Termination of Two-Dimensional Metallic Conduction near the Metal-Insulator Transition in a Si/SiGe Quantum Well

We report in this Letter our recent low-temperature transport results in a Si/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.