Hopping Conduction and Metal-Insulator Transition in Isotopically Enriched Neutron-Transmutation-Doped $^{70}$ Ge:Ga

We report on the electrical conductivity

σ

of a series of nominally uncompensated neutron-transmutation-doped isotopically enriched

^{70}

Ge:Ga samples with the Ga concentration [Ga] near

N_{c}

for the metal-insulator transition.

σ

of all insulating samples obeys

\ln σ \propto - (T_{0} / T)^{1 / 2}

with

T_{0} \propto (N_{c} - [Ga]) {/ N}_{c}

while the zero temperature conductivity

σ (0)

of the metallic samples is

σ (0) \propto {([Ga] - N_{c}) {/ N}_{c}}^{ν}

with the critical exponent

ν \approx 0.5

. The values of

N_{c}

obtained from the two independent scalings of

T_{0}

and

σ (0)

are identical, i.e.,

ν \approx 0.5

is established unambiguously for uncompensated Ge:Ga.

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Hopping Conduction and Metal-Insulator Transition in Isotopically Enriched Neutron-Transmutation-Doped70Ge:Ga