Robust phonon engineering and symmetry-selective lattice dynamics in CrSBr$_{1-x}$Cl$_{x}$

TL;DR

This work demonstrates that Br→Cl substitution in CrSBr$_{1-x}$Cl$_x$ enables controlled, symmetry-selective tuning of lattice dynamics in a low-symmetry 2D antiferromagnetic semiconductor. By combining polarization-resolved Raman spectroscopy with first-principles lattice-dynamics, including explicit supercell phonon calculations and Raman Γ-density-of-states simulations, the authors show that alloy disorder activates symmetry-lowered descendants of parent Ag modes and that anisotropic strain renormalizes Cr–S–dominated vibrations. The emergence of P$_1$–P$_3$ Raman features and the persistence of exciton-phonon–coupled, stimulated Raman scattering-like amplification under near-resonant excitation reveal a robust electron–phonon coupling landscape that can be engineered via composition and strain. These findings establish a practical pathway to tailor vibrational and nonlinear optical properties in low-symmetry van der Waals magnets, with potential applications in polarization-sensitive Raman lasers and photonic devices.

Abstract

Atomic substitution provides a controlled route to engineer lattice dynamics in low-symmetry two-dimensional materials. Here, by combining polarization-resolved Raman spectroscopy and first-principles calculations, we investigate the evolution of phonon characteristics in CrSBr$_{1-x}$Cl$_{x}$ ($0 \leq x \leq \sim 0.5$) upon partial substitution of Br with Cl atoms. Progressive Cl substitution of Br induces systematic shifts of parent CrSBr out-of-plane $A_\textrm{g}$ phonon modes and activates additional Raman features. These features persist across different polarization configurations and excitation energies, reflecting substitution-induced symmetry lowering and local lattice perturbations. Explicit supercell phonon calculations combined with Raman $Γ$-density-of-states simulations identify these features as symmetry-lowered descendants of parent modes arising from alloy disorder. Complementary strain-dependent calculations reveal that anisotropic lattice compression plays a key role in renormalizing Cr-S dominated phonons. Under near-resonant excitation, stimulated Raman scattering-like amplification remains observable with increasing Cl content, highlighting the resilience of anisotropic electron-phonon coupling in this system.

Figures (5)

• Figure 1: Phonon dispersion curves along the high-symmetry directions of the first Brillouin zone and the corresponding phonon density of states for bulk (a) CrSBr and (b) CrSCl. The phonon density of states is expressed in units of states/cm$^{-1}$. The high-symmetry points are defined as $\Gamma$(0, 0, 0), X($\frac{1}{2}$, 0, 0), S($\frac{1}{2}$, $\frac{1}{2}$, 0), Y(0, $\frac{1}{2}$, 0), Z(0, 0, $\frac{1}{2}$), U($\frac{1}{2}$, 0, $\frac{1}{2}$), R($\frac{1}{2}$, $\frac{1}{2}$, $\frac{1}{2}$), and T(0, $\frac{1}{2}$, $\frac{1}{2}$). Dashed lines in panel (b) show the dispersion calculated using mass approximation (MA).
• Figure 2: Raman spectra of Cl-substituted CrSBr measured under $E_\textrm{L}$ = 2.33 eV excitation at 100 µW for laser polarization along the crystallographic $a$-axis (a) and $b$-axis (b), as a function of Cl concentration. Solid curves represent the fitted spectra, while the lighter shaded curves correspond to the raw experimental data. Dashed curves represent the RGDOS-simulated spectra, while solid circles underneath the spectra mark the phonon energies calculated using the phonon DFT for various Cl substitution configurations, with horizontal bars indicating the standard deviation arising from local configurational disorder. Vertical solid (experimental parent $A_\textrm{g}$ modes) and dashed (RGDOS-simulated parent $A_\textrm{g}$ modes) lines are guides to the eye.
• Figure 3: Raman shifts as a function of compressive strain along the (a) $a$-axis and (b) $c$-axis for the three $A_\textrm{g}$ modes of CrSBr. Here, the solid circles are the data points, while the lines are the linear fits.
• Figure 4: Polarization-resolved Raman maps for $x = 0.0, 0.2$, and 0.5 compositions for $E_\textrm{L}$ = 2.33 eV (a-c) and $E_\textrm{L}$ = 1.96 eV (d-f) excitation energies at 100 µW power revealing the origin of the P modes and the characteristic $a$-axis to $b$-axis polarization switching of the $A_\textrm{g}^2$, P$_1$, and P$_2$ modes between off-resonance and on-resonance conditions.
• Figure 5: Evolution of the $A_\textrm{g}^2$ (a), P$_1$ (b), and P$_2$ (c) mode intensities as a function of laser power in the range of 100 µW -- 2500 µW, for $x = 0.5$ under $E_\textrm{L}$ = 1.96 eV excitation polarized parallel to crystalline $a$-axis.