Microscopic nature of coordination defects in amorphous silicon

²⁹Si spectra of the g=2.0055 spin resonance in undoped hydrogenated amorphous silicon containing the natural abundance of the ²⁹Si isotope have been measured with a sufficient signal-to-noise ratio to allow a quantitative modeling of the underlying hyperfine and g tensors. These experimental results are used to discuss the microscopic origin of the main deep electronic defect state in a-Si.