Interplay of vibrational, electronic, and magnetic states in CrSBr

The van der Waals antiferromagnet CrSBr exhibits coupling of vibrational, electronic, and magnetic degrees of freedom, giving rise to distinctive quasi-particle interactions. We investigate these interactions across a wide temperature range using polarization-resolved Raman spectroscopy at various excitation energies, complemented by optical absorption and photoluminescence excitation (PLE) spectroscopy. Under 1.96 eV excitation, we observe pronounced changes in the A$_g^1$, A$_g^2$, and A$_g^3$ Raman modes near the Néel temperature, coinciding with modifications in the oscillator strength of excitonic transitions and clear resonances in PLE. The distinct temperature evolution of Raman tensor elements and polarization anisotropy for Raman modes indicates that they couple to different excitonic and electronic states. The suppression of the excitonic state’s oscillation strength above the Néel temperature could be related to the magnetic phase transition, thereby connecting these excitonic states and Raman modes to a specific spin alignment. These observations make CrSBr a versatile platform for probing quasi-particle interactions in low-dimensional magnets and provide insights for applications in quantum sensing and quantum communication.

💡 Research Summary

This work investigates the intertwined vibrational, electronic, and magnetic degrees of freedom in the van‑der‑Waals antiferromagnet CrSBr. By combining temperature‑dependent, polarization‑resolved Raman spectroscopy with optical absorption, differential reflectance, and photoluminescence‑excitation (PLE) measurements, the authors map how phonon modes, excitonic transitions, and spin order influence each other across the paramagnetic (PM) → intermediate ferromagnetic (iFM) → antiferromagnetic (AFM) sequence (TC ≈ 165 K, TN ≈ 132 K).

Three Raman‑active out‑of‑plane Ag modes (A1g, A2g, A3g) are examined under two excitation energies, 1.96 eV and 2.33 eV, on flakes ranging from 5 nm to 31 nm. At 2.33 eV the polarization of A2g (a‑axis) and A3g (b‑axis) remains essentially unchanged with temperature, while their overall intensity monotonically decreases. In stark contrast, 1.96 eV excitation—close to the X B exciton resonance—produces a dramatic temperature‑driven reorientation of the Raman tensor: both A2g and A3g acquire a growing a‑axis component as the temperature is lowered, and below TN the a‑component of A3g even exceeds the b‑component. This evolution is quantified by the ratio of the Raman‑tensor amplitudes (b/a), which shows a distinct kink at the Néel temperature for each mode, with the kink being most pronounced for A3g.

PLE and differential reflectance spectra reveal that the 1.96 eV photon energy lies on the shoulder of a strong excitonic resonance (the X B exciton at ≈1.77 eV for b‑axis polarization). This exciton’s oscillator strength collapses above TN, accompanied by a broadening of its linewidth. The authors argue that the Raman scattering is mediated by such near‑resonant electronic states via a three‑step process (virtual/real electronic excitation → phonon emission → photon emission). Consequently, the suppression of the X B exciton above TN reduces the Raman‑tensor elements a and b, producing the observed temperature‑dependent polarization changes.

The thickness dependence shows that the b/a‑kink persists for all investigated thicknesses, though the sharpness diminishes as the flake becomes thicker, indicating that the quasi‑1D character and reduced dimensionality enhance the coupling. The phonon frequencies themselves shift only slightly across TN, suggesting that direct spin‑phonon coupling is weak; instead, the magnetic order influences the phonons indirectly through the exciton‑phonon (electron‑phonon) interaction.

Overall, the study demonstrates that in CrSBr the magnetic phase transition modulates the electronic band structure and excitonic oscillator strengths, which in turn control the Raman‑tensor anisotropy of specific phonon modes. This indirect spin‑phonon coupling mediated by excitons provides a versatile platform for probing quasi‑particle interactions in low‑dimensional magnets and points toward potential applications in quantum sensing, communication, and spintronic devices where control of spin, charge, and lattice degrees of freedom is essential.