Just-metallic germanium doped with arsenic : magnetic properties

We report results of an investigation into the magnetic properties of the electron spins in a sample of germanium heavily doped with arsenic; the doping process is effected by the neutron transmutation technique, on germanium isotopically enriched to 95% Ge-74. The process leaves the sample with 9% compensation, and with a true random distribution of donor spins. Temperatures down to 40 mK and uniaxial stresses up to 0.36 GPa are utilized, and the integrated electron spin resonance is used to monitor the magnetic properties. Electrical characterization of the sample at ambient stress shows hopping behaviour, placing the density of the sample below the critical density for the metal-non-metal transition at this pressure. Electron spin resonance (esr) in the dilution refrigerator is only observed with stress applied; this minimal stress drives the sample metallic. A large 'stress-tuning' effect is inferred. The data on the electron spin susceptibility in the just-metallic sample, as monitored by the integrated area of the esr line at the lowest stress, shows at most only a small variation with temperature; a small broadening as T is lowered appears to be matched by a similar decrease of intensity. Further, [110] uniaxial stress enhances the intensity of the esr line. The sharp contrast with Si:P is discussed. We speculate that the experiment is detecting only the spins in the strongly metallic portions of the sample.