Tuning inelastic light scattering via symmetry control in the two-dimensional magnet CrI3

Abstract

The coupling between spin and charge degrees of freedom in a crystal imparts strong optical signatures on scattered electromagnetic waves. This has led to magneto-optical effects with a host of applications, from the sensitive detection of local magnetic order to optical modulation and data storage technologies. Here, we demonstrate a new magnetooptical effect, namely, the tuning of inelastically scattered light through symmetry control in atomically thin chromium triiodide (CrI3). In monolayers, we found an extraordinarily large magneto-optical Raman effect from an A1g phonon mode due to the emergence of ferromagnetic order. The linearly polarized, inelastically scattered light rotates by ~40⁰, more than two orders of magnitude larger than the rotation from MOKE under the same experimental conditions. In CrI3 bilayers, we show that the same A1g phonon mode becomes Davydov-split into two modes of opposite parity, exhibiting divergent selection rules that depend on inversion symmetry and the underlying magnetic order. By switching between the antiferromagnetic states and the fully spin-polarized states with applied magnetic and electric fields, we demonstrate the magnetoelectrical control over their selection rules. Our work underscores the unique opportunities provided by 2D magnets for controlling the combined time-reversal and inversion symmetries to manipulate Raman optical selection rules and for exploring emergent magneto-optical effects and spin-phonon coupled physics. Main text:Raman scattering measures light inelastically scattered from collective quasiparticle excitations. Since it is highly sensitive to material parameters such as crystal symmetry and local electronic states, Raman spectroscopy has provided a powerful probe of a broad range of condensed matter phenomena, such as charge density waves 1 , superconductivity 2 , ferroelectricity 3 , and topological physics 4 . In particular, Raman scattering from spin-phonon excitations has yielded incisive information on magnetic materials. For instance, in recently developed 2D van der Waals magnets, Raman scattering has been used to reveal magnetic order and phase transitions 5-7 down to a single layer [8][9][10] .Chromium triiodide (CrI3), a van der Waals magnet, was shown to be a layered antiferromagnet in its few-layer form: spins within each layer are ferromagnetically (FM) coupled with strong outof-plane anisotropy, while the interlayer exchange is antiferromagnetic (AFM) 11 . For bilayers, the system undergoes a spin-flip transition upon the application of a moderate magnetic field 11 , switching from a layered AFM state to a fully spin-polarized state. In addition, magneto-optical effects manifest strongly and in distinctly novel ways in CrI3. Examples include the very large magnetic-optical Kerr effect (MOKE) 11,12 and spontaneous helical light emission 13 from ferromagnetic monolayers, and electric-field induced Kerr rotation [14][15][16] and giant second-order nonreciprocal optical effects 17 in antiferromagnetic bilayers. Content:Extended Data...

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

“…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.…”

Magnetic-Field-Induced Quantum Phase Transitions in a van der Waals Magnet

“…In order to probe both the magnetic and crystallographic degrees of freedom in CrI 3 , we carry out polarized Raman spectroscopy to detect the symmetry-resolved collective excitations of spin precessions (i.e., magnons) [17] and lattice vibrations (i.e., phonons) [18][19][20][21], respectively. To further reveal the interplay between these 2 degrees of freedom, we perform both temperature-and magnetic fielddependent Raman measurements covering a temperature (T) range from room temperature down to 10 K and a magnetic field (B) range from 0 up to 7 T. Because the stray magnetic fields cause the Faraday rotation of linearly polarized light transmitted through the objective in close proximity to the magnet [22], we choose circularly polarized light to perform reliable selection rule measurements (see the Supplemental Material Sec.…”

Tunable electronic and magnetic properties of monolayer and bilayer Janus Cr2Cl3I3: a first-principles study

“…Moreover, CrX 3 (X = I, Br, Cl) is usually synthesized with multilayers stacked together by vdW interactions. 6,9,14,[22][23][24][25][26][27][28]30,31,35,53,73,74 So the stacking-induced modifications of the properties need to be systemically discussed. Taking BL MoSSe as a reference, 11 six different stacking patterns (AA-II, AA-ClI, AA-ClCl, AB-ClI, AB-II and AB-ClCl) of BL Cr 2 Cl 3 I 3 are considered, and the optimized geometries are shown in Fig.…”

“…The interlayer vertical distance of the plane of the I (Cl) and Cl (I) atoms d is defined to describe the interaction between the Cr 2 Cl 3 I 3 layers. The corresponding distance and binding energies calculated by the DFT-D2, 58 DFT-D3, [30][31][32] and DFT-TS 33 methods are shown in Table 1. We can find that the AB-ClCl configuration has the smallest d = 3.01 Å Fig.…”

“…41,43,46 A deep understanding of the intrinsic physical mechanism of Janus chromium trihalides and practical methods to control their spin states are highly required. 48,49 Moreover, strain 33,50 and stacking order 19,33,[48][49][50][51][52] are widely used methods to modulate the tunneling magneto-resistances, 29,53 magneto-optical effects, 54 light scattering 31 and topological properties of materials. 15,55,56 In this article, using first-principles methods with vdW interaction corrections, [57][58][59] we investigate the geometric, magnetic, and electronic properties and stability of ML Janus Cr 2 Cl 3 I 3 using hybrid functional-HSE06.…”