After the preparation of 2D electronic flat band (EFB) in van der Waals (vdW) superlattices, recent measurements suggest the existence of 1D electronic flat bands (1D-EFBs) in twisted vdW bilayers. However, the realization of 1D-EFBs is experimentally elusive in untwisted 2D layers, which is desired considering their fabrication and scalability. Herein, the discovery of 1D-EFBs is reported in an untwisted in situ-grown two atomic-layer Bi(110)superlattice self-aligned on an SnSe(001) substrate using scanning probe microscopy measurements and density functional theory calculations. While the Bi-Bi dimers of Bi zigzag (ZZ) chains are buckled, the epitaxial lattice mismatch between the Bi and SnSe layers induces two 1D buckling reversal regions (BRRs) extending along the ZZ direction in each Bi(110)-11 x 17 supercell. A series of 1D-EFBs arises spatially following BRRs that isolate electronic states along the armchair (AC) direction and localize electrons in 1D extended states along ZZ due to quantum interference at a topological node. This work provides a generalized strategy for engineering 1D-EFBs in utilizing lattice mismatch between untwisted rectangular vdW layers.
Fig.1 Interlayer distance and interlayer number regulating interlayer magnetism. a: The interlayer electron kinetic energy term, Pauli and Coulomb repulsion competition determine the interlayer magnetic ground state. b: Total-energy differences between the interlayer AFM/FM configurations as a function of interlayer Se-Se distance. c: Evolution of energy differences between interlayer FM and AFM configurations as a function of the layer number. T d: Experimental confirmation of ferromagnetism of CrSe2 enhanced with the number of layers. e: CrSe2 remains stable after exposure to air for several months.
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Nature Materials
Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order
It is found that in weakly interlayer coupled CrI3 system, the horizontal sliding change stack can regulate interlayer magnetism, and the vertical change of interlayer distance can also be another dimension to regulate two-dimensional magnetism. The researchers found that MX2 (M=V, Cr, Mn; X=S, Se, Te) in the double layer, the phenomenon of interlayer antiferromagnetic — ferromagnetic transition will occur with the change of the layer spacing, which is the result of the competition between the kinetic energy of electrons moving in the layer, the Pauli repulsion of electrons and the Coulomb repulsion of electrons[1] (FIG. 1 a-b).
This work is the first to show that interlayer distance can be used as a new degree of freedom to regulate two-dimensional magnetism, highlighting the importance of non-metallic elements and their interlayer Pauli repulsion not considered in previous models.
We found that the above magnetic coupling mechanism is also applicable to the layers. With the increase of layer thickness, the kinetic energy of electrons moving along the direction of CrSe2 layer increases, gradually represses the interlayer antiferromagnetic coupling caused by Pauli and Coulomb repulsion, and macro ferromagnetism gradually takes the upper hand. In cooperation with the experimental groups, we confirmed the above theoretical image, which is the first international confirmation that CrSe2 is a two-dimensional magnetic material with thickness-dependent magnetic coupling transition between layers and is highly stable in air[2] (FIG. 1 c-e). It has initially overcome the difficulty of insufficient air stability of two-dimensional magnetic materials that had puzzled the scientific community for a long time. As soon as this work was published, it quickly gained academic attention. Professor Andrew T.S. Wee from the National University of Singapore quoted and positively appraised this work for a long time, and Ki Kang Kim from Sungkyunkwan University in Korea wrote a review article affirming this work.
REFERENCES
1. Wang, C. et al. Bethe-Slater-curve-like behavior and interlayer spin-exchange coupling mechanisms in two-dimensional magnetic bilayers. Phys. Rev. B 102, 020402 (2020).
2. Li, B. et al. Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order. Nature Materials 20, 818-825 (2021).
Fig.1 Theoretical prediction of new magnetic materials. a: The Tc of twelve magnetic single-layer materials is predicted to reach 100K to 500K. b: Strong anisotropy with spin-, dichroism- and mobility-anisotropy locking. c: CrOCl bulk phase magnetic ground state. d-e: The computational simulation is highly consistent with the experimentally measured anisotropic Raman diagrams.
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Science Bulletin
A family of high-temperature ferromagnetic monolayers with locked spin-dichroism-mobility anisotropy: MnNX and CrCX (X = Cl, Br, I; C = S, Se, Te)
The search for room temperature ferromagnetic semiconductors is one of 125 important questions posed in Science. In previous studies, researchers found that strong electron doping (1 e/Cr) of CrS2 can lead to its transformation from AFM metal to FM semiconductor, and replacing S with Cl atoms to form CrSCl monolayer is one of the feasible ideas to realize such doping.
Based on the above thought, 12 kinds of single-layer ferromagnetic materials with high Tc are predicted: MnNX and CrCX (where X=Cl,Br,I; C=S,Se,Te)[1]. Based on the anisotropic Heisenberg model and renormalized spin wave theory, Tc of this series of materials is predicted to range from 100K to 500K (Figure 1a). Eight members among them show semiconducting bandgaps varying from roughly 0.23 to 1.85 eV. These semiconducting monolayers also show extremely large anisotropy along the two in-plane lattice directions of these layers. Additional orbital anisotropy leads to a spin-locked linear dichroism, in different from previously known circular and linear dichroisms in layered materials. Together with the mobility anisotropy, it offers a spin-, dichroism- and mobility-anisotropy locking. These results manifest the potential of this 2D family for both fundamental research and high performance spin-dependent electronic and optoelectronic devices (Figure 1b).
