Isotope tuning of the superconducting dome of strontium titanate

Abstract

Doped strontium titanate SrTiO3 (STO) is one of the most dilute superconductors known today. The fact that superconductivity occurs at very low carrier concentrations is one of the two reasons that the pairing mechanism is not yet understood, the other being the role played by the proximity to a ferroelectric instability. In undoped STO, ferroelectric order can in fact be stabilized by substituting O16 with its heavier isotope O18. Here, we explore the superconducting properties of doped and isotope-substituted SrTi(18Oy16O1−y)3−δ for 0≤y≤0.81 and carrier concentrations between 6×1017 and 2×1020cm−3 (δ<0.02). We show that the superconducting Tc increases when the O18 concentration is increased. For carrier concentrations around 5×1019cm−3 this Tc increase amounts to almost a factor 3, with Tc as high as 580 mK for y=0.74. When approaching SrTiO318 the maximum Tc occurs at much smaller carrier densities than for pure SrTiO316. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode.