Low energy electrodynamics of CrI3 layered ferromagnet

Abstract

We report on the optical properties from terahertz (THz) to Near-Infrared (NIR) of the layered magnetic compound CrI3 at various temperatures, both in the paramagnetic and ferromagnetic phase. In the NIR spectral range, we observe an insulating electronic gap around 1.1 eV which strongly hardens with decreasing temperature. The blue shift observed represents a record in insulating materials and it is a fingerprint of a strong electron-phonon interaction. Moreover, a further gap hardening is observed below the Curie temperature, indicating the establishment of an effective interaction between electrons and magnetic degrees of freedom in the ferromagnetic phase. Similar interactions are confirmed by the disappearance of some phonon modes in the same phase, as expected from a spin-lattice interaction theory. Therefore, the optical properties of CrI3 reveal a complex interaction among electronic, phononic and magnetic degrees of freedom, opening many possibilities for its use in 2-Dimensional heterostructures.

Comment on “Distinct spin–lattice and spin–phonon interactions in monolayer magnetic CrI” by L. Webster, L. Liang and J.-A. Yan, Phys. Chem. Chem. Phys., 2018, 20, 23546

“…Comment PCCP the infrared spectra of layered CrI 3 in ref. 3. Our interpretation for this phenomenon is that the infrared spectra of layered CrI 3 in ref.…”

“…Similarly, the A 2u mode also transforms into an A u mode in the low-temperature (R% 3) structure and B u in the high-temperature (C2/m) structure, which is still infrared active in both cases. 8 The absence of the highest-frequency A u or B u peak in single-phonon absorption spectra of bulk CrI 3 might be attributed to its negligible intensity or attributed to the uncertainty of its frequency arising from significant spin-phonon coupling, [1][2][3] which cannot be discriminated within group theory only. Finally, we classify the lattice vibrational modes of CrI 3 at the G according to the irreducible representations of D 3d within group theory.…”

“…There is an infrared peak at 133 cm −1 (near 15 mode, Fig. 4(j) 134.5 cm −1 ) whose intensity increases with temperature, but there is no infrared peak near 264.7 cm −1 (21 mode) in the infrared spectra of layered CrI 3 in ref. 3.…”

“…Furthermore, some of these mistakes have been cited by later studies and led to further confusion. 2,3 Thus, it is necessary and urgent to point out these mistakes in the following four items explicitly.…”

“…It is interesting that a recent experimental investigation on the temperature dependence of the far-infrared vibrational modes of layered CrI 3 , 3 which citing ref. 1, seems to support Webster's identification of optical modes in Fig.…”