but suffer from prolonged recovery due to their stronger gas-surface interactions. This work establishes a theoretical basis for the design of phosphorene-based gas sensors.
