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

Yasuhide Tomioka,Naoki Shirakawa,Isao H Inoue,Keisuke Shibuya

doi:10.1038/s41467-019-08693-1

Yasuhide Tomioka, Naoki Shirakawa + Show 2 more

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https://doi.org/10.1038/s41467-019-08693-1

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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 SrTiO3 regarded as being close to a ferroelectric QCP. Here we prepare high-quality Sr1−xLaxTi(16O1−z18Oz)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 × 1018 cm−3. Because of the QCP, the superconducting dome of Sr1−xLaxTiO3 can be raised upwards. Furthermore, remarkable enhancement of Tc (~0.6 K) is achieved by 18O exchange on the Sr1−xLaxTiO3 crystals. These findings provide a new knob for observing intriguing physics around the ferroelectric QCP.

Highlights

Studies on quantum critical points (QCP) have focused on magnetic QCPs to date
It is generally believed that the missing ferroelectricity, even at low temperatures, is entirely due to quantum fluctuations: i.e., zero-point motion preventing the complete softening of the transverse optic phonons[6,8]
Near the QCP, any residual interactions may drive the system to a superconducting state[8,12,13]

Summary

Introduction

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 SrTiO3 regarded as being close to a ferroelectric QCP. For the 18O-exchanged La-substituted SrTiO3 single crystals, the maximum Tc is enhanced to 0.55 K at almost the same n of ~6.0 × 1019 cm−3 These values of n are slightly lower than n ~ 1 × 1020 cm−3, at which SrTiO3−δ exhibits the optimal Tc (δ is the amount of oxygen off-stoichiometry that provides two electrons per δ in a formula unit.)[20,21]. These enhancements of Tc are investigated in this work, and the results suggest a hidden QCP even in a metallic region which may contribute to the Tc enhancement

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