Seminarium Fizyki Ciała Stałego

While hexagonal transition-metal dichalcogenides require external fields to break valley degeneracy, anisotropic materials such as tin sulfide (SnS) offer intrinsically nondegenerate valleys addressable with linearly polarized light—but understanding valley-specific dynamics remains elusive. Here, we demonstrate that resonant Raman scattering unveils a direct fingerprint of valley identity through strong reorganization of exciton-phonon coupling. At the X1 valley (1.37 eV), the A3g breathing mode dominates with fourfold enhancement over A4g, while this hierarchy completely inverts at the X3 valley (1.68 eV) where A4g shows sixfold stronger coupling. This valley-specific coupling originates from the distinct orbital characters, as revealed by our density functional theory calculations: px orbitals at X1 couple efficiently to symmetric breathing motions, whose symmetric compression/extension modulates the px-px overlap. Conversely, the mixed s-p orbitals at X3 favor complex displacement patterns. Beyond conventional Raman modes, we observe resonant activation of infrared-active phonons through Fröhlich coupling, with valley-dependent polarization selection rules and activation energies of 4.6–13.2 meV. Power dependent measurements reveal valley-specific saturation behaviors, with X3 saturating at three times lower carrier densities than X1 due to differences in band curvature. These findings establish phonon scattering as an intrinsic probe of valley physics, enabling optical identification and potentially manipulation of valley states.