Electrical properties of isotopically enriched neutron-transmutation-doped near the metal-insulator transition

We report low-temperature carrier transport properties of a series of nominally uncompensated neutron-transmutation-doped (Formula presented) samples very close to the critical concentration (Formula presented) for the metal-insulator transition. The nine samples closest to (Formula presented) have Ga concentrations (Formula presented) in the range (Formula presented) The electrical conductivity σ has been measured in the temperature range (Formula presented) On the metallic side of the transition the standard (Formula presented) with (Formula presented) was observed for all the samples except for the two that are closest to (Formula presented) with (Formula presented) between (Formula presented) and (Formula presented) These samples clearly show (Formula presented) An extrapolation technique has been developed in order to obtain the zero-temperature conductivity σ(0) from (Formula presented) with different dependence on (Formula presented) Based on the analysis, (Formula presented) in the familiar form of (Formula presented) has been found. On the insulating side of the transition, variable range hopping resistivity (Formula presented) with (Formula presented) has been observed for all the samples having (Formula presented) In this regime (Formula presented) with (Formula presented) as (Formula presented) The values of (Formula presented) agree very well with theoretical estimates based on the modified Efros and Shklovskii relation (Formula presented) where (Formula presented) and (Formula presented) are the dielectric constant and the Bohr radius, respectively. The insulating samples very close to the transition (Formula presented) exhibit quite a different behavior. In this range (Formula presented) increases rapidly as (Formula presented) changes from (Formula presented) to (Formula presented) The relevance of our findings to the collapsing of the Coulomb gap is discussed.