The work was published in Sci.Bull. 2019, making it one of the 10 most downloaded articles of that year. The material predictions has been verified and cited by several different experimental groups in Nat. Mat (1 paper), Nat. Nanotech. (2 papers), Adv. Mater. (2 papers) and Nano Letters (1 paper). The magnetic properties and anisotropic electronic structure were confirmed to be consistent with our theoretical prediction.
On this basis, the researchers, in collaboration with experimental collaborators, determined the bulk phase magnetic ground state and magnetic field evolution of CrOCl[2] (FIG. 1c-e). Combined with the magnetic transport measurements of experimental collaborators, the layered dependent magnetic field evolution behavior of the CrSBr system was investigated[3].
REFERENCES
1. Wang, C. et al. A family of high-temperature ferromagnetic monolayers with locked spin-dichroism-mobility anisotropy: MnNX and CrCX (X = Cl, Br, I; C = S, Se, Te). Science Bulletin 64, 293-300 (2019)
2. Gu, P. et al. Magnetic Phase Transitions and Magnetoelastic Coupling in a Two-Dimensional Stripy Antiferromagnet. Nano Letters 22, 1233-1241 (2022)
3. Ye, C. et al. Layer-Dependent Interlayer Antiferromagnetic Spin Reorientation in Air-Stable Semiconductor CrSBr. ACS Nano 16, 11876-11883 (2022)
Fig.1 Layer number regulated intra-layer magnetism. a-b: differential charge density between layers. Red represents the accumulation of charge in the interlayer region after the two layers are stacked on top of each other. c-d: Single layer CrTe2 zigzag antiferromagnetic ground state and easy magnetization axis direction. e-g: Spin reorientation transformation of CrTe2 monolayer.
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Nature Communications
Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2
Charge redistribution induced by interlayer interaction is relatively small at the vdW gap of a CrI3 bilayer in comparison with other bilayers like BP, Te, PtSe2, PtS2, and CrS2, suggesting a limited overlap of interlayer wave functions in the CrI3 bilayer. It is exceptional that such a small overlap could even appreciably affect the interlayer magnetism through direct exchange between two interlayer I atoms separated by 4.20 Å2.
A question then arises of how do strongly overlapped interlayer wave functions affect interlayer magnetism and whether there are any generalized spin-exchange coupling mechanisms solely for such a noncovalent interaction. The interlayer differential charge density shows that the interlayer electron coupling of of CrX2(X = S, Se, Te) systems is significantly stronger than that of CrI3. It is expected that different interlayer magnetic coupling mechanisms exist in CrX2. The bulk phase of this kind of material shows ferromagnetism, and many studies speculate that the single layer should also be the ferromagnetic ground state. However, the theoretical prediction of the researchers found that the single layer of this kind of material is striped antiferromagnetic ground state. The in-plane magnetic orders of CrX2 mono and few layers are tunable between striped antiferromagnetic (sAFM) and ferromagnetic (FM) orders by manipulating charge transfer between Cr t2g and eg orbitals. Such charge transfer is realizable through interlayer coupling, direct charge doping, or substituting S with Cl atoms[1,2].
However, due to the fact that antiferromagnetic orders usually do not exhibit macroscopic magnetic moments, experimental detection is very difficult, and the antiferromagnetic ground state of CrX2(X=S,Se,Te) materials mentioned in the above theoretical prediction are still lacking of experimental evidence in the two-dimensional limit. By combining spin polarised scanning tunnelling microscopy and first-principles calculations, we investigate the magnetism of vdW ML CrTe2, which has been successfully grown through molecular-beam epitaxy. We observe a stable antiferromagnetic (AFM) order at the atomic scale in the ML crystal, whose bulk is ferromagnetic, and correlate its imaged zigzag spin texture with the atomic lattice structure. confirming the theoretical prediction of the researchers (FIG. 1 c). Previous studies of two-dimensional magnetic materials usually assumed that the material’s easy magnetization axis was along the conventional lattice base vector or out of plane direction. The researchers calculated that the magnetic moment direction of the CrTe2 single layer was in the y-z plane at an Angle of 70 degrees from the z direction (FIG. 1 d), resulting in a novel spin reorientation transition mode under magnetic field (FIG. 1 e-g). The real space intrinsic antiferromagnetic order discovered and confirmed in this study, the magnetic order transformation in the single-layer limit, and the spin reorientation under external magnetic fields have not been previously reported before[3].
REFERENCES
1. C. Wang, X. Zhou, Y. Pan, J. Qiao, X. Kong, C.-C. Kaun, and W. Ji, Layer and Doping Tunable Ferromagnetic Order in Two-Dimensional CrS2 Layers, Phys. Rev. B 97, 245409 (2018).
2. Wang, C. et al. Bethe-Slater-curve-like behavior and interlayer spin-exchange coupling mechanisms in two-dimensional magnetic bilayers. Phys. Rev. B 102, 020402 (2020).
3. Xian, J.-J. et al. Spin mapping of intralayer antiferromagnetism and field-induced spin reorientation in monolayer CrTe2. Nature Communications 13, 257 (2022)
