Ge film thermometers at ultralow temperatures in high magnetic fields

The conduction mechanisms and effect of magnetic field for Ge-film thermome ters in the 25 mK to 4.2 K temperature range and magnetic fields up to 6 T are studied. At low temperatures, the variable-range hopping conductivity p revails. At temperatures below 0.3 K, a gigantic negative magneto-resistanc e (up to 100%) is observed. The temperature dependence of resistance and ma gneto-resistance shows that localization behavior near the mobility edge ex ists. To explain experimental results, we adopt a phenomenological model ba sed on variable-range hopping below the mobility edge. We also use a scalin g theory of localization with a localization length depending on the mobili ty edge location. (C) 2000 Published by Elsevier Science B.V. All rights re served.