| 研究課題/領域番号 |
21K13889
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| 研究機関 | 国立研究開発法人理化学研究所 |
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研究代表者 |
王 子謙 国立研究開発法人理化学研究所, 創発物性科学研究センター, 特別研究員 (00898934)
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| 研究期間 (年度) |
2021-04-01 – 2024-03-31
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| キーワード | Symmetry crossover / 2D materials |
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| 研究実績の概要 |
Recent synchrotron-based optical studies on antiferromagnetic MnPS3 have revealed a thickness-dependent symmetry crossover, suggesting possible different stackings in few-layer crystals from the bulk, which, however, has not been explicitly identified. We analyzed the atomic-scale structure of MnPS3 down to bilayer in real space using scanning transmission electron microscopy and unveiled that rippling effect disrupts layer stacking order locally, lifting the symmetry of combined few layers while preserving the symmetry of individual component monolayers. Combining the structure characterization with optical selection rule analyses and theoretical calculations of the substantially weak interlayer interactions of MnPS3, we proposed the new phenomenon of “ripple-induced effective layer-decoupling”, i.e., monolayer properties are to a large extent preserved in rippled few-layer MnPS3, and suggested that this phenomenon is most likely the origin of the puzzling symmetry crossover observed in previous optical studies. Our finding reveals the possibility of achieving monolayer-like physical properties in ultrathin van der Waals materials with weak interlayer interactions and raises the possibility of approaching the paradigmatic two-dimensional Neel-type antiferromagnetic honeycomb lattice model in MnPS3 without reaching monolayer thickness. This study provides a deeper understanding of the structure-optical property relationship in van der Waals magnets.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
1. One of last year’s goals was to set up a Sagnac interferometer. An ultra-sensitive scanning Sagnac interferometer has been set up successfully. The Kerr rotation sensitivity of the Sagnac interferometer can reach about 100 nrad, which is three orders of magnitude more sensitive than conventional Kerr microscopes. It has been confirmed that the Sagnac interferometer can image the magnetic domains of ultrathin Cr2Ge2Te6, which is known to have a rather weak magnetization. Currently, the Sagnac interferometer is used to investigate the ferromagnetic domains and topological domain walls in vdW magnets and their heterostructures.
2. Another goal of last year was to realize magnetic domain imaging of MnPS3 by second-harmonic generation (SHG) and challenge the magnetic domain manipulation. Our SHG spectroscopic studies have confirmed the presence of antiferromagnetic domains in our MnPS3 sample. However, the domain size was found to be smaller than the spatial resolution of the SHG microscope. Research focus was then switched to resolving the electronic and magnetic excitations in MnPS3 using resonant second-harmonic generation (SHG) spectroscopy, which is of higher scientific significance than the original plan.
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| 今後の研究の推進方策 |
Based on the progress made so far, future works are planned as follows:
1. Excitons and exciton-magnons in MnPS3: Understanding the behavior of magnetic excitations, such as excitons, magnons and their complex exciton-magnons, is crucial as it lays the foundation for manipulating the magnetic degrees of freedom of vdW magnets. As the on-going and future work, SHG spectroscopy studies are being conducted to understand the behaviors of excitons and exciton-magnons in magnetic fields and to investigate the possible nonreciprocity of magnons in MnPS3.
2. Symmetry breaking in FePS3 and NiPS3: In recent literature, intense SHG signal has been observed in centrosymmetric (SHG-inactive) FePS3 and NiPS3, indicating significant symmetry breaking hidden in these materials. We plan to unveil the origin of this symmetry breaking and its effects on the lattice, spin, orbital and magnetic systems using resonant SHG spectroscopy.
3. Magnon Nernst effect in MnPS3: Neel-type antiferromagnetic honeycomb lattice is expected to host the magnon Nernst effect, i.e., the spin Nernst effect mediated by magnons. However, uncertainty exists in the existence and magnitude of the magnon Nernst effect in MnPS3. We plan to investigate the magnon Nernst effect in MnPS3 by directly detecting the opposite spin accumulation at different sample edges using modified ultrasensitive Sagnac interferometer.
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| 次年度使用額が生じた理由 |
The current Sagnac interferometer setup only allows the application of magnetic fields below 0.1 T, which greatly limits the possibility to study the magnetic transitions and symmetry breaking in vdW magnets that typically occurs at higher magnetic fields. To continue the study under higher magnetic fields, we plan to assemble the Sagnac interferometer with our in-house superconducting magnet. I plan to use the remaining amount to purchase necessary optical and mechanical parts needed for the assembly in the next year.
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