Isotope tuning of the superconducting dome of strontium titanate

Abstract

Doped strontium titanate SrTiO_{3} (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 ^{16}O with its heavier isotope ^{18}O. Here, we explore the superconducting properties of doped and isotope-substituted SrTi(^{18}O_{y}^{16}O_{1−y})_{3−δ} for 0≤y≤0.81 and carrier concentrations between 6×10^{17} and 2×10^{20}cm^{−3} (δ<0.02). We show that the superconducting T_{c} increases when the ^{18}O concentration is increased. For carrier concentrations around 5×10^{19}cm^{−3} this T_{c} increase amounts to almost a factor 3, with T_{c} as high as 580 mK for y=0.74. When approaching SrTi^{18}O_{3} the maximum T_{c} occurs at much smaller carrier densities than for pure SrTi^{16}O_{3}. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode.