Metal-insulator transition of isotopically enriched neutron-transmutation-doped Ge-70 : Ga in magnetic fields

We have investigated the temperature dependence of the electrical conductiv ity sigma(N,B,T) of nominally uncompensated, neutron-transmutation-doped Ge -70:Ga samples in magnetic fields up to B = 8 T at low temperatures (T = 0. 05 - 0.5 K). In our earlier studies at B = 0, the critical exponent mu = 0. 5 defined by sigma(N,0,0)proportional to(N-N-c)(mu) has been determined for the same series of Ge-70:Ga samples with the doping concentration N rangin g from 1.861 X 10(17) cm(-3) to 2.434 X 10(17) cm(-3). In magnetic fields, the motion of carriers loses time-reversal symmetry, the universality class may change and with it the value of mu. In this work, we show that magneti c fields indeed affect the value of mu (mu changes from 0.5 at B = 0 to 1.1 at B greater than or equal to 4 T). The same exponent mu' = 1.1 is also fo und in the magnetic-field-induced MIT for three different Ge-70: Ga samples , i.e., sigma(N,B,0)proportional to[B-c(N)-B](mu') where B-c(N) is the conc entration-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, we d erive from a simple mathematical argument that mu = mu' as has been observe d in our experiment. [S0163-1829(99)10447-8].