The paramagnetic susceptibility of crystals with bistable In centers was measured at temperatures ranging from 4 to 300K. In the case of crystals which were cooled (in the dark) to liquid He temperatures, the susceptibility was determined by trace Mn impurities. Illumination of the sample with ultra-violet or visible light resulted in the appearance of a photo-induced susceptibility signal that was due to the formation of centers with a magnetic moment of 1/2. This confirmed the absence of paramagnetism from the deep state of the bistable In center, and its presence in the shallow state; thus proving the negative-U nature of the deep state.

S.A.Kazanskii, A.I.Ryskin, V.V.Romanov: Applied Physics Letters, 1997, 70[10], 1272-4