On the origin of oxygen isotope exchange induced ferroelectricity in strontium titanate

The quantum paraelectric strontium titanate can be made ferroelectric through replacing oxygen atoms with their heavy isotopes. Although suppressed quantum fluctuations have been widely believed to be the origin, the details remain unsettled. The controveries are often framed using Barrett formula, which involves two quantities termed T 0 and T 1, respectively. The obervations can be equally explained by assuming either a decrease in T 1 or an increase in T 0 upon isotope replacement. The conventional view holds a direct connection between quantum fluctuations and the T 1 and hence adopts the decreasing T 1 picture. In this paper, we offer a different opinion, in which the T 1 bears a different meaning and quantum fluctuations are attached to another quantity to be denoted ω 0. We show that a decrease in ω 0 could diminish quantum fluctuations and simultaneously enhance T 0. A vibronic mechanism is presented as a possible route to the ω 0 change. The isotope effects are argued to be rather non-local. The dynamics of the system could be highly quasi-one-dimensional and this is then employed in discussing the relation between a soft mode and a recently observed central mode.