ELECTRICAL-PROPERTIES OF ISOTOPICALLY ENRICHED NEUTRON-TRANSMUTATION-DOPED GE-70-GA NEAR THE METAL-INSULATOR-TRANSITION

We report low-temperature carrier transport properties of a series of nominally uncompensated neutron-transmutation-doped Ce-70:Ga samples v ery close to the critical concentration N-c for the metal-insulator tr ansition. The nine samples closest to N-c have Ga concentrations N in the range 0.99N(c)<N<1.01N(c). The electrical conductivity a has been measured in the temperature range T = 0.02-1 K. On the metallic side o f the transition the standard sigma(T) = a + bT(q) with q = 1/2 was ob served for all the samples except for the two that are closest to N-c with N between N-c and 1.0015N(c). These samples clearly show q = 1/3. An extrapolation technique has been developed in order to obtain the zero-temperature conductivity sigma(0) from sigma(T) With different de pendence on T. Based on the analysis, nu approximate to 0.5 in the fam iliar form of sigma(0)proportional to(N/N-c-1)(nu) has been found. On the insulating side of the transition, variable range hopping resistiv ity rho(T)proportional to exp(T-0/T)(p) with p = 1/2 has been observed for all the samples having N<0.991N(c). In this regime T(0)proportion al to(1 - N/N-c)(alpha) with alpha approximate to 1 as N-->N-c. The va lues of T-0 agree very well with theoretical estimates based on the mo dified Efros and Shklovskii relation k(B)T(0)approximate to(2.8e(2)/4 pi epsilon(0)kappa(0)xi(0))(1 - N/N-c)(alpha), where kappa(0) and xi(0 ) are the dielectric constant and the Bohr radius, respectively. The i nsulating samples very close to the transition (0.991N(c)<N<N-c) exhib it quite a different behavior. In this range lip increases rapidly as N changes from 0.991N(c) to N-c. The relevance of our findings to the collapsing of the Coulomb gap is discussed. [S0163-1829(98)05339-9].