Experimental Signatures of Ground State and Photoexcited Edge Magnetism in Phosphorene Nanoribbons*

Zig-zag edge terminated phosphorene nanoribbons (z-PNRs) are mono-elemental intrinsically anisotropic two-dimensional (2D) semiconductors with a plethora of predicted exotic properties e.g. room-temperature edge ferromagnetism. Recently z-PNRs have been produced using a top-down chemical synthesis. Using femtosecond impulsive stimulated Raman spectroscopy on these z-PNRs, we demonstrate coupling between an excited state and a symmetry forbidden edge phonon mode allowing us to isolate signatures of excitations at the edge of the ribbons. Using polarisation sensitive pump-probe spectroscopy with sub-1T magnetic fields we demonstrate signatures of room temperature ground state ferromagnetism in these materials via a magnetic field-induced anisotropy. Calculations reveal spin-spin correlation lengths of at least 500 lattice sites in z-PNRs. We further find that the excited state dynamics cannot be fully resolved by a ground state orientation effect, suggesting the excited state edge-excitations additionally couple to the magnetic field. These results present a significant step towards room temperature optically controlled magnetism in semiconductors.