Localized versus itinerant electrons at the metal-insulator transition in Si:P

The specific heat C of uncompensated phosphorus-doped silicon with P concentration N between 0.34 and 7.3×{}10¹⁸ cm^-3, i.e., in the vicinity of the metal-insulator transition, has been measured over a large range of temperature (0.04 K{≤}T{≤}3 K), allowing the unambiguous detection of an anomalous contribution {Δ}C{~}T^α, with {α} becoming negative for small N. This is attributed to exchange-coupled clusters. The magnetic-field dependence of C (up to 5.7 T) allows us to deduce the relative contributions of localized and delocalized electrons and gives evidence for interactions and correlations between these.