Physical origin of giant excitonic and magneto-optical responses in two-dimensional ferromagnetic insulators

Abstract

The recent discovery of magnetism in atomically thin layers of van der Waals crystals has created great opportunities for exploring light–matter interactions and magneto-optical phenomena in the two-dimensional limit. Optical and magneto-optical experiments have provided insights into these topics, revealing strong magnetic circular dichroism and giant Kerr signals in atomically thin ferromagnetic insulators. However, the nature of the giant magneto-optical responses and their microscopic mechanism remain unclear. Here, by performing first-principles GW and Bethe-Salpeter equation calculations, we show that excitonic effects dominate the optical and magneto-optical responses in the prototypical two-dimensional ferromagnetic insulator, CrI 3 . We simulate the Kerr and Faraday effects in realistic experimental setups, and based on which we predict the sensitive frequency- and substrate-dependence of magneto-optical responses. These findings provide physical understanding of the phenomena as well as potential design principles for engineering magneto-optical and optoelectronic devices using two-dimensional magnets.

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

“…A single PL mode at 1.11 eV and three prominent absorbance peaks at 1.51 eV, 1.96 eV, and 2.68 eV (denoted as A, B, and C, respectively) are observed across the entire temperature range. These three energies are in good agreement with the ligand-field electronic transitions assigned by differential reflectance measurements on monolayer CrI 3 26 and bulk CrI 3 35,36 and have been later revealed to be bright exciton states through sophisticated first principle GW and Bethe-Salpeter equation calculations 24 . The large Stokes shift (~400 meV) between the PL and A exciton absorption peak…”

“…The realization of a long-range magnetic order in 2D semiconducting CrI 3 paves the way to engineer optical and optoelectronic properties of 2D magnetic semiconductors [16][17][18][19][20][21][22][23] . The large excitonic effect 24 from the localized molecular orbitals, of neither Wannier-type in 2D TMDCs nor Frenkel-type in ionic crystals, can be considered as the microscopic origin of the giant magneto-optical Kerr effect 25 and magnetic circular dichroism 26 signals in 2D CrI 3 . Meanwhile, the strong e-ph coupling is suggested to cause the large Stokes shift, profound broadness, and skewed lineshape in the photoluminescence (PL) spectra of 2D CrI 3 26 .…”

Artificial Multiferroics and Enhanced Magnetoelectric Effect in van der Waals Heterostructures

“…[20][21][22] Meanwhile, 2D ferromagnetism has been recently achieved in vdW materials, such as few-layer Cr2Ge2Te3, 23 CrI3, 24 and Fe3GeTe2, 25,26 spurring substantial interests to fabricate ultra-thin magnetic devices and study fundamental properties, such as magnons, spin liquids, and many other quantum states in reduced-dimensional structures. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] All these advances pave the way for exploring the existence of 2D multiferroics and magnetoelectronic couplings.…”

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

“…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.…”