Characterization of Superconductivity in Compressively Strained SrTiO3 Thin Films

Although superconductivity in strontium titanate (SrTiO3) was discovered in the 1960s, the origin of superconductivity in SrTiO3 has not been completely understood. Moreover, its unusual superconducting characteristics have made SrTiO3 a fascinating playground to potentially study multiple aspects of unconventional superconductivity. For example, SrTiO3 is one of the most dilute superconductors where standard Bardeen-Cooper-Schrieffer (BCS theory) is believed to be no longer valid. Furthermore, the presence of superconductivity and ferroelectricity and their interplay possibly leads to an unconventional pairing mechanism in SrTiO3. In this dissertation, high-quality, doped, ferroelectric, compressively strained SrTiO3 thin films were grown using hybrid molecular beam epitaxy (MBE) to unveil unique superconducting properties. First, we present unconventional characteristics of relatively thick (160-180 nm) strained SrTiO3 films. In some strained films, we show signatures of unusual superconducting states such as in-plane critical fields far above the Pauli limiting field and second harmonic resistances. The study of superconductivity in ultra-thin SrTiO3 films is hindered by strong surface carrier depletion, which makes thin films insulating. We solve this issue by introducing thin (~10 nm) EuTiO3 capping layers. We show that the EuTiO3 capping layer effectively prevents strong carrier depletion in SrTiO3 films and that a capped 40 nm-thick SrTiO3 film becomes superconducting. We utilize the EuTiO3 capping layer to elucidate the connection between superconductivity and ferroelectricity in SrTiO3 films. Systematic film thickness study shows that superconductivity and ferroelectricity are suppressed simultaneously in the SrTiO3 films of thicknesses less than 40 nm. We speculate that this result implies that broken inversion symmetry (spin-orbit coupling) plays an important role in the superconductivity of strained SrTiO3 films