Electrical transport studies of quench condensed Bi films at the initial stage of film growth: A structural transition and the possible formation of electron droplets

The electrical transport properties of amorphous Bi films prepared by sequential quench deposition have been studied in situ. A superconductor-insulator transition was observed as the film was made increasingly thicker, consistent with previous studies. Unexpected behavior was found at the initial stage of film growth, a regime not explored in detail prior to the present work. As the temperature was lowered, a positive temperature coefficient of resistance (formula presented) emerged, with the resistance reaching a minimum before the (formula presented) became negative again. This behavior was accompanied by a nonlinear and asymmetric current-voltage characteristic. As the film became thicker, conventional variable-range hopping was recovered. We attribute the observed crossover in the electrical transport properties to an amorphous to granular structural transition. The positive (formula presented) found in the amorphous phase of Bi formed at the initial stage of film growth is qualitatively explained by the formation of metallic droplets within the electron glass.