Abstract

Abstract Substitution of Nb5+ for Ti4+ in SrTiO3 (denoted as Nb:SrTiO3) is an excellentelectron doping method with reduceddisorder, superior mobility,and higher Tc, comparedwith the electrondoping by creatingthe oxygen defectsdenoted as SrTiO3-δ. We applied the Nb doping to the two differentferroelectric materials, Sr0.95Ba0.05TiO3 and Sr0.985Ca0.015TiO3, confirmed, by the xray structural analyses, they turned into polar metals with broken centrosymmetry. We can define a particular carrier density n for the polar metals. When the carrier density n < n, the resistance shows an upturn below the temperature TK. The values of TK increase monotonically as the carrier density decreases. Interestingly, they coincide with the ferroelectric Curie temperature at the zero carrier density limit. Although the values of n for Nb:Sr0.985Ca0.015TiO3 and Nb:Sr0.95Ba0.05TiO3 are different by one order of magnitude, both materials show typical superconducting domes with single peaks commonly located around 1020 cm−3. Compared with the nonpolar matrix Nb:SrTiO3, the values of Tc are enhanced and reach 0.75 K for Nb:Sr0.95Ba0.05TiO3. However, the Tc enhancement was insignificant (less than 0.1 K) at around n. The enhancement becomes much more prominent further inside the dilute carrierdensity region, where the screening is less effective. These results suggest that centrosymmetry breaking, i.e., the ferroelectric nature, does not kill the superconductivity. Instead, it enhances the superconductivity directly, despite the absence of strong quantum fluctuations. Our results call for a reconsideration of the existing microscopic models of superconductivity in SrTiO3. *This study was supported by JSPS KAKENHI (19H01844 and 18H03686) and was partially supported by the JST CREST (JPMJCR19K2).