Observation of Giant Surface Second-Harmonic Generation Coupled to Nematic Orders in the van der Waals Antiferromagnet FePS₃

Abstract

Second-harmonic generation has been applied to study lattice, electronic, and magnetic proprieties in atomically thin materials. However, inversion symmetry breaking is usually required for the materials to generate a large signal. In this work, we report a giant second-harmonic generation that arises below the Neél temperature in few-layer centrosymmetric FePS 3 . A layer-dependent study indicates the detected signal is from the second-order nonlinearity of the surface. The magnetism-induced surface secondharmonic response is 2 orders of magnitude larger than those reported in other magnetic systems, with the surface nonlinear susceptibility reaching 0.08−0.13 nm 2 /V in 2−5 L samples. By combing linear dichroism and second-harmonic generation experiments, we further confirm the giant second-harmonic generation is coupled to nematic orders formed by the three possible Zigzag antiferromagnetic domains. Our study shows that the surface secondharmonic generation is also a sensitive tool to study antiferromagnetic states in centrosymmetric atomically thin materials.

Evidence for Topological Magnon–Phonon Hybridization in a 2D Antiferromagnet down to the Monolayer Limit

“…Stable magnons have been found in ferromagnetic monolayers including Fe and CrI 3 through electron energy loss spectroscopy and Raman spectroscopy, , but magnons in similar AFM have much smaller scattering cross section and have not been clearly detected in thicknesses below four layers. , Therefore, we employ a cavity which enhances the Raman cross section by about 15 times (Experimental Methods and Figure S1) and protects the ultrathin crystals (Figures b and S2) from degradation. FePSe 3 belongs to a family of 2D transition phosphorus trichalcogenides MPX 3 (where M = Mn, Fe, Ni, etc., and X = S, Se) with stable magnetic order in the monolayer limit − and unique light–matter interactions, such as magnetic-order-dependent photoluminescence, linear dichroism, and second-harmonic generation. − Despite the same honeycomb structure, the difference in exchange interactions leads to distinctive magnetic orders and group symmetry among the MPX 3 materials . We discovered a symmetry-breaking phase transition near T N = 98 K in monolayer FePSe 3 (compared with 110 K in the bulk), with two prominently enhanced features around 75 cm –1 (P1) and 115 cm –1 (P2) (Figure c).…”

“…The major challenge is that in typical magnetic materials the coherent coupling between magnon and phonon bands (denoted by g ) is weak compared to the dissipation rates of the respective bosons (denoted by γ and κ, respectively), i.e., the quantum cooperativity C = 4 g 2 /γκ ≲ 1. Consequently, the phonons and magnons can incoherently convert into each other but do not form isolated topologically nontrivial bands. − Very recently, van der Waals (vdW) magnetic materials, − particularly antiferromagnets (AFMs) such as FePS 3 , were discovered to exhibit much higher magnon–phonon cooperativity than conventional materials and may overcome this challenge. − However, it is unclear if the cooperativity can survive in the monolayer limit or if the hybridized magnon–phonon bands exhibit topological properties.…”

Probing the Néel‐Type Antiferromagnetic Order and Coherent Magnon–Exciton Coupling in Van Der Waals VPS3

“…We observe a finite SHG signal across the whole temperature region, on the other hand, the quasi two-fold SHG pattern above T N also suggests the possible contribution from surface ED mechanism. [28,29] Obviously, a substantial increase of SHG intensity can be observed due to the emergence of the ED allowed susceptibility under the broken inversion symmetry when the temperature is below T N (Figure 3b). Note that such a common phenomenon has also been illustrated in other Néel-type AFM MnPS 3 and MnPSe 3 , [25,26,29] whereas no changes for the SHG intensity could be observed from zigzag-type AFM [57] the SHG intensity can be fitted to yield 𝛽 = 0.36 (2), which is close to the value of 0.369 for 3D Heisenberg model.…”

“…have attracted much interests recently and have been a typical representative of 2D antiferromagnets. [1][2][3][25][26][27][28] A significant fact is that the magnetic Hamiltonian hosts strong dependence on metal elements in such a homogenous system, leading to distinct physical phenomena. For instance, Heisenbergtype MnPS 3 with Néel-type AFM order embodies a linear magnetoelectric effect in the long-range ordered phase as well as a sensitive response to SHG; [29] XXZ-type van der Waals (vdW) antiferromagnet NiPS 3 with zigzag-type AFM order, on the other hand, appears coherent many-body exciton below the AFM transition temperature (T N ).…”

Floquet formulation of the dynamical Berry-phase approach to nonlinear optics in extended systems

“…For example, second harmonic generation (SHG), due to its sensitivity to electric fields and symmetry properties, has been traditionally used on surfaces and interfaces [1][2][3] and more recently for the characterisation and imaging of two-dimensional flakes [4][5][6][7]. In addition, SHG is being used as a highly sensitive probe of magnetic ordering in atomically thin materials and in multiferroics [8][9][10]. Due to the sensitivity to changes in the electric polarisation, SHG can also probe the dynamic of excited systems, tracking-for instance-the formation of excitons, exciton-phonon coupling and demagnetisation of antiferromagnets [11,12].…”