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We have investigated the temperature dependence of the electricalconductivity sigma(N,B,T) of nominally uncompensated,neutron-transmutation-doped ^{70}Ge:Ga samples in magnetic fields up to B=8 Tat low temperatures (T=0.05-0.5 K). In our earlier studies at B=0, the criticalexponent mu=0.5 defined by sigma(N,0,0) \propto (N-N_c)^{mu} has beendetermined for the same series of ^{70}Ge:Ga samples with the dopingconcentration N ranging from 1.861 \times 10^{17} cm^{-3} to 2.434 \times10^{17} cm^{-3}. In magnetic fields, the motion of carriers loses time-reversalsymmetry, the universality class may change and with it the value of mu. Inthis work, we show that magnetic fields indeed affect the value of mu (muchanges from 0.5 at B=0 to 1.1 at B \geq 4 T). The same exponent mu'=1.1 isalso found in the magnetic-field-induced MIT for three different ^{70}Ge:Gasamples, i.e., sigma(N,B,0) \propto [B_c(N)-B]^{mu'} where B_c(N) is theconcentration-dependent critical magnetic induction. We show that sigma(N,B,0)obeys a simple scaling rule on the (N,B) plane. Based on this finding, wederive from a simple mathematical argument that mu=mu' as has been observed inour experiment.Comment: 9 pages, 5 figures, published versio

physical review. b, condensed matter

Metal-insulator transition of isotopically enriched neutron-transmutation-doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow /><mml:mrow><mml:mn>70</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">Ge</mml:mi><mml:mo>:</mml:mo><mml:mi mathvariant="normal">Ga</mml:mi></mml:math>in magnetic fields

Metal-insulator transition of isotopically enriched neutron-transmutation-doped70Ge:Gain magnetic fields