A method for predicting the evolution characteristics of two-dimensional Janus material magnetic coupling based on first principles

Abstract

The application belongs to the field of material calculation simulation, and relates to a prediction method for magnetic coupling evolution characteristics of two-dimensional Janus materials based on first principles. The technical problem to be solved is that the existing technology fails to uniformly analyze the magnetic freedom and structural evolution behavior, resulting in interlayer abnormal shrinkage, spontaneous bond collapse and lack of judgment basis for the coupling relationship between magnetism and interlayer structure. The technical scheme comprises the following steps: establishing a crystal structure model taking MA2Z4 material as a mother body; cutting and constructing an asymmetric Janus single layer; constructing a double-layer AA stacking initial model; performing geometric structure optimization and static self-consistent calculation under non-spin polarization and spin polarization conditions respectively; comparing the interlayer spacing, magnetic moment distribution and electronic state characteristics under the two conditions; analyzing the inhibitory effect of interface magnetism on interlayer bonding tendency by combining the electron localization function and the crystal orbital Hamiltonian population; and finally establishing the cooperative evolution relationship among magnetism, interlayer coupling and electronic structure, thereby providing a theoretical basis for identifying stable double-layer van der Waals configuration.

Description

Technical Field

[0001] This invention belongs to the field of computational materials science and theoretical design of two-dimensional functional materials, and provides a method for predicting the magnetic coupling evolution characteristics of two-dimensional Janus materials based on first principles. Background Technology

[0002] Two-dimensional van der Waals bilayer materials, due to their tunable interlayer coupling and significant interfacial response, hold significant research value in areas such as slip ferroelectricity, piezoelectricity, interlayer polarization control, and low-dimensional functional devices. Existing research indicates that MA2Z4 and its Janus derivatives possess strong designability in layered configurations. By symmetrically tailoring the parent structure and constructing bilayer stacks, new structural foundations can be provided for controlling slip behavior, polarization reversal, and interlayer coupling. For these Janus bilayer systems, maintaining a relatively independent van der Waals bilayer configuration is a prerequisite for realizing subsequent slip ferroelectricity, interlayer polarization response, and related functional properties.

[0003] However, this study found that not all Janus-MA2Z4 bilayer structures can stably maintain a layered stacking state in the ground state. Due to the loss of equivalence in the chemical environments of the upper and lower surfaces after symmetry trimming, local coordination relationships, charge distribution, and interatomic interactions at the interface all change. Some candidate systems exhibit significant interlayer shrinkage after structural optimization, further manifested as abnormally close interatomic proximity, enhanced interlayer bonding, and even bilayer collapse and fusion. This evolution disrupts the original bilayer separation characteristics, compresses or even eliminates interlayer slip degrees of freedom, thus hindering the subsequent realization of slip ferroelectric, piezoelectric, and interlayer coupling functions.

[0004] Current analyses of such systems often treat magnetic and structural stability issues separately, lacking a method to integrate them as coupled variables within a unified analytical framework. In fact, in Janus bilayer systems, magnetism is not a secondary phenomenon but a crucial degree of freedom directly influencing whether interlayer spacing is maintained, whether interface atoms form stable bonds, and whether the bilayer retains van der Waals characteristics. The introduction of magnetic states alters local electron occupancy, orbital hybridization, and interlayer interaction competition, further affecting the final convergent configuration and electronic structure distribution. Therefore, based solely on non-magnetic structure optimization results, it is difficult to accurately determine whether a candidate system will maintain a stable bilayer or evolve towards spontaneous interlayer bonding and collapse configurations.

[0005] To address the aforementioned problems, this invention proposes a first-principles-based method for predicting the magnetic coupling evolution characteristics of two-dimensional Janus materials. Taking the Janus-MA2Z4 bilayer system as the research object, this method compares the structural evolution behavior under uniform initial interlayer spacing and uniform stacking conditions, comparing the non-spin-polarized and spin-polarized states respectively. Furthermore, it determines the competitive relationship between interlayer magnetism and interlayer bonding by combining information on interface atomic magnetic moments, localized electron distribution, and bonding. The study shows that for some Janus bilayer systems that are prone to interlayer collapse under non-spin-polarized conditions, when the interface forms appropriate magnetism relative to the atomic layers, the tendency for abnormal interlayer proximity and interlayer bonding can be significantly suppressed. This allows the system to regain its bilayer layering characteristics and van der Waals stacking properties, thus preserving the necessary structural foundation for the subsequent realization of related interlayer functions.

