Superconductivity mediated by polar modes in doped ferroelectrics

The occurrence of superconductivity in doped SrTiO₃ and related materials at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades.  We present experimental results showing the pressure dependence of the superconducting transition temperature, T_c, that sheds light on the nature of this interaction.  We find that T_c increases dramatically when the energy gap of ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached, in a way reminiscent of behaviour observed in magnetic counterparts.  However, in contrast to the latter, the coupling of itinerant electrons to the critical modes in ferroelectrics is predicted to be small.  We present a superconductivity model that shows quantitative agreement with experiment without the use of adjustable parameters which involves electron pairing via the virtual exchange of longitudinal hybrid-polar-modes.  The model accounts for the domes of superconductivity observed as function of carrier density and pressure and the enhancement of T_c close to quantum criticality even in the absence of a direct coupling to the transverse critical modes.