LOCAL MOMENTS AT THE METAL-INSULATOR-TRANSITION IN GE-SB AS PROBED WITH SPECIFIC-HEAT MEASUREMENTS IN MAGNETIC-FIELDS

Specific-heat measurements at low temperatures (0.07 less than or equal to T less than or equal to 1.6 K) are reported for Ge:Sb with donor concentrations between 0.6 and 4.0 x 10(17) cm(-3) spanning across the metal-insulator transition at N-c = 1.44 x 10(17) cm(-3). The electronic coefficient gamma of the specific heat C is roughly twice as large as expected for donor electrons with the Ge conduction-band effective mass. Below about 0.3 K an additional contribution to C is found which is attributed at least partly to exchange-coupled clusters of local moments. This is supported by measurements of C in magnetic fields where this contribution turns into a Schottky-like anomaly. Local moments persist also in the metallic region of Ge:Sb as previously inferred from magnetic susceptibility measurements. The dependence of the density of local moments on donor concentration is similar to that found previously in Si:P.