We report in this Letter our recent low-temperature transport results in a $\mathrm{Si}/\mathrm{SiGe}$ quantum well with moderate peak mobility. An apparent metal-insulating transition is observed. Within a small range of densities near the transition, the conductivity $\sigma$ displays a nonmonotonic temperature dependence. After an initial decrease at high temperatures, $\sigma$ first increases with decreasing temperature $T$, showing a metallic behavior. As $T$ continues decreasing, a downturn in $\sigma$ is observed. This downturn shifts to a lower $T$ at higher densities. More interestingly, the downturn temperature shows a power-law dependence on the mobility at the downturn position, suggesting that a similar downturn is also expected to occur deep in the apparent metallic regime at albeit experimentally inaccessible temperatures. This thus hints that the observed metallic phase in 2D systems might be a finite temperature effect.

• Received 14 February 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.126403