  Transparent conductive indium tin oxide (ITO) thin films, electrochemically
intercalated with sodium or other cations, show tunable superconducting
transitions with a maximum $T_c$ at 5 K. The transition temperature and the
density of states, $D(E_F)$ (extracted from the measured Pauli susceptibility
$\chi_p$ exhibit the same dome shaped behavior as a function of electron
density. Optimally intercalated samples have an upper critical field $\approx
4$ T and $\Delta/{k_BT_c} \approx 2.0$. Accompanying the development of
superconductivity, the films show a reversible electrochromic change from
transparent to colored and are partially transparent (orange) at the peak of
the superconducting dome. This reversible intercalation of alkali and alkali
earth ions into thin ITO films opens diverse opportunities for tunable,
optically transparent superconductors.
