Direct observation of two-dimensional magnons in atomically thin CrI3
Direct observation of two-dimensional magnons in atomically thin CrI3
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DOI：
10.1038/s41567-020-0999-1
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发表时间：
2020-08-10
期刊：
影响因子：
19.6
通讯作者：
Xu, Xiaodong
中科院分区：
文献类型：
--
作者：
Cenker, John;Huang, Bevin;Xu, Xiaodong
磁子是具有长程磁序的晶体中的集体自旋激发。新出现的范德华磁体(1-3)提供了一个高度可调的平台来探索二维极限中的磁激发，从它们的蜂窝状晶格结构和可切换的磁组态中可以看出它具有有趣的性质。在这里，我们报道了用磁拉曼光谱直接观测二维磁子，光学选择规则由原子薄CrI_3中晶体对称性、层数和磁态之间的相互作用决定。在单分子膜中，我们观察到类似于0.3 meV的声学磁振子模式。它具有严格的交叉圆极化选择规则，锁定在磁化方向，该磁化方向源于由三重旋转对称性决定的光子和磁子的角动量守恒(4)。此外，我们还发现了类似于17 meV的光学磁振子模式。这种模式在单层膜中是拉曼无声的，但在双层膜和块体中是光学活跃的，这是由于层折射率引起的奇偶判据的松弛。在层状反铁磁态中，我们直接解出了两个具有相反角动量和共轭光学选择规则的简并光学磁振子模。通过这些测量，我们定量地提取了自旋波隙、磁各向异性以及层内和层间交换常数，并建立了二维磁体作为探索磁振子物理的新平台。
Magnons are collective spin excitations in crystals with long-range magnetic order. The emergent van der Waals magnets(1-3) provide a highly tunable platform to explore magnetic excitations in the two-dimensional limit with intriguing properties, manifesting from their honeycomb lattice structure and switchable magnetic configurations. Here, we report the direct observation of two-dimensional magnons through magneto-Raman spectroscopy with optical selection rules determined by the interplay between crystal symmetry, layer number and magnetic states in atomically thin CrI3. In monolayers, we observe an acoustic magnon mode at similar to 0.3 meV. It has strict cross-circularly polarized selection rules locked to the magnetization direction that originates from the conservation of angular momentum of photons and magnons dictated by three-fold rotational symmetry(4). Additionally, we reveal optical magnon modes at similar to 17 meV. This mode is Raman silent in monolayers, but optically active in bilayers and bulk due to a relaxation of the parity criterion resulting from the layer index. In the layered antiferromagnetic states, we directly resolve two degenerate optical magnon modes with opposite angular momentum and conjugate optical selection rules. From these measurements, we quantitatively extract the spin-wave gap, magnetic anisotropy and intralayer and interlayer exchange constants, and establish two-dimensional magnets as a new platform for exploring magnon physics.