Raman Polarization Switching in CrSBr

Abstract

Semiconducting CrSBr is a layered A-type antiferromagnet, with individual layers antiferromagnetically coupled along the stacking direction. Due to its unique orthorhombic crystal structure, CrSBr exhibits highly anisotropic mechanical and optoelectronic properties acting itself as a quasi-1D material. CrSBr demonstrates complex coupling phenomena involving phonons, excitons, magnons, and polaritons. Here we show through polarization-resolved resonant Raman scattering the intricate interaction between the vibrational and electronic properties of CrSBr. For samples spanning from few-layer to bulk thickness, we observe that the polarization of the A$_g^2$ Raman mode can be rotated by 90 degrees, shifting from alignment with the crystallographic a (intermediate magnetic) axis to the b (easy magnetic) axis, depending on the excitation energy. In contrast, the A$_g^1$ and A$_g^3$ modes consistently remain polarized along the b axis, regardless of the laser energy used. We access real and imaginary parts of the Raman tensor in our analysis, uncovering resonant electron-phonon coupling.

Figures (16)

• Figure 1: CrSBr crystal structure and photoluminescence. (a) Optical image of investigated CrSBr sample on SiO$_2$/Si substrate. The white arrows indicate the $a$ and $b$ crystallographic axes of the depicted flake, and the red box denotes the area covered by the AFM scan. (b) Atomic force microscopy image of the sample. The atomic layer thicknesses of different areas are marked in red. (c) Sketch of CrSBr crystal structure, crystallographic axes $a$, $b$, and $c$ are indicated. Red arrows in the top panel show an in-plane spin orientation in a single layer. (d) Photoluminescence spectra of four-layer (4L) thick CrSBr flake at T = 300 K, PL emission is polarized along the $a$ (dashed line) and $b$ (solid line) crystalographic directions. (e) Polar plot of the PL intensities as a function of light polarization orientation to a $b$ axis (0 degrees).
• Figure 2: Raman modes polarization switching at different laser energies. All results are obtained from a four-monolayer (4L) thick CrSBr sample. (a, b) Raman spectra at T = 300 K under an excitation laser energy of (a) $E_L$ = 2.33 eV and (b) $E_L$ = 1.96 eV for polarization along the $b$ axis (top) and $a$ axis (bottom). (c, d) Color mapping of Raman scattering signal for (c) $E_L$ = 2.33 eV and (d) $E_L$ = 1.96 eV as a function of polarization orientation angle, $a$ and $b$ axes are indicated as horizontal dotted lines and correspond to the individual spectra shown in (a, b). Under excitation of 2.33 eV A$_g^2$ mode reveals the polarization along $a$ direction rotated by 90 degrees concerning A$_g^1$ and A$_g^3$ modes polarized along $b$ axis. While under 1.96 eV all phonon modes are polarized along the $b$ axis.
• Figure 3: Polarization dependence of Raman modes of 6L CrSBr for different excitation energies. The top panel presents the polar plot of Raman scattering signal intensity dependent on the polarisation rotation angle of A$_g^2$ (blue color), while the bottom panel shows the same for A$_g^3$ mode (pink color) at different excitation energies ranging from 2.33 to 1.96 eV (each column reflects a certain excitation energy). Dots correspond to an experimental spectrum, and the solid lines are fitting curves using equation \ref{['fit']}. The angle of the polarization orientation of the Raman signal is measured from the direction of the $b$ axis (0 degrees). The radial axis corresponds to the signal intensity, which differs for each plot, the center point is 0 counts on all plots. The A$_g^2$ mode polarization behavior switches from being polarized along $a$ to along $b$ direction at around 2.07 eV excitation energy.
• Figure 4: Polarization dependence of Raman modes for different sample thickness. The top panel demonstrates bright field imaging of the samples of different thicknesses from $\sim$3.6 to 62 nm, columns under each image correspond to the results measured on the area marked with a red dashed line. The middle panel shows the polarization polar plots of all three Raman modes at the excitation of 2.33 eV, and the bottom panel illustrates the same Raman modes under 1.96 eV excitation. Dots correspond to experimental results, and the solid lines are fitting curves using equation \ref{['fit']}. The angle of the polarization orientation of the Raman signal is measured from the direction of the $b$ axis (0 degrees). The radial axis corresponds to the signal intensity, which differs for each plot. The polarization orientation of each Raman mode is independent of the sample thickness.
• Figure S1: Atomic force microscopy (AFM). On the left the AFM image with a green line indicating the position of the thickness profile.The corresponding thickness profile on the right, where the monolayer thickness is measured as 1.01 nm.