A ferroelectric quantum phase transition inside a superconducting dome*

The quantum paraelectric SrTiO₃ becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms [1]. Furthermore, it turns into a ferroelectric upon substitution of a tiny fraction of strontium atoms with calcium [2]. These two orders seem to be more intimately connected than intuitively anticipated at first glance. We show (by means of magnetic, transport, thermal expansion, ultrasound, dielectric, and spectroscopic measurements) that in Sr_1-xCa_xTiO_3-δ (0.002 < x < 0.009, δ < 0.001) the "ferroelectric" order, in the sense of a screened but longrange polar lattice distortion coexists with dilute metallicity and its superconducting instability in a finite window of doping. At a critical carrier density, which scales with the Ca content, a quantum phase transition destroys the ferroelectric order. An upturn in the normal-state scattering and a significant modification of the superconducting dome can be detected in the vicinity of this quantum phase transition. The enhancement of the superconducting transition temperature with calcium substitution documents the role played by "ferroelectric" polar lattice distortions for the precocious emergence of superconductivity in this system, restricting possible theoretical scenarios for pairing.

[1] J.F. Schooley, W.R. Hosler, and M.L. Cohen, Phys. Rev. Lett. 12, 474 (1964)
[2] J.G. Bednorz and K.A. Müller, Phys. Rev. Lett. 52, 2289 (1984)
[3] C.W. Rischau et al., Nature Physics 13, 643 (2017), doi:10.1038/nphys4085

E-mail for contributing author: hemberger@ph2.uni-koeln.de