Knight shifts in Si:P and Si:(P,B) in the vicinity of the metal-insulator transition

Recent experimental and theoretical work has suggested that a two-fluid model can be used to describe the magnetic properties of the just-metallic phase of metal-insulator systems such as Si:P. Alternative descriptions based on a Fermi liquid approach have also been put forward to explain the magnetic and other behaviors of these systems. In the present work ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ Knight-shift measurements have been made on a number of samples of Si:P and Si:(P,B) in the just-metallic region as a function of temperature and electron concentration. The magnetization of the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ spin system follows Curie law behavior down to 50 mK. The mean Knight shift is found to be temperature independent in the range 50 mK–4.2 K for both compensated and uncompensated systems. This result is consistent with a proposal that the ${}_{}{}^{29}{}_{}{}^{}\mathrm{Si}$ spins that are observed in the Knight-shift measurements interact only with the itinerant electron fluid, even though the susceptibility is controlled by the other fluid of localized moments.