Enhanced superconductivity close to a non-magnetic quantum critical point in electron-doped strontium titanate

Abstract

Studies on quantum critical points (QCP) have focused on magnetic QCPs to date. Remarkable phenomena such as superconductivity due to avoided criticality have been discovered, but we focus here on the non-magnetic counterpart, i.e., the superconductivity of SrTiO ₃ regarded as being close to a ferroelectric QCP. Here we prepare high-quality Sr _{1− x} La _x Ti( ¹⁶O _{1− z} ¹⁸O _z ) ₃ single crystals without localisation at low temperatures, which allow us to systematically investigate the La substitution of Sr as an alternative to introducing oxygen vacancies. Analysis of our data based on a theoretical model predicts an appearance of the ferroelectric QCP around 3 × 10 ¹⁸ cm ⁻³. Because of the QCP, the superconducting dome of Sr _{1− x} La _x TiO ₃ can be raised upwards. Furthermore, remarkable enhancement of T _c (~0.6 K) is achieved by ¹⁸O exchange on the Sr _{1− x} La _x TiO ₃ crystals. These findings provide a new knob for observing intriguing physics around the ferroelectric QCP.

Abstract

Among its interesting properties, SrTiO ₃ can show both superconductivity and ferroelectric quantum criticality at low temperatures. Tomioka et al. use La and oxygen-isotope doping to tune electron-doped SrTiO ₃ to the critical region and observe enhanced superconductivity, suggesting a link between them.