Isotope tuning and the pairing mechanism of superconducting strontium titanate*

Pure SrTiO3 is a para-electric insulator, in which ferro-electric order can be stabilized by substituting the (natural) 16O atoms with the heavier isotope 18O. Weak doping with electrons renders SrTiO3 superconducting. We have determined the influence of 18O substitution on the superconducting critical temperature and observed that the effect of substituting 16O with the heavier 18O isotope is that Tc increases - opposite to the BCS prediction. We furthermore observe a downward shift of the optimal carrier density. The unusually large size of the observed isotope effect supports a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft phonon, possibly by pairs of these phonons.