Ligand-field helical luminescence in a 2D ferromagnetic insulator

Abstract

Bulk chromium tri-iodide (CrI 3 ) has long been known as a layered van der Waals ferromagnet 1 . However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet 2 , providing a new platform for investigating light-matter interactions and magnetooptical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI 3 under linearly polarized excitation, with heli city determined by the monolayer magnetization direction. In contrast, the bilayer CrI 3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI 3 bilayers 2 . Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors 3 , our absorption and layer-dependent photoluminescence measurements reveal the importance of ligandfield and charge-transfer transitions to the optoelectronic response of atomically thin CrI 3 . We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr 3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.Van der Waals layered materials offer fascinating opportunities for studying light-matter interactions in the 2D limit. For instance, monolayer semiconducting transition metal dichalcogenides (for example, WSe 2 ) enable coupling between the helicity of light and the valley degree of freedom 4 . In all non-metallic 2D materials to date, it has been established that tightly bound Wannier-Mott excitons dominate the intrinsic optical response 3 , and there has been rapid progress in studying 2D excitonic interactions, dynamics and spin/valley physics 3,5 . However, none of these 2D materials possesses long-range magnetic order. A monolayer semiconductor or insulator with intrinsic magnetism would enable the study of novel photo-physical phenomena and the interplay with underlying magnetic order, possibly involving physics incompatible with the Wannier-Mott excitonic picture.On the other hand, the exploration of ferromagnetism in non-metallic bulk materials has a long history. Early studies examined the intrinsic ferromagnetic ordering of a variety of insulating and semiconducting materials, including, for example, the ferrites and ferrospinels 6 , Cr trihalides 7 , Eu chalcogenides 8 and Cr spinels 9 . Later, with the introduction of magnetic dopants into non-magnetic II-VI and III-V semiconductors, diluted magnetic semiconductors captured widespread attention 10 , boosted by the discovery of ferromagnetism in Mn-doped InAs (ref.11 ) and GaAs (ref.1 ) in the 1990s 12 . Central to progress in these fields, optical experiments have led to a deep understanding of electronic structure, magnetization dynamics and interactions between magnetism and light 8,[13][14][15] . While the fascinating physics in the quantum structures of diluted magnet...

Magnetism and magneto-optical effects in bulk and few-layer CrI3: a theoretical GGA + U study

“…It can be seen from figure 6(a) that twe peaks (A 2 and A 3 ) in the experimental xx 1 s of bulk CrI 3 [2] agree well with the theoretical ones in both shape and position, although the magnitude of the experimental spectrum is slightly smaller. Moreover, three peaks (A 1 , A 2 and A 3 ) in the experimental differential reflection spectrum of bulk CrI 3 [7] also agree very well with the theoretical ones (see figure 6(a)). This indicates that our GGA + U calculations could describe the optical properties of bulk CrI 3 quite well.…”

“…The optical conductivity spectra calculated with the SC are displayed in figure S8 in the SI. Surprisingly, the optical conductivity spectra of bulk and ML CrI 3 calculated with the SC are in worse agreement with the corresponding experimental values [2,7] (see figures S8(a), S8(b) and S8(j)). This could be explained as follows.…”

“…For example, we could relate the peak P 1 at 1. For comparison with the available experimental data, we plot in figures 6 (a) and (b) the experimental differential reflection spectra (in arbitrary units) of bulk and ML CrI 3 , respectively [7], which are proportional to xx 1 s . Moreover, we also display the xx 1 s of bulk CrI 3 derived from the experimental refraction index and extinction coefficient [2] in figure 6(a).…”

“…We notice that the Kerr rotation (θ K ) and Kerr ellipticity (ε K ) of all the structures (figure 8) are, respectively, similar to that of the real part ( xy 1 s ) and imaginary part ( xy 2 s ) of the offdiagonal conductivity element ( figure 6). This could be expected because the Kerr effect and the off-diagonal conductivity element are connected through equations (7) and (8) In fact equations (7) and (8) indicate that the complex Kerr rotation angle would be proportional to the off-diagonal optical conductivity of σ xy if the conductivity σ xx of the sample and substrate is roughly a constant of photon energy. For the CrI 3 multilayers, the dielectric constant of ε=2.25 of the SiO 2 substrate is a constant and thus the Kerr rotation angle is proportional to the σ xy .…”

“…attracting much renewed interest. For example, Seyler et al [7] recently reported their observation of magnetophotoluminescence in monolayer and bilayer CrI 3 . Excitingly, manipulation of 2D magnetism (e.g.…”