Enhancement of the superconducting T_c upon isotopic substitution in Strontium Titanate

SrTiO₃, a quantum paraelectric, becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms. It turns into a ferroelectric upon isotopic ¹⁸O substitution exceeding a quantum critical point (QCP) located at a substitution level of around 33 at% ¹⁸O. The exceptionally dilute superconducting and the ferroelectric order may be accidental neighbors or intimately connected, as in the picture of quantum critical ferroelectricity.
We developed an ¹⁸O substitution process to control substitution levels in SrTi(¹⁶O_1-x¹⁸O_x)₃ beyond the critical point. Dielectric measurements evidence a ferroelectric order below the Curie temperature. In metallic SrTi(¹⁶O_1-x¹⁸O_x)_3-δ with ¹⁸O-substitution levels above the QCP, we find that the superconducting critical temperature seen in resistivity measurements is enhanced by more than a factor two and reaches values of up to 700 mK. This enhancement strongly supports the role played by the vicinity to the ferroelectric order in the precocious emergence of superconductivity, restricting possible theoretical scenarios for pairing.