Electronic structure observed by magnetic resonance

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Abstract
Magnetic resonance experiments on phosphorus-doped silicon, notably those published in 1965 by Jérome, Ryter and Winter, are recalled and discussed in the light of more recent low-temperature conductivity measurements by the Bell laboratories group on the same type of dilute solutions. The resonance experiments carried out above 1 K clearly show that, at doping levels above 1018 phosphorus atoms/cm3, electrons are not localized on specific P impurities; they show also that a band structure with a (local) continuum of states at the Fermi level exists at concentrations below the metal-insulator transition at 3·7 × 1018cm−3. This agrees with recent low-temperature electronic specific heat measurements. The large dispersion of the Si and P Knight shift and the Stoner enhancement factor deduced from the Korringa relation in this intermediary range can be explained by a narrow and half-filled impurity band; the fraction of P impurities with a permanent paramagnetic moment cannot be large and there is no evidence of antiferromagnetic couplings. Resonance measurements in the conducting regime suggest that this impurity band merges into the conduction band only at higher concentrations; the continuity of results across the critical concentration indicates that localization does not play an essential role in the spatial dispersion of Knight shifts. Similar detailed studies at lower temperatures could help to clarify the exact role of electronic correlations, in this regime.