Excitonic optical absorption in strained monolayer CrSBr

TL;DR

This work addresses how strain modulates excitonic optical responses in the 2D magnetic semiconductor CrSBr. By solving the Bethe-Salpeter equation with an anisotropic Rytova–Keldysh screening, it reveals that bound excitons form mainly along the B direction and experience substantial energy shifts and lineshape changes under different strain configurations, influencing the sub-bandgap optical conductivity. Circular dichroism remains negligible within the gap, while magnetization orientation can enhance MCD above the gap, highlighting a route to strain- and magnetization-driven control of excitonic and magneto-optical properties. The findings have implications for strain-engineered optoelectronic and spintronic devices based on CrSBr and related 2D magnets, and point to future real-time studies of exciton–magnon interactions under dynamic strain.

Abstract

Recently, the isolation of 2D magnetic materials has opened several avenues for possible new ap- plications in spintronics. Among these materials, CrSBr has sparked interest due to its relatively high Curie temperature, highly anisotropic lattice structure, and high structural stability. These properties ran along others shared by any atomically thin material such as its outstanding defor- mation capacity and a strong optical response dominated by excitonic effects. The combination of these properties provides a fairly uncharted playground where to explore the interplay between magnetism and optical excitations. Here, we focus our attention on the theoretical optical response of CrSBr under several distinct strain configurations, analyzing the resulting changes to both the excitonic peaks and overall shape of the diagonal components of the linear conductivity tensor.

Figures (8)

• Figure 1: Top (left) and side views (right) of the unit-cell of a single $\mathrm{CrSBr}$ layer, where green, yellow, and pink represent the Cr, S, and Br atoms, respectively. In the plots, the $A$, $B$, and $C$ directions correspond to $x$, $y$, and $z$ axis, respectively.
• Figure 2: Top: Band structure of CrSBr along the $\mathrm{X}-\Gamma-\mathrm{Y}-\mathrm{S}-\Gamma$ path in the first Brillouin zone. Red bands were directly utilized for the BSE calculations in this work. Bottom: Contour plots of highlighted bands (increasing in energy from top left to bottom right) in the FBZ (red rectangle). Gray, green, blue, and orange points represent the high--symmetry $\Gamma$, $\mathrm{X}$, $\mathrm{Y}$, and $\mathrm{S}$ points, respectively.
• Figure 3: Contour plots of the two bands closest to the Fermi level (conduction top, valence bottom) in the FBZ (red rectangle) for strain configurations $95\%\, A$, $105\%\, A$, $95\%\, B$, $105\%\, B$ (left to right respectively).
• Figure 4: Wave functions of the $n=1$ (optically dark), $n=7$, $n=14$, $n=19$, and $n=26$ exciton states plotted in the FBZ (top panel), together with the subgap region of the optical conductivity (bottom panel). In the bottom panel, vertical dashed line at $\hbar\omega=2.1\,\mathrm{eV}$ corresponds to the bandgap at the $\Gamma$ point for the unstrained system, solid vertical lines correspond to the optically bright states and dot-dashed lines correspond to optically dark states until $n=7$ (blue dot-dashed line highlights the lowest energy exciton state, $n=1$).
• Figure 5: Excitonic (solid lines) and IPA (dashed lines) contribution to the diagonal tensor elements of the linear optical conductivity (top: $\sigma_{xx}$, bottom: $\sigma_{yy}$). Left panels correspond to strain in the $A$ direction ($x$-axis), while right panels correspond to strain in the $B$ direction ($y$-axis). Blue, black, and red lines correspond to $95\%$ strain, unstrained, and $105\%$ strain in the corresponding column's direction, respectively. In all panels, vertical dashed line at $\hbar\omega=2.1\,\mathrm{eV}$ corresponds to the bandgap at the $\Gamma$ point for the unstrained system.