Electrical properties of isotopically enriched neutron-transmutation-doped $^{70} G e : G a$ near the metal-insulator transition

We report low-temperature carrier transport properties of a series of nominally uncompensated neutron-transmutation-doped

^{70} G e : G a

samples very close to the critical concentration

N_{c}

for the metal-insulator transition. The nine samples closest to

N_{c}

have Ga concentrations

N

in the range

{0.99 N}_{c} < N < {1.01 N}_{c} .

The electrical conductivity σ has been measured in the temperature range

T = 0.02 - 1 K .

On the metallic side of the transition the standard

σ {(T) = a + b T}^{q}

with

q = 1 / 2

was observed for all the samples except for the two that are closest to

N_{c}

with

N

between

N_{c}

and

{1.0015 N}_{c} .

These samples clearly show

q = 1 / 3.

An extrapolation technique has been developed in order to obtain the zero-temperature conductivity σ(0) from

σ (T)

with different dependence on

T .

Based on the analysis,

ν \approx 0.5

in the familiar form of

σ (0) \propto {(N / N}_{c} - {1)}^{ν}

has been found. On the insulating side of the transition, variable range hopping resistivity

ρ (T) \propto \exp {(T}_{0} {/ T)}^{p}

with

p = 1 / 2

has been observed for all the samples having

N < {0.991 N}_{c} .

In this regime

T_{0} \propto (1 - {N / N}_{c})^{α}

with

α \approx 1

\vec{N} N_{c} .

The values of

T_{0}

agree very well with theoretical estimates based on the modified Efros and Shklovskii relation

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Electrical properties of isotopically enriched neutron-transmutation-doped70Ge:Ganear the metal-insulator transition