Abstract
We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of Tc. Also the magnetic critical field Hc2 is increased by a factor ~2. Such a strong impact on Tc and Hc2, 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 SrTiO3.
Highlights
We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O
SrTiO3 is a para-electric insulator which becomes ferroelectric when 35% or more of the oxygen is substituted with the isotope 18O1–3
Due to electron-phonon coupling doped charge carriers form a polaronic liquid at small concentration[4,5,6,7,8,9,10,11,12,13] and the material becomes superconducting with a doping dependent critical temperature (Tc) below ~1 Kelvin both for bulk[14,15,16,17,18,19] and interfaces[20,21]
Summary
These measurements confirmed that oxygen has been substituted throughout the bulk of the material, and indicated that the amount of 18O in the (ceramic and crystalline) samples is about 35%. The red-shift of the phonon mode is found to be of the same magnitude in ceramic samples as in crystals, the former having being processed by oxidation of metal elements in pure 18O2, and confirms well the amount of substitution measured by the mass spectrometry and thermogravimetry. The crucial observation is that, after the back-substitution, the superconducting transition temperature of the back-substituted samples has become nearly the same This allows us to conclude that the Tc in SrTiO3 is tuned by the mass of oxygen isotope. Due to the anti-adiabatic conditions, the expression for Tc in the relevant doping range reads[9]
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