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 PH2 in the range 5 · 10−6 to 2 · 10−5 Torr) onto a substrate cooled by liquid helium. The dissolved hydrogen leads to a significant increase in the resistivity, a lowering of Tc, and an increase in the temperature of the transformation from the amorphous phase to the crystal line. If PH2 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 °K 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 Tc is elevated (the change approaches 2 °K). In many cases this process appears as a sharp transition on heating from a resistive to a superconducting state.

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