Transition from a normal to a superconducting state observed on heating in an amorphous bismuth--hydrogen system

We report a study of the electrical and superconducting properties of bismuth films obtained by condensation in a hydrogen atmosphere (at pressure P/sub H/2 in the range 5 x 10-6 to 2 x 10-5 Torr) onto a substrate cooled by liquid helium. The dissolved hydrogen leads to a significant increase in the resistivity, a lowering of T/sub c/, and an increase in the temperature of the transformation from the amorphous phase to the crystal line. If P/sub H/2 exceeds some critical value, then the films obtained take the form of a strongly hydrogen saturated metastable phase a-Bi (H). This phase transforms around 5 0K to the usual amorphous bismuth phase, which is also metastable. At this transition, the resistivity falls sharply (by two orders of magnitude) and the value of T/sub c/ is elevated (the change approaches 2 0K). In many cases this process appears as a sharp transition on heating from a resistive to a superconducting state