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

Abstract

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 T0 and T1, respectively. The obervations can be equally explained by assuming either a decrease in T1 or an increase in T0 upon isotope replacement. The conventional view holds a direct connection between quantum fluctuations and the T1 and hence adopts the decreasing T1 picture. In this paper, we offer a different opinion, in which the T1 bears a different meaning and quantum fluctuations are attached to another quantity to be denoted ω0. We show that a decrease in ω0 could diminish quantum fluctuations and simultaneously enhance T0. A vibronic mechanism is presented as a possible route to the ω0 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.