Origin of superconductivity in disordered tungsten thin films
Vivas Bagwe, Rishabh Duhan, Bhagyashree A. Chalke, Jayesh B. Parmar, Somak Basistha, Pratap Raychaudhuri
Abstract
The most common allotrope of tungsten, $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{W}$, has a superconducting transition at a temperature of $\ensuremath{\sim}11\phantom{\rule{0.16em}{0ex}}\mathrm{mK}$. However, two other forms of tungsten have been reported to have superconducting transitions in the temperature range ${T}_{c}\ensuremath{\sim}2\ensuremath{-}5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ when synthesized as thin films: crystalline $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{W}$ and amorphous W ($a\text{\ensuremath{-}}\mathrm{W}$). 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 $a\text{\ensuremath{-}}\mathrm{W}$ is indeed a conventional superconductor, $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{W}$ is not a superconductor down to 2.3 K. Superconductivity with ${T}_{c}>3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in the putative $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{W}$ films probably originates from an amorphous phase that forms underneath the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{W}$ phase. Our findings reconcile some of the anomalies earlier reported in $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{W}$, such as the very small superconducting gap and the decrease of ${T}_{c}$ with increase in film thickness.