Magnetic order induced polarization anomaly of Raman scattering in CrI$_3$ layers
arxiv(2019)
摘要
Raman scattering has been widely used to probe the symmetry, strain, doping condition and electron-phonon coupling in various condensed matter systems. The polarization properties of the Raman signal are typically determined by the lattice symmetry. However, here we show that the lattice structure alone cannot fully account for the Raman polarization in CrI$_3$ layers while the magnetic order plays a crucial role in the two-dimensional magnetic crystals as dimensionality reduces. For monolayer samples, the dominate A$\rm_{1g}$ peak at about 128 cm$^{-1}$ shows abnormally high intensity in the cross polarization channel at low temperature, which is supposed to be forbidden by the selection rule based on the lattice symmetry. While for bilayer, this peak is absent in the cross polarization channel for the layered antiferromagnetic (AFM) state and reappears when the bilayer is tuned to the ferromagnetic (FM) state by an external magnetic field. These observations indicate the polarization selection rules of optical phonons can be modified by the magnetic ordering. Our study shows that Raman spectroscopy is a very convenient and readily available technique to probe magnetic order in the emerging 2D magnets.
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