Staros Daniel, Hu Guoxiang, Tiihonen Juha, Nanguneri Ravindra, Krogel Jaron, Bennett M Chandler, Heinonen Olle, Ganesh Panchapakesan, Rubenstein Brenda
Department of Chemistry, Brown University, Providence, Rhode Island 02912, USA.
Department of Chemistry and Biochemistry, Queens College, City University of New York, Flushing, New York 11367, USA.
J Chem Phys. 2022 Jan 7;156(1):014707. doi: 10.1063/5.0074848.
The first magnetic 2D material discovered, monolayer (ML) CrI, is particularly fascinating due to its ground state ferromagnetism. However, because ML materials are difficult to probe experimentally, much remains unresolved about ML CrI's structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin-phonon and spin-lattice coupling of ML CrI. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI's ML geometry with DMC accuracy, yielding lattice parameters in good agreement with recently published STM measurements-an accomplishment given the ∼10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find that previous DFT predictions of ML CrI's magnetic spin moments are correct on average across a unit cell but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts that magnetic moments in ML CrI are 3.62 μ per chromium and -0.145 μ per iodine, both larger than previous DFT predictions. The large disparate moments together with the large spin-orbit coupling of CrI's I-p orbital suggest a ligand superexchange-dominated magnetic anisotropy in ML CrI, corroborating recent observations of magnons in its 2D limit. We also find that ML CrI exhibits a substantial spin-phonon coupling of ∼3.32 cm. Our work, thus, establishes many of ML CrI's key properties, while also continuing to demonstrate the pivotal role that DMC can assume in the study of magnetic and other 2D materials.
首个被发现的磁性二维材料——单层（ML）CrI，因其基态铁磁性而格外引人入胜。然而，由于单层材料在实验上难以探测，关于ML CrI的结构、电子和磁性特性仍有许多未解决的问题。在此，我们利用密度泛函理论（DFT）和高精度扩散蒙特卡罗（DMC）模拟来预测ML CrI的晶格参数、磁矩以及自旋 - 声子和自旋 - 晶格耦合。我们采用最近开发的替代海森矩阵DMC线搜索技术，以DMC精度确定CrI的单层几何结构，得到的晶格参数与最近发表的扫描隧道显微镜（STM）测量结果吻合良好——鉴于先前基于DFT得出的估计值因泛函不同而存在约10%的变化性，这是一项了不起的成就。引人注目的是，我们发现先前对ML CrI磁自旋矩的DFT预测在整个晶胞上平均是正确的，但遗漏了更精确的DMC所揭示的自旋密度中关键的局部空间波动。DMC预测ML CrI中每个铬的磁矩为3.62 μ，每个碘的磁矩为 -0.145 μ，两者均大于先前的DFT预测值。这些大不相同的磁矩以及CrI的I - p轨道的大自旋 - 轨道耦合表明，ML CrI中存在以配体超交换为主导的磁各向异性，这证实了最近在其二维极限下对磁振子的观测结果。我们还发现ML CrI表现出约3.32 cm的显著自旋 - 声子耦合。因此，我们的工作确定了ML CrI的许多关键特性，同时也继续证明了DMC在磁性和其他二维材料研究中可以发挥的关键作用。