[0006] The technical contribution of this invention lies not in providing static results for a single material, but in proposing a path for predicting and screening the structural stability of the Janus-MA2Z4 bilayer system. This involves first determining whether candidate structures exhibit interlayer collapse tendency under non-spin-polarized conditions, and then further examining whether the interfacial magnetism introduced with spin polarization can suppress interlayer bonding and maintain the bilayer configuration. For candidate systems that can maintain van der Waals bilayer properties under magnetic influence, their interlayer slip space is preserved, making them more suitable as a material basis for subsequent research on slip ferroelectric, piezoelectric responses, and interlayer coupling modulation. Therefore, this invention essentially provides a theoretical prediction and screening approach for suppressing interlayer collapse and maintaining a slippable bilayer structure through interfacial magnetism, providing a basis for structural feasibility and screening directions for the design of related low-dimensional functional materials and device selection. Summary of the Invention

[0007] The purpose of this invention is to solve the problems in the prior art caused by the failure to analyze magnetic degrees of freedom and structural evolution behavior within a unified framework, resulting in abnormal interlayer shrinkage, spontaneous bonding collapse, and a lack of clear criteria for determining the coupling relationship between magnetism and interlayer structure in two-dimensional Janus materials.

[0008] To achieve the above objectives, the present invention employs the following technical means:

[0009] This invention provides a method for predicting the magnetic coupling evolution characteristics of two-dimensional Janus materials based on first-principles calculations, characterized by comprising the following steps:

[0010] Step S1: Select MA2Z4 material with a seven-layered ZAZMZAZ structure as the initial parent material and establish the original crystal structure model.

[0011] Step S2: Perform bottom AZ two-layer trimming on the seven-layer structure of the original crystal structure model to construct an asymmetric Janus monolayer with a ZAZMZ five-layer atomic arrangement sequence;

[0012] Step S3: Construct a double-layer AA stacked initial model based on the asymmetric Janus single layer, and set the initial layer spacing to obtain the double-layer Janus initial model;

[0013] Step S4: Optimize the geometry of the initial bilayer Janus model under non-spin polarization conditions to obtain a stable configuration and corresponding structural parameters under non-spin polarization conditions.

[0014] Step S5: Optimize the geometry of the same double-layer Janus initial model under spin polarization conditions to obtain a stable configuration and corresponding magnetic and structural parameters under spin polarization conditions.

[0015] Step S6: Perform static self-consistent electronic structure calculations on the optimized structures obtained in steps S4 and S5 respectively to obtain the total energy, magnetic moment distribution and electronic structure parameters of the corresponding system.

[0016] Step S7: Compare the total energy, interlayer spacing, magnetic moment distribution and electronic state characteristics near the Fermi level under non-spin polarization and spin polarization conditions to determine whether the candidate Janus bilayer system tends to be a state of anomalous interlayer contraction and spontaneous bonding collapse, or a magnetically stable van der Waals state that maintains a large interlayer spacing.

[0017] Step S8 further analyzes the selected Janus bilayer system with M=Ca by combining its band structure, density of states, electronic localization function (ELF), and crystal orbital Hamiltonian population (COHP). By comparing the distribution of electronic states near the Fermi level, the localization characteristics of electronic interfaces, and the strength of interlayer bonding interactions under non-spin polarization and spin polarization conditions, the inhibitory effect of interfacial magnetism on interlayer bonding trends is determined. The co-evolution relationship between magnetism, interlayer coupling, and electronic structure in the two-dimensional Janus bilayer system is established. That is, the generation or enhancement of interfacial magnetism will change the distribution of electronic states near the Fermi level and orbital hybridization characteristics, thereby weakening interlayer bonding, increasing interlayer spacing, and inhibiting abnormal interlayer shrinkage or collapse. This provides a basis for determining the structural stability of the Janus bilayer system and analyzing interlayer interactions.

