Isotope effect in superconducting n-doped SrTiO(3)

We report the influence on the superconducting critical temperature T(c) in doped SrTiO(3) of the substitution of the natural (16)O atoms by the heavier isotope (18)O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of T(c). Also the magnetic critical field H(c2) is increased by a factor ~2. Such a strong impact on T(c) and H(c2), with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO(3).