Disappearance of the metal-like behavior in GaAs two-dimensional holes below 30 mK

The zero-field temperature-dependence of the resistivity of two-dimensional holes are observed to exhibit two qualitatively different characteristics for a fixed carrier density for which only the metallic behavior of the so-called metal-insulator transition is anticipated. As $T$ is lowered from 150 mK to 0.5 mK, the sign of the derivative of the resistivity with respect to $T$ changes from being positive to negative when the temperature is lowered below $sim$30 mK and the resistivity continuously rises with cooling down to 0.5 mK, suggesting a crossover from being metal-like to insulator-like.