[0018] In the above scheme, step S7 determines whether the Janus bilayer system tends towards an anomalous interlayer shrinkage and spontaneous bonding collapse state, or towards a magnetically stable van der Waals state that maintains a large interlayer spacing. Specifically:

[0019] The interlayer spacing is determined by the closest distance between Z atoms in the interlayer: when the interlayer spacing is greater than or equal to 3.0 Å, the system is considered to maintain bilayer separation characteristics; when the interlayer spacing is less than 3.0 Å but greater than or equal to 2.0 Å, the system is considered to have significant interlayer contraction and a tendency to collapse; when the interlayer spacing is less than 2.0 Å, the system is considered to have abnormal contraction and a tendency to collapse. Based on this, the strength of interlayer bonding is further quantitatively compared by combining the COHP integral value, and the local distribution characteristics of interlayer electrons are verified by combining ELF. Finally, it is determined whether the system tends to spontaneous bonding collapse state or magnetically stable van der Waals state.

[0020] In the above scheme, the MA2Z4 material mentioned in step S1 is a two-dimensional layered compound with layered hexagonal symmetry. Its original monolayer structure presents a seven-layer atomic arrangement sequence of ZAZMZAZ along the vertical plane direction, where M is an alkaline earth metal element, A is a group IV element, and Z is a group V element.

[0021] In the above scheme, M is selected from Ca, A is selected from one or two of Si and Ge, Z is selected from one or more of N, P and As, and the candidate system is a Janus bilayer system derived from the MA2Z4 matrix formed by the combination of the above elements.

[0022] In the above scheme, in step S2, the bottom continuous A and Z atomic layers are removed from the original seven-layer structure along the stacking direction, so that the original symmetrical structure is transformed into an asymmetric Janus monolayer with a five-layer atomic arrangement sequence of ZAZMZ, thereby forming a two-dimensional Janus configuration with inequivalent upper and lower surface elemental composition and local coordination environment.

[0023] In the above scheme, in step S3, a double-layer Janus initial model is constructed using the AA stacking method, so that the upper and lower Janus single layers are aligned in the in-plane lattice projection direction, and a predetermined initial interlayer spacing is set as the starting condition for interlayer coupling evolution; the initial interlayer spacing is a modeling parameter of 3.0 Å to maintain the initial separation state of the double layers, and a vacuum layer is set in the vertical plane direction to reduce the interaction between adjacent repeating units under periodic boundary conditions.

[0024] In the above scheme, steps S4 to S6 respectively optimize the geometric structure of the same bilayer Janus initial model under non-spin polarization and spin polarization conditions. After the structural optimization is completed, static self-consistent calculations are further performed on the optimized configuration to obtain the interlayer distance, total magnetic moment, local magnetic moment of the nearest Z atom pair in the interlayer, electronic state distribution near the Fermi level, and charge density distribution of the corresponding system after optimization. The distance between the nearest Z atom pairs in the interlayer and their local magnetic moments are used as key parameters for determining the interlayer bonding tendency and structural stability.

[0025] Because the present invention employs the above-mentioned technical means, it has the following beneficial effects:

[0026] 1. This invention addresses the technical problem of lacking a microscopic mechanism for quantifying the competition between magnetism and interlayer bonding in existing technologies by combining static self-consistent calculations (steps S6 and S8) using band structure, density of states, electronic localization function (ELF), and crystal orbital Hamiltonian population (COHP). From the microscopic level of electron orbital hybridization and localized distribution of interface charge, it confirms that chemical bond formation can only be effectively suppressed when the opposing Z-atom layers in the interlayer possess localized magnetism. This method can reveal the bandgap change trend of the system during its evolution, such as accurately assessing whether the material maintains a metallic or half-metallic state rather than completely transforming into a semiconductor, providing theoretical support for subsequent modulation of the electronic transport properties of low-dimensional materials.

