Indirect Band Edge and Chain-Locked Linear Dichroism in the Quasi-1D Van der Waals Antiferromagnet AgCrP2S6

AgCrP2S6 (ACPS) is an exciting antiferromagnetic material in which van der Waals structure combines with low crystal symmetry and a quasi-1D Cr-based spin-chain network, giving rise to pronounced directional anisotropy in its optical and electronic properties. Despite its promise for exploring 1D physics, uncertainties in its band-edge electronic structure limit its application potential. In this work, we combine polarization-resolved optical spectroscopy, photoemission, and photocurrent measurements to establish the hierarchy of band-edge transitions in ACPS. We find that the fundamental gap is indirect, appearing as a photoluminescence band at ≈1.35 eV without a corresponding absorption resonance. Instead, the first allowed direct transitions emerge at higher energies (≈1.63–1.76 eV) for light polarized along the chain axis. Resonant Raman excitation profiles, together with photoemission measurements, support these findings by locating Γ-coupled electronic resonances and placing the valence-band maximum ≈1.56 eV below EF. Finally, integration of ACPS with graphene into a heterostructure enables polarization-sensitive photocurrent collection that mirrors the optical selection rules with an onset ≳1.5 eV and an anisotropy ratio of ≈0.53.