Electrical conduction and metal-insulator transition of indium nanowires on Si(111)

We have studied the metal-insulator (MI) transition of indium nanowires on the Si(111) surface by electrical conductivity measurements with a four-point probe. Upon cooling, the sheet conductivity of the high-temperature (4 x 1) phase, which is known to have metallic electron bands, exhibited a gradual decrease in quantitative agreement with the Mott's variable range hopping conduction. Upon further cooling, the conductivity exhibited a sharp drop at 120 K, indicating the transition into the insulating (8 x 2) phase. The conductivity upon heating from 65 K did not trace the curve during cooling but showed a thermal hysteresis with a width of 8 K. The observation of the hysteresis agrees with the previous electron diffraction experiments, showing that the MI transition is first order. It was further found that, instead of the superheating behavior usually observed in first-order transitions, the transition upon heating starts below T-c, while the supercooling is always observed. This indicates a specific heterogeneous nucleation process only during heating. It is suggested that this is due to the destabilization of the nanowires near the domain boundaries. This is corroborated by the significant decrease of the transition temperature observed on a substrate with a high step density.