[0027] 2. This invention combines five-layer model construction with interlayer spacing monitoring and the dynamic introduction of spin polarization degrees of freedom, resulting in a significant predictive synergy effect. This solves the problem of functional failure caused by the loss of interlayer independence in two-dimensional Janus bilayer materials during practical applications. Unified under the same analytical framework, the invention discovers and utilizes the Pauli incompatibility repulsion force brought about by interfacial local magnetism, proving that this repulsion force can effectively antagonize the tendency for spontaneous interlayer bonding in the non-magnetic ground state. The combination of these two methods not only statically reveals the electronic properties of the material but also dynamically constructs a magnetically coupled anti-collapse prediction defense. This allows us to use magnetism to block interlayer collapse and maintain a large interlayer sliding space, clearing the underlying structural obstacles for subsequent development of functional devices such as two-dimensional slip ferroelectrics and multiferroic heterojunctions that rely on bilayer separation. This achieves a closed-loop prediction from mechanism discovery to material screening and device structure preservation. Attached Figure Description

[0028] Figure 1 The flowchart of the magnetic coupling prediction and screening method for two-dimensional Janus bilayer materials described in this invention shows the overall process from model construction, structural optimization under non-spin polarization conditions, structural optimization under spin polarization conditions, to the determination of the correlation between magnetic properties, interlayer spacing changes and structural stability.

[0029] Figure 2 This is a schematic diagram of the two-dimensional Janus-MA2Z4 material described in this invention, cut from the parent material to a double-layer stack; where (a) is the original seven-layer ZAZMZAZ structure, (b) is the cut five-layer Janus single-layer structure, and (c) is the double-layer AA stacked geometry.

[0030] Figure 3This is a comparison diagram of the structural evolution of the Janus bilayer of CaSiN3 under non-spin polarized and spin polarized conditions.

[0031] Figure 4 The diagram shows a comparison of the electronic localization functions of the Janus-MA2Z4 bilayer interface region under (a) spin polarization and (b) non-spin polarization conditions. Detailed Implementation

[0032] The embodiments of the present invention will be described in detail below. Although the present invention will be described and illustrated in conjunction with some specific embodiments, it should be noted that the present invention is not limited to these embodiments. On the contrary, any modifications or equivalent substitutions made to the present invention should be covered within the scope of the claims of the present invention.

[0033] In existing technologies, research on Janus-derived bilayer materials typically fails to analyze magnetic degrees of freedom and structural evolution behavior within a unified framework. This results in a lack of clear criteria for determining the coupling relationship between magnetism and interlayer structure in such systems. Furthermore, there is a lack of effective preliminary prediction methods for whether the system experiences abnormal interlayer spacing contraction during optimization or further evolves into a spontaneous interlayer bonding configuration. Simultaneously, the evolutionary law regarding how the electronic structure of the system is reconstructed with changes in interlayer coupling after spin polarization participation remains unclear. Therefore, establishing a standardized theoretical prediction method for Janus-derived bilayer systems to accurately identify the correlation between magnetism, interlayer spacing, and electronic structure, and thereby determine the stable configuration and evolution trend of the material, constitutes the technical problem to be solved by this invention.

[0034] This invention belongs to the field of computational materials science and theoretical design of two-dimensional functional materials, specifically involving a first-principles-based method for predicting the magnetic coupling evolution characteristics of two-dimensional Janus materials. This method takes a five-layer Janus structure derived from the MA2Z4 family as the research object. First, a Janus monolayer with disrupted top-to-bottom symmetry is obtained by trimming the original seven-layer structure. Then, a bilayer stacking model is constructed, and structural optimization and electronic structure calculations are performed under both non-spin polarization and spin polarization conditions. By comparing the interlayer spacing, magnetic moment distribution, electronic localization characteristics, and bonding information of the system under different calculation conditions, the inhibitory effect of interfacial magnetism on interlayer anomalous contraction and interlayer bonding trends is determined, establishing a predictive relationship for the co-evolution of magnetism, interlayer coupling, and electronic structure in a two-dimensional Janus bilayer system. Studies have shown that some Janus bilayer systems are prone to excessive interlayer proximity or even collapse under non-spin polarization conditions. However, with the introduction of spin polarization, the local magnetism of the interface-related atomic layers can significantly suppress the interlayer bonding driving force, enabling the system to maintain a large interlayer spacing and a stable bilayer van der Waals configuration. This is accompanied by a significant modulation of the electronic state distribution near the Fermi level and an evolutionary trend from metallic to near-zero bandgap states. This invention proposes a magnetic coupling prediction and screening method for two-dimensional Janus materials, which can be used to identify candidate materials with stable bilayer configuration preservation capabilities and tunable interlayer interaction characteristics. This provides a theoretical basis for subsequent material design and experimental research on slip ferroelectricity, interlayer polarization modulation, and related two-dimensional functional devices.

