Group Theory Analysis of Phonons in Monolayer Chromium Trihalides and Their Janus Structures

Abstract

A contrastive investigation of the symmetry aspects of phonons in monolayer chromium trihalides and their Janus structures Y${}_3$-Cr${}_2$-X${}_3$ (X, Y = F, Cl, Br, I) by group theory is presented. We first classify all phonons at the Brillouin-zone center ($\Gamma$) into the irreducible representation. Then the infrared and Raman activity of optic phonons, Raman tensors, and the possible polarization assignments of R active phonons are predicted. Base on these results, we clarify the the discrepancy about the Raman activity o optic modes in monolayer CrI${}_3$. Besides, we find that the Raman and infrared spectra for X${}_3$-Cr${}_2$-X${}_3$ are exclusive, whereas that for Janus Y${}_3$-Cr${}_2$-X${}_3$ are coincident. This distinction is vital for optic spectra identification of Janus Y${}_3$-Cr${}_2$-X${}_3$ monolayer from X${}_3$-Cr${}_2$-X${}_3$ monolayer. In addition, we derive the symmetry-matched phonon eigenfunctions and corresponding schematic representations of the eigenvectors for both F${}_3$-Cr${}_2$-I${}_3$ and I${}_3$-Cr${}_2$-I${}_3$ monolayer, which demonstrate intuitively the origin of phonon chirality and magnetism. At last, our analysis indicates that the spin-phonon coupling, the magneto-optical effect of infrared and Raman active phonons, and phonon chirality should be observed in Janus Y${}_3$-Cr${}_2$-X${}_3$ monolayer as that and even easier than that in X${}_3$-Cr${}_2$-X${}_3$ monolayer. Our work provides a detailed guiding map for experimental characterization of Y${}_3$-Cr${}_2$-X${}_3$ monolayer, and also reveals important effects of optic phonons in Janus Y${}_3$-Cr${}_2$-X${}_3$ monolayer.

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Cited by in corpus (1)

• Discovery of an ultrastable antiferromagnetic two-dimensional CrF3 phase with anisotropic quasi-one-dimensional mechanical, electronic, and thermal properties