A rhombic spin-1 interpretation of a room-temperature GaN emitter

Single defects in GaN show optically detected magnetic resonance at room temperature. In the more abundant class, a minimal $S=3/2$ model leaves a g≈ 4 line unexplained. For one defect, this line lies near the sum of the two outer lines, consistent with transitions among three shared levels. A rhombic $S=1$ model reproduces the three line positions with rms errors of 8.1-9.8 MHz. The tested second-order $S=3/2$ fits containing all three transitions and the published minimal model do not reproduce these positions. A ladder fitted to the strong lines remains viable if the g≈ 4 line is a separate transition near the sum of the outer two. An unresolved frequency-window shift prevents the existing data from establishing the $S=1$ assignment uniquely. We propose a zero-field sweep that directly tests the isolated $S=1$ assignment.