Origin of superconductivity in disordered tungsten thin films

The most common allotrope of tungsten, alpha -W, has a superconducting transition at a temperature of similar to 11 mK. However, two other forms of tungsten have been reported to have superconducting transitions in the temperature range T- c similar to 2 - 5 K when synthesized as thin films: crystalline beta -W and amorphous W ( a -W). In this work we carry out a systematic study of W films synthesized using dc magnetron sputtering, using transport, low frequency magnetic shielding response, and transmission electron microscopy. Our results show that while a -W is indeed a conventional superconductor, beta -W is not a superconductor down to 2.3 K. Superconductivity with T- c > 3 K in the putative beta -W films probably originates from an amorphous phase that forms underneath the beta -W phase. Our findings reconcile some of the anomalies earlier reported in beta -W, such as the very small superconducting gap and the decrease of T- c with increase in film thickness.