Electrical properties of isotopically enriched neutron-transmutation-doped {sup 70}Ge:Ga near the metal-insulator transition
- Cryogenic Center, University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-0032 (Japan)
- Department of Applied Physics and Physico-Informatics, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522 (Japan)
- Lawrence Berkeley National Laboratory and University of California at Berkeley, Berkeley, California 94720 (United States)
We report low-temperature carrier transport properties of a series of nominally uncompensated neutron-transmutation-doped {sup 70}Ge:Ga samples very close to the critical concentration N{sub c} for the metal-insulator transition. The nine samples closest to N{sub c} have Ga concentrations N in the range 0.99N{sub c}{lt}N{lt}1.01N{sub c}. The electrical conductivity {sigma} has been measured in the temperature range T=0.02{endash}1thinspK. On the metallic side of the transition the standard {sigma}(T)=a+bT{sup q} with q=1/2 was observed for all the samples except for the two that are closest to N{sub c} with N between N{sub c} and 1.0015N{sub c}. These samples clearly show q=1/3. An extrapolation technique has been developed in order to obtain the zero-temperature conductivity {sigma}(0) from {sigma}(T) with different dependence on T. Based on the analysis, {nu}{approx}0.5 in the familiar form of {sigma}(0){proportional_to}(N/N{sub c}{minus}1){sup {nu}} has been found. On the insulating side of the transition, variable range hopping resistivity {rho}(T){proportional_to}exp(T{sub 0}/T){sup p} with p=1/2 has been observed for all the samples having N{lt}0.991N{sub c}. In this regime T{sub 0}{proportional_to}(1{minus}N/N{sub c}){sup {alpha}} with {alpha}{approx}1 as N{r_arrow}N{sub c}. The values of T{sub 0} agree very well with theoretical estimates based on the modified Efros and Shklovskii relation k{sub B}T{sub 0}{approx}(2.8e{sup 2}/4{pi}{epsilon}{sub 0}{kappa}{sub 0}{xi}{sub 0})(1{minus}N/N{sub c}){sup {alpha}}, where {kappa}{sub 0} and {xi}{sub 0} are the dielectric constant and the Bohr radius, respectively. The insulating samples very close to the transition (0.991N{sub c}{lt}N{lt}N{sub c}) exhibit quite a different behavior. In this range 1/p increases rapidly as N changes from 0.991N{sub c} to N{sub c}. The relevance of our findings to the collapsing of the Coulomb gap is discussed. {copyright} {ital 1998} {ital The American Physical Society}
- OSTI ID:
- 662247
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 58, Issue 15; Other Information: PBD: Oct 1998
- Country of Publication:
- United States
- Language:
- English
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