Search for superconductivity in surface-doped WO$_{3}$ films

We report the search for superconductivity in surface-doped WO$_{3}$ films. Possible evidence for high temperature superconductivity in this system has been reported in the literature. In our work, WO$_{3}$ films were grown by MBE and characterized by \textit{in-situ} XPS, UPS, and \textit{ex-situ} XRD and resistivity measurements. For some films, Na was deposited on the surface at room temperature, and the resultant Na+WO$_{3-x}$ films annealed in vacuum ($\sim $ 10$^{-8}$ Torr) for 1h at various temperatures (300 -- 800C). With increasing thickness of Na, the intensity of the Na1$s$ peak in XPS spectra and area of the W5$d$ state in UPS spectra increased, suggesting some charge transfer to the WO$_{3}$ film. After annealing below 500C, XPS and UPS spectra did not change, while after annealing above 500C, the area of the W5$d$ state drastically increased. The same behavior was observed for pure WO$_{3-x}$ films, however, suggesting that oxygen vacancies are created during the annealing. The films started to decompose above 700C. The resistivity of our films decreased with increasing annealing temperature. However, so far we have not observed any sign of superconductivity down to 2 K in any of our films. This work is supported by Air Force Office of Scientific Research.