Magnetic Order-Induced Polarization Anomaly of Raman Scattering in 2D Magnet CrI₃

Abstract

The recent discovery of 2D magnets has revealed various intriguing phenomena due to the coupling between spin and other degree of freedoms (such as helical photoluminescence, nonreciprocal SHG). Previous research on the spin-phonon coupling effect mainly focuses on the renormalization of phonon frequency. Here we demonstrate that the Raman polarization selection rules of optical phonons can be greatly modified by the magnetic ordering in 2D magnet CrI 3 . For monolayer samples, the dominant A 1g peak shows abnormally high intensity in the cross polarization channel at low temperature, which is 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 it is tuned to the ferromagnetic (FM) state by an external magnetic field. Our findings shed light on exploring the emergent magneto-optical effects in 2D magnets.

Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

“…2a. The 1st-order single-phonon peaks appear in the relatively low frequency range of 50-150 cm −1 , and are assigned to be of either A g or E g symmetries under the C 3i point group (see Supplementary Note 1), which is consistent with earlier work 34,[37][38][39][40][41][42][43][44] and proves the high quality of our samples. The 2nd-order two-phonon and the 3rd-order three-phonon modes show up in slightly higher frequency ranges of 190-290 cm −1 and 310-410 cm −1 , respectively, and show decreasing mode intensities at higher-order processes, same as typical multiphonon overtones under harmonic approximation 45 or cascade model 46 .…”

“…Figure 2a displays a representative Raman spectrum acquired in the crossed linear polarization channel at 40 K (slightly below T C = 45 K). Note that this spectrum covers a much wider frequency range than earlier Raman studies on CrI 3 34,[37][38][39][40][41][42][43][44] . The multiphonon scattering is visible up to the 3rd order, and their zoom-in Raman spectra are shown in the inset of Fig.…”

Distinct magneto-Raman signatures of spin-flip phase transitions in CrI3

“…Of note, the origins of P 1 and P 2 as zone-folded phonons predict they would disappear for a monolayer sample (no zone-folding) or a sample with an odd number of layers because the AFM state preserves inversion symmetry with an odd number of layers. Recent work 41 , 42 has shown P 1 and P 2 are not present in monolayers of CrI 3 , yet data by Jin et al 34 suggests the presence of P 1 and P 2 for odd layer thicknesses. One possibility is that the encapsulation of thin layers of CrI 3 in hBN breaks inversion symmetry naturally, leading to the presence of P 1 and P 2 in all thicknesses.…”

Air‐Stable Low‐Symmetry Narrow‐Bandgap 2D Sulfide Niobium for Polarization Photodetection

“…The main Raman peaks are significantly higher in the parallel (VV) direction than in the cross‐polarization channels (VH) direction (Figure 2d), and the ratios (VH/VV) of each peak are 0.36, 0.31, 0.35, 0.39, 0.41, and 0.55, respectively (Figure 2d). [ 35 ] The ratio is closer to 0.75, indicating that the polarization characteristics of scattered radiation are stronger and NbS 3 has significant anisotropic characteristics. A typical azimuth‐dependent reflectance difference microscopy (ADRDM) measurement of a 2D NbS 3 flakes (Figure S4, Supporting Information) further demonstrates the in‐plane anisotropic optical reflection.…”