Threshold for superconductivity in ultrathin amorphous gallium films

Systematic studies of the onset of superconductivity in ultrathin amorphous Ga films have revealed the existence of a threshold dependent only on the normal-state sheet resistance. Global superconductivity, or zero resistance, develops only when the normal-state sheet resistance falls below 6000 /-. This result agrees with previous observations on crystalline Sn films and further supports the notion of a universal resistance threshold. © 1986 The American Physical Society.