Josephson junction array type I-V characteristics of quench-condensed ultra thin films of Bi

In this communication we report studies of dc current-voltage (I-V) characteristics of ultra thin films of Bi, quench condensed on single crystal sapphire substrates at T = 15 K. The hysteretic I-V characteristics are explained using a resistively and capacitively shunted junction (RCSJ) model of Josephson junction arrays. The Josephson coupling energy (E_J) and the charging energy (E_c) are calculated for different thickness (d) values. A low resistance state is found in the low current regime below the critical current, I_c. This resistance R₀ is found to have a minimum at a particular thickness (d_c) value. Reflection high energy electron diffraction (RHEED) studies are done on these films. A distinct appearance of a diffuse ring near d_c is observed in the diffraction images, consistent with the recent STM studies. These films show an irreversible annealing when temperature is increased. The annealing temperature (T_a) also has a maximum at the same thickness. Although the R_s vs T of quench-condensed Bi films suggest that the films are uniform, our results indicate that even in thick films, the order parameter is not fully developed over the complete area of the film. These results are discussed qualitatively.