Anomalous magnetic properties of triplet excited states in crystalline and amorphous arsenic triselenide

The transient photoluminescence in crystalline arsenic triselenide (c-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$) is strongly magnetic field dependent at low magnetic fields (H<1 T) and low temperature (T=2 K), proving that the excited state of the luminescence center is a magnetic multiplet state. This is consistent with earlier results which suggested that the luminescence arises from a spin-triplet excited state. Although highly anisotropic, the magnetic field dependence does not include the level crossings expected for a triplet state. The luminescence centers have also been studied by the technique of optically detected magnetic resonance (ODMR). No resonances are observed at the intrinsic luminescence center in c-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$; the absence of resonances is surprising in view of the magnetic field dependence of the recombination. Resonances characteristic of a spin-triplet state are, however, observed at an impurity-related luminescence center present in iodine-doped c-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$. In amorphous arsenic triselenide (a-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$), both magnetic-field-dependent recombination and triplet magnetic resonance are observed. The triplet ODMR spectrum in a-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ is extremely broad, suggesting a broad distribution of zero-field splittings. Possible explanations for these results are discussed, in particular the possibility that the spin-triplet excited state is also orbitally degenerate, and comparisons are made to spin-triplet states in other systems.