The temperature-quantum tuning parameter phase diagram of SrTiO 3 and related materials has recently been discussed in terms of a phenomenological model involving the interaction of the local ferroelectric order-parameter field with itself, and with the strain field of the lattice 3,S1–S13 (see Fig. Strontium titanate is an incipient ferroelectric insulator, which can be tuned essentially continuously into the ferroelectric phase via a ‘quantum’ tuning parameter, such as chemical substitution, isotopic substitution or applied stress (see, e.g., refs. We present a quantitative model involving the dynamical screening of the Coulomb interaction and show that an enhancement of T c near to a ferroelectric quantum critical point can arise due to the virtual exchange of longitudinal hybrid-polar-modes, even in the absence of a strong coupling to the transverse critical modes. However, in contrast to the latter, the coupling of the carriers to the critical modes in ferroelectrics is predicted to be small.
We find that T c increases dramatically when the energy gap of the ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached in a way reminiscent to behaviour observed in magnetic counterparts.
We report experimental results showing the pressure dependence of the superconducting transition temperature, T c, near to optimal doping that sheds light on the nature of this interaction. The occurrence of superconductivity in doped SrTiO 3 at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades.
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