Superconductivity Near a Polar Instability in Incipient Ferroelectrics

A combination of broken inversion symmetry and spin-orbit coupling gives rise to a wide range of exotic superconducting states, such as mixed-parity superconductivity, superconducting Weyl state, and superconducting diode effect. Incipient ferroelectrics e.g., SrTiO₃, KTaO₃, EuTiO₃, and PbTe are near a polar instability. The emergence of superconductivity has been reported in some of these incipient ferroelectrics upon doping. Many unconventional superconductors, such as the cuprates, pnictides, and heavy fermion systems, occur in close proximity to magnetic fluctuations or magnetic orders, suggesting that these are important ingredients in the superconductivity in these materials. Here, ferroelectricity and superconductivity could be connected or accidental neighbors. Furthermore, the intersection of superconductivity and topologically nontrivial states is a fertile landscape for exciting quantum phenomena, including non-abelian excitations. Topologically nontrivial states have been reported in some of these polar superconductors. First, I will focus on the emergence of superconductivity and its interplay with ferroelectricity in tuned SrTiO₃ thin films. I will discuss the ferroelectric enhancement of superconductivity in epitaxially strained SrTiO₃ films. The ferroelectric films reveal a doubling in the critical temperature of superconductivity. We show that suppression of ferroelectricity in over-doped samples also suppresses superconductivity. Next, I will present our recent results on the emergence of 2D superconductivity in KTaO₃(111) interfaces. Signatures of weak anti-localization are observed in both superconducting, KTaO₃(111), and non-superconducting, KTaO₃(100), interfaces. The observed weak anti-localization correction, however, is stronger in superconducting interfaces.<br/>A.H. Al-Tawhid, J. Kanter, M. Hatefipour, D.P. Kumah, J. Shabani, K. Ahadi, ArXiv:2109.09786 (2021).<br/>S Salmani-Rezaie, K Ahadi, S Stemmer Nano Letters 20 (9), 6542-6547 (2020).<br/>K Ahadi, L Galletti, Y Li, S Salmani-Rezaie, W Wu, S Stemmer, Science advances 5 (4), eaaw0120 (2019).