Abstract:Exploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of two-dimensional (2D) magnetism in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter r… Show more
“…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 .…”
Section: Resultssupporting
confidence: 91%
“…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.…”
Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI3, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm−1. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI3. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors.
“…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 .…”
Section: Resultssupporting
confidence: 91%
“…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.…”
Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI3, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm−1. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI3. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors.
“…A thin flake of CrI 3 (≈7 nm from atomic force microscopy, or ≈10 layers) was encapsulated between two 20 nm and 30 nm flakes of hBN using the dry transfer technique 22 , 23 . In the ab plane, the Cr 3+ atoms are arranged in a honeycomb lattice, where each chromium atom is bonded with six iodine atoms to form a distorted octahedron (see Fig.…”
Section: Resultsmentioning
confidence: 99%
“…Raman spectroscopy is a powerful technique to study a variety of phenomena in 2D quantum materials, including effects of strain 16 , electron–phonon coupling 17 , phase transitions 18 , spin-phonon coupling 19 , and magnetic excitations 20 – 22 . In addition, the diffraction-limited spot size allows for the investigation of atomically thin samples and heterostructures using a non-contact probe.…”
The discovery of 2-dimensional (2D) materials, such as CrI
3
, that retain magnetic ordering at monolayer thickness has resulted in a surge of both pure and applied research in 2D magnetism. Here, we report a magneto-Raman spectroscopy study on multilayered CrI
3
, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI
3
.
“…It undergoes a layered-AFM to ferromagnetic (FM) phase transition upon applying a moderate magnetic field (1-7), or electric field (8)(9)(10), or electrostatic doping (11), or hydrostatic pressure (12,13). The strong coupling between spin and charge degrees of freedom in 2D CrI 3 allows magneto-optical effects manifested in a variety of ways including large magneto-optical Kerr effect (5) and magnetic circular dichroism (8)(9)(10)(11)(12)(13), spontaneous helical photoluminescence (14), giant nonreciprocal second harmonic generation (15), and anomalous magneto-optical Raman effect (7,(16)(17)(18)(19). All of these magneto-optical effects can be tuned across the layered-AFM to FM phase transition, making 2D CrI 3 a promising candidate for applications in magnetic sensors, optical modulation, and data storage.…”
We used a combination of polarized Raman spectroscopy experiment and model magnetism–phonon coupling calculations to study the rich magneto-Raman effect in the two-dimensional (2D) magnet CrI3. We reveal a layered-magnetism–assisted phonon scattering mechanism below the magnetic onset temperature, whose Raman excitation breaks time-reversal symmetry, has an antisymmetric Raman tensor, and follows the magnetic phase transitions across critical magnetic fields, on top of the presence of the conventional phonon scattering with symmetric Raman tensors in N-layer CrI3. We resolve in data and by calculations that the first-order Ag phonon of the monolayer splits into an N-fold multiplet in N-layer CrI3 due to the interlayer coupling (N≥2) and that the phonons within the multiplet show distinct magnetic field dependence because of their different layered-magnetism–phonon coupling. We further find that such a layered-magnetism–phonon coupled Raman scattering mechanism extends beyond first-order to higher-order multiphonon scattering processes. Our results on the magneto-Raman effect of the first-order phonons in the multiplet and the higher-order multiphonons in N-layer CrI3 demonstrate the rich and strong behavior of emergent magneto-optical effects in 2D magnets and underline the unique opportunities of spin–phonon physics in van der Waals layered magnets.
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