[0035] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art will understand that the present invention can be practiced without these specific details.

[0036] This invention provides a method for designing and predicting two-dimensional phase transition materials based on spin polarization-driven structural evolution, comprising the following steps:

[0037] Step 1, Model Construction (corresponding to steps S1-S3).

[0038] MA2Z4 material with a seven-layered ZAZMZAZ structure was selected as the initial parent material to establish the original crystal structure model. Based on this, the bottom two AZ layers were cut to construct an asymmetric Janus monolayer with a five-layer atomic arrangement sequence of ZAZMZ. Furthermore, a double-layer AA stacked initial model was established, and the initial interlayer spacing was set to obtain the double-layer Janus initial structure.

[0039] Step 2, optimization of non-spin polarized and spin polarized structures (corresponding to steps S4-S5).

[0040] The geometry of the same bilayer Janus initial model was optimized under both non-spin-polarized and spin-polarized conditions to obtain stable configurations and corresponding structural and magnetic parameters under both conditions.

[0041] Step 3, Static self-consistent calculation and structural state determination (corresponding to steps S6-S7).

[0042] Static self-consistent electronic structure calculations were performed on the optimized structures obtained under non-spin polarization and spin polarization conditions to obtain the total energy, interlayer spacing, magnetic moment distribution, and electronic state characteristics near the Fermi level. The closest distance between Z atoms in the interlayer of the optimized interface was used as the interlayer spacing criterion. The COHP integral value and ELF were combined with the interlayer bonding interaction and electronic localization characteristics of the interface to determine whether the system tends to a spontaneous bonding collapse state or a magnetically stable van der Waals state.

[0043] Step 4, Analysis of magnetic coupling mechanism (corresponding to step S8).

[0044] The selected Janus bilayer system with M=Ca was further analyzed by combining band structure, density of states, electronic localization function (ELF), and crystal orbital Hamiltonian population (COHP). By comparing the distribution of electronic states near the Fermi level, the localized characteristics of electronic interfaces, and the strength of interlayer bonding interactions under non-spin polarization and spin polarization conditions, the inhibitory effect of interfacial magnetism on the interlayer bonding trend was determined, and the co-evolution relationship between magnetism, interlayer coupling, and electronic structure in the two-dimensional Janus bilayer system was established.

[0045] This invention, through first-principles calculations, reveals a significant magnetic coupling competition mechanism in two-dimensional Janus-MA2Z4 bilayer systems (especially Ca-based systems). The physical logic of this invention is explained below with specific experimental data.

[0046] Example 1: Typical magnetic coupling-driven structure preservation system – CaSiN3

[0047] This embodiment selects CaSiN3 as the research object, focusing on demonstrating how magnetic degrees of freedom act as a "structural skeleton" to prevent the collapse of the Janus double-layer structure.

[0048] First, using a CaSi₂N₄ parent monolayer with a seven-layered ZAZMZAZ structure as the initial model, the bottom continuous Si and N atomic layers were removed along the stacking direction to construct an asymmetric Janus monolayer with a five-layer atomic arrangement sequence of N-Si-N-Ca-N. Then, based on this Janus monolayer, a two-layer initial model was established using an AA stacking method, with the initial interlayer spacing set to 3.0 Å. A 20 Å vacuum layer was placed in the vertical direction to reduce the interaction between adjacent repeating units under periodic boundary conditions. First-principles calculations were performed, with a plane wave cutoff energy set to 500 eV. A 9×9×1 Monkhorst-Pack k-point grid was used for Brillouin zone sampling, and the electronic self-consistent convergence accuracy was set to 10. -5 eV, with the atomic force convergence threshold set to 0.01 eV / Å.

