Zeeman Splitting of Anomalous Shallow Bound States in ZnO

Apparent $1s$-to-$2p$ transitions, observed with far-infrared spectrometers at 5.9 and 7.2 meV, exhibit linear Zeeman splitting but are not otherwise consistent with the effective-mass hydrogenic model because the activation energy is an order of magnitude too small and the quadratic Zeeman shift is absent. Cyclotron resonance was observed at 5 K but not at 1.3 K, which confirms the existence of shallow bound states.