Ferroelectric quantum phase transition inside the superconducting dome of Ca doped SrTiO_3-x

Ferroelectricity and superconductivity are mutually exclusive. Ferroelectricity implies the alignment of electric dipoles in an insulator, and while superconductivity requires pairing of mobile electrons in a metal. However, recently a number of 'ferroelectric metals' have emerged during the past few years (See for example Nature Materials 12, 1024 (2013)). By employing a variety of experimental probes (electric conductivity, electric permittivity, Raman spectroscopy, thermal expansion and sound velocity), we find that calcium doped oxygen-reduced SrTiO₃, is the first solid in which ferroelectric-like and superconducting phase transitions occur one after each order. A variety of superconducting domes have been found in the vicinity of a competing order during the past three decades. In this context, we find evidence for interplay between superconductivity in strontium titanate and a neighboring ferroelectric order. When the ferroelectric order is destroyed by doping, the superconducting critical temperature enhances. This observation may be the first experimental signature of a role played by ferroelectric quantum criticality in the formation of Cooper pairs.