Polar Order and Superconductivity in SrTiO₃

Currently, there exist many different theoretical proposals that link ferroelectricity and superconductivity in SrTiO₃.  This talk will focus on our experiments on strained SrTiO₃ films, which can undergo successive ferroelectric and superconducting transitions, resulting in a factor of two enhancement of the superconducting transition temperature.  The ferroelectric transition is an order-disorder transition and local polar order persists to temperatures far above the ferroelectric transition and exist even in unstrained SrTiO₃.  We show that these local polar regions, along with superconductivity, are suppressed in the over-doped regime.  We show that local polar order is key to the enhanced superconducting transition temperature.  We also discuss recent insights into the nature of the superconducting state, including its surprising robustness against alloying with large concentrations of magnetic elements.