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arXiv 2105.04531cond-mat.mtrl-sci
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磁振子-声子相互作用增强 CrI$_3$ 二维磁体自旋波谱中 Dirac 点处的能隙

Magnon-phonon interactions enhance the gap at the Dirac point in the spin-wave spectra of CrI$_3$ two-dimensional magnets

Pietro Delugas, Oscar Baseggio, Iurii Timrov, Stefano Baroni, Tommaso Gorni

AI总结:

本研究结合 TDDFpT 与 DFpT 模拟单层 CrI$_3$,发现仅靠 DM/K 相互作用所得磁振子能隙远小于实验值,并通过磁振子-声子耦合模型解释约 4 meV 的 Dirac 点能隙增强。

摘要:

近期对蜂窝状 CrI$_3$ 准二维铁磁体开展的中子衍射实验表明,其自旋波谱中的 Dirac 点处存在能隙。该能隙的存在一直被归因于强面内 Dzyaloshinskii-Moriya 或 Kitaev(DM/K)相互作用,并被认为可为拓扑保护边缘态支撑无耗散自旋输运创造条件。我们基于含时密度泛函微扰理论(TDDFpT),对单层 CrI$_3$ 的自旋波谱进行了最先进的模拟,并充分考虑了 DM/K 相互作用最终所源自的自旋-轨道耦合(SOC)。尽管我们的结果与实验在定性上一致,但计算得到的 Dirac 点处 TDDFpT 磁振子能隙为 0.47 meV,比最新实验估计值约小 6 倍,因此令人质疑仅凭层内各向异性是否能够解释所观测到的能隙。在密度泛函微扰理论(DFpT)框架内进行的晶格动力学计算表明,该体系中存在显著的简并以及振动激发与磁激发之间的强耦合,从而为自旋波谱提供了一种可能的额外能隙打开机制。为沿此方向深入研究,我们引入了一个相互作用磁振子-声子哈密顿量,其特征是在 SOC 诱导的磁各向异性作用下,晶格涨落与自旋涨落之间形成线性耦合。在通过 DFpT 和超胞计算确定相关相互作用常数后,该模型使我们能够提出:磁振子-声子相互作用是一种重要的微观机制,负责将 CrI$_3$ 单层 Dirac 点附近的能隙增强至约 4 meV 的范围。

英文摘要:

Recent neutron-diffraction experiments in honeycomb CrI$_3$ quasi-2D ferromagnets have evinced the existence of a gap at the Dirac point in their spin-wave spectra. The existence of this gap has been attributed to strong in-plane Dzyaloshinskii-Moriya or Kitaev (DM/K) interactions and suggested to set the stage for topologically protected edge states to sustain non-dissipative spin transport. We perform state-of-the-art simulations of the spin-wave spectra in monolayer CrI$_3$, based on time-dependent density-functional perturbation theory (TDDFpT) and fully accounting for spin-orbit couplings (SOC) from which DM/K interactions ultimately stem. While our results are in qualitative agreement with experiments, the computed TDDFpT magnon gap at the Dirac point is found to be 0.47~meV, roughly 6 times smaller than the most recent experimental estimates, so questioning that intralayer anisotropies alone can explain the observed gap. Lattice-dynamical calculations, performed within density-functional perturbation theory (DFpT), indicate that a substantial degeneracy and a strong coupling between vibrational and magnetic excitations exist in this system, providing a possible additional gap-opening mechanism in the spin-wave spectra. In order to pursue this path, we introduce an interacting magnon-phonon Hamiltonian featuring a linear coupling between lattice and spin fluctuations, enabled by the magnetic anisotropy induced by SOC. Upon determination of the relevant interaction constants by DFpT and supercell calculations, this model allows us to propose magnon-phonon interactions as an important microscopic mechanism responsible for the enhancement of the gap in the range of $\approx 4$~meV around the Dirac point of the CrI$_3$ monolayer.

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