Ligand field luminescence in the honeycomb Mott insulator α-RuCl3

Novel electronic structure is central to Kitaev-like quantum spin liquid ground state in the van der Waals antiferromagnet α -RuCl _3 . In this octahedral low-spin $4d^5$ system, the rich bandstructure includes Mott–Hubbard insulating bandgap of ∼1.1 eV and ligand field induced gap of ∼2 eV. Though the bandgap is extensively reported in previous optical and photoemission studies, the latter gap is less understood. In this regard, we probed the ligand field-induced electronic structure of bulk α -RuCl _3 single crystals via unpolarized photoluminescence (PL) spectroscopy under linearly polarized excitation in the visible region. We observed the broad principal PL peak at E _PL $\approx$ 1.87 eV, close to the energy required for the optical transition across the ligand field splitting gap (∼2 eV). This gap ( E _PL ) monotonically increases with temperature, described by the thermal expansion of lattice parameters dominating the electron–phonon interaction in this Honeycomb Mott insulator. Moreover, we observed the multiplet structure of the PL spectra, which can be explained by parity-forbidden d–d transitions in the light of single ion ligand-field theory. We attribute the broad PL linewidths to strong vibronic coupling modeled by the Huang–Rhys parameter. Further we observed the signature of structural transitions, indicated by thermal hysteresis in normalized integrated PL intensity.