[0049] (1) Structural collapse and spontaneous interlayer bonding in the nonmagnetic (NM) state:

[0050] Without considering spin polarization, the initial Janus bilayer model of the aforementioned AA stack was geometrically optimized, and further static self-consistent electronic structure calculations were performed. The results show that the CaSiN3 bilayer exhibits significant interlayer shrinkage behavior under non-spin polarization conditions, with the interlayer spacing decreasing significantly from the initial modeling value of 3.0 Å to approximately 1.35 Å. Simultaneously, the interfaces show closer proximity between N atoms, and the system exhibits a clear interlayer bonding trend, indicating that the system is more prone to evolving from the initial layered bilayer configuration to a compact collapsed configuration under non-spin polarization conditions.

[0051] (2) Interlayer separation is maintained in the spin polarization (SP) state:

[0052] Under spin polarization, the geometry of the same initial model was optimized, and initial magnetic moments were assigned to interface-related atoms to introduce magnetic degrees of freedom. Static self-consistent electronic structure calculations were then performed after optimization. The results show that the system converges to a stable configuration with a non-zero total magnetic moment of approximately 4.193 μB under spin polarization. The interlayer spacing significantly increases after optimization and recovers to approximately 3.42 Å. Compared to the non-spin polarization results, this configuration retains a larger interlayer separation, indicating that the introduction of interlayer magnetism can significantly suppress anomalous interlayer proximity and spontaneous bonding tendencies, thereby maintaining the van der Waals layering characteristics of the bilayer Janus system.

[0053] (3) Conclusion:

[0054] By comparing the interlayer spacing, total energy, and electronic state characteristics under NM and SP conditions, it was determined that the system tends towards an interlayer bonding collapse state under non-spin polarization, while tending towards a magnetically stable van der Waals state under spin polarization. In the specific microscopic analysis, firstly, by comparing the electronic orbital hybridization near the Fermi level through band structure and density of states analysis, it was found that after introducing spin polarization, the electronic state distribution underwent significant reconstruction, accompanied by a trend of evolution from metallic to near-zero bandgap states. Subsequently, the localization distribution characteristics of the interface charge were observed using the electronic localization function (ELF). It was found that under spin polarization, the interlayer electronic localization bridging that originally existed in the NM state disappeared, and the charge re-localized towards the atomic nucleus. Finally, by integrating and quantifying the bonding and antibonding contributions of interlayer atomic pairs using the crystal orbital Hamiltonian population (COHP), it was confirmed that the local magnetism generated at the interface produced a strong exchange repulsion force through the Pauli exclusion principle, significantly suppressing the contribution of strong bonding states below the Fermi level, thus effectively antagonizing the driving force of interlayer atomic bonding. This result establishes a synergistic evolution relationship between magnetism, interlayer coupling, and electronic structure. That is, the generation of interfacial magnetism can weaken interlayer bonding and suppress anomalous interlayer contraction, thus preserving the physical premise for the subsequent use of interlayer slip to achieve ferroelectric polarization reversal in this system.

[0055] Example 2: Universal screening of slip functional materials using magnetic coupling mechanisms – CaGeN3

[0056] This embodiment selects CaGeN3 as the research object, focusing on illustrating the universal role of magnetic degrees of freedom in maintaining the Janus double-layer sliding functional platform, and further clarifying the guiding significance of this mechanism for the subsequent screening and design of functional devices.

[0057] First, using a CaGe2N4 parent monolayer with a seven-layered ZAZMZAZ structure as the initial model, the bottom continuous Ge and N atomic layers were removed along the stacking direction to construct an asymmetric Janus monolayer with a five-layer atomic arrangement sequence of N-Ge-N-Ca-N. Then, based on this Janus monolayer, a two-layer initial model was established using an AA stacking method, with the initial interlayer spacing set to 3.0 Å. A 20 Å vacuum layer was placed in the vertical direction to reduce the interaction between adjacent repeating units under periodic boundary conditions. First-principles calculations were performed, with a plane wave cutoff energy set to 500 eV. A 9×9×1 Monkhorst-Pack k-point grid was used for Brillouin zone sampling, and the electronic self-consistent convergence accuracy was set to 10. -5 eV, with the atomic force convergence threshold set to 0.01 eV / Å.

