Superconductivity in Eu_xSr_1-xTiO₃ films grown by molecular beam epitaxy

Electron-doped SrTiO₃ is a dilute superconductor which violates standard BCS theory and for which the origin of superconductivity remains unsolved. Numerous studies have suggested a correlation between incipient ferroelectricity and superconductivity. Strained films of SrTiO₃ undergo a ferroelectric transition. Their solid solutions with EuTiO₃, which exhibits multiferroicity under strain, are interesting because there are multiple quantum critical points. Here, we report on the evolution of superconducting, structural, electronic, and magnetic properties in a series of compressively strained Eu_xSr_1-xTiO₃ films grown by molecular beam epitaxy in which the Eu concentration (x) is varied. We demonstrate that superconductivity survives to Eu concentrations greater than 10% and we examine how its subsequent destruction as x is increased correlates with other properties, such as ferroelectricity. We discuss the implications for the superconducting mechanism in this system.