Origin of superconductivity in disordered tungsten thin films

The most common allotrope of tungsten, α-W, has a superconducting transition at a temperature of ~110.16em0exmK. However, two other forms of tungsten have been reported to have superconducting transitions in the temperature range Tc~2-50.16em0exK when synthesized as thin films: crystalline β-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, β-W is not a superconductor down to 2.3 K. Superconductivity with Tc>30.16em0exK in the putative β-W films probably originates from an amorphous phase that forms underneath the β-W phase. Our findings reconcile some of the anomalies earlier reported in β-W, such as the very small superconducting gap and the decrease of Tc with increase in film thickness.