[0058] (1) Data presentation and functional relationship:

[0059] Without considering spin polarization, the initial model of the aforementioned AA-stacked Janus bilayer was geometrically optimized, and further static self-consistent electronic structure calculations were performed. The results show that the CaGeN3 bilayer also exhibits significant interlayer anomalous contraction behavior under non-spin polarization conditions, with the interlayer spacing decreasing significantly from the initial modeling value of 3.0 Å to approximately 1.14 Å. This extremely short interlayer distance indicates that the interface atoms have come into strong proximity, and the system tends towards a compact collapsed configuration, with the original van der Waals layering characteristics essentially disappearing. Further analysis from a functional realization perspective reveals that this type of collapsed structure significantly increases the potential barrier during the interlayer relative slip process, making polarization reversal, strain response, and field modulation behavior based on interlayer slip kinetically difficult to achieve, thus weakening the usability of this type of Janus bilayer as a slip functional material platform.

[0060] (2) Interlayer separation recovery and functional platform preservation in spin polarization (SP) state:

[0061] Under spin polarization, the same initial model was geometrically optimized, and initial magnetic moments were assigned to interface-related atoms to introduce magnetic degrees of freedom. Static self-consistent electronic structure calculations were then performed after optimization. The results show that the system converges to a stable configuration with a non-zero total magnetic moment of approximately 4.182 μB under spin polarization. The interlayer spacing significantly increases after optimization and recovers to approximately 3.27 Å. Compared to the non-spin polarization result, this configuration re-establishes a more typical interlayer separation state, indicating that the introduction of interfacial magnetism effectively suppresses anomalous interlayer proximity and spontaneous collapse, allowing the system to regain the van der Waals bilayer characteristics with slip space. These results demonstrate that magnetic degrees of freedom not only alter the structural stability of the system but also directly determine whether the Janus bilayer possesses the geometric degrees of freedom required for subsequent functional realization.

[0062] (3) Conclusion:

[0063] By comparing the interlayer spacing, total energy, and structural evolution characteristics under NM and SP conditions, it was determined that the CaGeN3 system tends towards an interlayer collapse state under non-spin polarization, while under spin polarization it tends towards a magnetically stable van der Waals layered state. This result demonstrates that magnetic coupling is not limited to individual systems but can serve as a universal criterion for screening Janus bilayer materials with slip functionality. For candidate systems represented by CaGeN3, only with the intervention of the magnetic state can the material maintain a sufficiently large interlayer spacing, thus providing the necessary spatial degrees of freedom for interlayer relative sliding, strain control, and electric field-induced phase transitions. Based on this understanding, the ability to recover a stable van der Waals gap after spin polarization can be used as an important basis for screening candidate slip ferroelectric materials, and further provide clear theoretical guidance for the design of magnetically controlled slip ferroelectric memories, multiferroic two-dimensional heterojunctions, and related tunable low-dimensional devices.

