Interplay between ferroelectricity and superconductivity

Abstract

This project studied the interplay between ferroelectricity, metallicity, superconductivity, and heat transport in quantum-paraelectric perovskite oxides, Sr₁₋ₓCaₓTiO₃₋d and EuTiO₃₋d. Motivated by the proximity of competing ground states in SrTiO₃, the work aimed to clarify whether ferroelectric order can coexist with itinerant charge carriers and superconductivity. In Sr₁₋ₓCaₓTiO₃₋d (x = 0.009), thermodynamic, transport, and spectroscopic measure-ments demonstrate that ferroelectric-like lattice instabilities persist deep into the metallic re-gime and coexist with superconductivity (Tc < 0.3 K). High-resolution thermal expansion data show robust ferroelectric-like anomalies up to carrier densities ~10¹⁹ cm⁻³, providing clear evidence for polar metallic behavior rather than a simple competition between ferroelectricity and metallicity. Broadband microwave and optical spectroscopy reveal non-Drude charge transport at low frequencies, consistent with partial carrier localization and the formation of large charge puddles promoted by the exceptionally high dielectric constant of SrTiO₃. Com-plementary studies on EuTiO₃₋d establish a somewhat different behavior. While oxygen re-duction induces metallicity and a T² low-temperature resistivity, the critical carrier density for metallic conduction is significantly higher than in SrTiO₃. This difference is traced to stronger, antiferroelectric-type quantum fluctuations in EuTiO₃, which limit the dielectric constant and effective Bohr radius. Scaling analyses across SrTiO₃, EuTiO₃, and related oxides reveal a universal Mott-like criterion for the metal–insulator transition and a systematic decrease of the T² resistivity prefactor with increasing carrier density. The project uncovered glass-like ther-mal conductivity in EuTiO₃ despite being crystalline, which is attributed to phonon scattering by randomly oriented Eu²⁺ magnetic moments. Furthermore, a sizable thermal Hall effect was observed in EuTiO₃, with distinct signatures in the paramagnetic, antiferromagnetic, and field-polarized phases, pointing to phonon- and magnon-related mechanisms. Overall, the project has established highly dilute polar metals as an informative platform for studying unconventional charge and heat transport behavior. Moreover, it was possible to verify experimentally that quantum paraelectricity and polar metallicity at least partially coex-ist in samples, which at even lower temperatures still exhibit superconductivity