Claims

1. A method for predicting the magnetic coupling evolution characteristics of two-dimensional Janus materials based on first-principles calculations, characterized in that, Includes the following steps: Step S1: Select MA2Z4 material with a seven-layered ZAZMZAZ structure as the initial parent material and establish the original crystal structure model. Step S2: Perform bottom AZ two-layer trimming on the seven-layer structure of the original crystal structure model to construct an asymmetric Janus monolayer with a ZAZMZ five-layer atomic arrangement sequence; Step S3: Construct a double-layer AA stacked initial model based on the asymmetric Janus single layer, and set the initial layer spacing to obtain the double-layer Janus initial model; Step S4: Optimize the geometry of the initial bilayer Janus model under non-spin polarization conditions to obtain a stable configuration and corresponding structural parameters under non-spin polarization conditions. Step S5: Optimize the geometry of the same double-layer Janus initial model under spin polarization conditions to obtain a stable configuration and corresponding magnetic and structural parameters under spin polarization conditions. Step S6: Perform static self-consistent electronic structure calculations on the optimized structures obtained in steps S4 and S5 respectively to obtain the total energy, magnetic moment distribution and electronic structure parameters of the corresponding system. Step S7: Compare the interlayer spacing, magnetic moment distribution, and electronic state characteristics near the Fermi level under non-spin polarization and spin polarization conditions to determine whether the Janus bilayer system tends towards anomalous interlayer contraction and spontaneous bonding collapse, or towards a magnetically stable van der Waals state maintaining a large interlayer spacing. Specifically: The interlayer spacing is determined by the closest distance between Z atoms in the interlayer: when the interlayer spacing is greater than or equal to 3.0 Å, the system is considered to maintain bilayer separation characteristics; when the interlayer spacing is less than 3.0 Å but greater than or equal to 2.0 Å, the system is considered to have significant interlayer contraction and a tendency to collapse; when the interlayer spacing is less than 2.0 Å, the system is considered to have abnormal contraction and a tendency to collapse. Based on this, the strength of interlayer bonding is further quantitatively compared by combining the COHP integral value, and the local distribution characteristics of interlayer electrons are verified by combining ELF. Finally, it is determined whether the system tends to spontaneous bonding collapse state or magnetically stable van der Waals state. Step S8 involves further analyzing the selected Janus bilayer system with M=Ca by combining its band structure, density of states, electronic localization function (ELF), and crystal orbital Hamiltonian population (COHP). By comparing the distribution of electronic states near the Fermi level, the localized characteristics of electronic interfaces, and the strength of interlayer bonding interactions under non-spin polarization and spin polarization conditions, the inhibitory effect of interfacial magnetism on the interlayer bonding trend is determined, and the co-evolutionary relationship between magnetism, interlayer coupling, and electronic structure in the two-dimensional Janus bilayer system is established.

2. The prediction method according to claim 1, characterized in that: The MA2Z4 material mentioned in step S1 is a two-dimensional layered compound with layered hexagonal symmetry. Its original monolayer structure presents a seven-layer atomic arrangement sequence of ZAZMZAZ along the vertical direction, where M is an alkaline earth metal element, A is a group IV element, and Z is a group V element.

3. The prediction method according to claim 2, characterized in that: M is selected from Ca, A is selected from one or two of Si and Ge, Z is selected from one or more of N, P and As, and the candidate system is a Janus bilayer system derived from the MA2Z4 matrix formed by the combination of the above elements.

4. The prediction method according to any one of claims 1 to 3, characterized in that: In step S2, the bottom continuous A and Z atomic layers are removed from the original seven-layer structure along the stacking direction, so that the original symmetric structure is transformed into an asymmetric Janus monolayer with a five-layer atomic arrangement sequence of ZAZMZ, thereby forming a two-dimensional Janus configuration with inequivalent upper and lower surface elemental composition and local coordination environment.

5. The prediction method according to claim 4, characterized in that: In step S3, a double-layer Janus initial model is constructed using the AA stacking method, so that the upper and lower Janus single layers are aligned and coincident in the in-plane lattice projection direction, and a predetermined initial interlayer spacing is set as the starting condition for interlayer coupling evolution; the initial interlayer spacing is a modeling parameter of 3.0 Å to maintain the initial separation state of the double layers, and a vacuum layer is set in the vertical plane direction to reduce the interaction between adjacent repeating units under periodic boundary conditions.

6. The prediction method according to claim 5, characterized in that: In steps S4 to S6, the geometric structure of the same bilayer Janus initial model is optimized under both non-spin polarization and spin polarization conditions. After the structural optimization is completed, static self-consistent calculations are performed on the optimized configuration to obtain the interlayer distance, total magnetic moment, local magnetic moment of the nearest Z atom pair in the interlayer, electronic state distribution near the Fermi level, and charge density distribution of the corresponding system after optimization. The distance between the nearest Z atom pairs in the interlayer and their local magnetic moments are used as key parameters for determining the interlayer bonding tendency and structural stability.

Images