SrTiO3 is an incipient ferroelectric on the verge of a polar instability, which is avoided at low temperatures by quantum fluctuations. Within this unusual quantum paraelectric phase, superconductivity persists despite extremely dilute carrier densities. Ferroelectric fluctuations have been suspected to play a role in the origin of superconductivity by contributing to electron pairing. To investigate this possibility, we used optical second harmonic generation to measure the doping and temperature dependence of the ferroelectric order parameter in compressively strained SrTiO3 thin films. At low temperatures, we uncover a spontaneous out-of-plane ferroelectric polarization with an onset that correlates perfectly with normal-state electrical resistivity anomalies. These anomalies have previously been associated with an enhancement of the superconducting critical temperature in doped SrTiO3 films, directly linking the ferroelectric and superconducting phases. We develop a long-range mean-field Ising model of the ferroelectric phase transition to interpret the data and extract the relevant energy scales in the system. Our results support a long-suspected connection between ferroelectricity and superconductivity in SrTiO3, but call into question the role played by ferroelectric fluctuations.

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Doped SrTiO 3 is a superconductor whose pairing mechanism is still not fully understood. The response of a superconductor to impurities has long been used to obtain insights into the nature of the superconducting state. Here, the superconductivity of SrTiO 3 films that are doped or alloyed with different rare earth ions, which carry a magnetic moment, is investigated. It is shown that large concentrations (up to a few percent) of rare earth ions with unpaired f-electrons, such as Sm and Eu, do not reduce the superconducting critical temperature and critical fields. The finding is independent of whether the rare earth ion acts as a dopant or is an isovalent impurity. The interactions between the superconducting condensate and the magnetic dopants that could result in the observed insensitivity to magnetic impurities are discussed.

The possible connection between superconductivity and polar order in SrTiO3 has been discussed extensively in the recent literature. Here, the thickness dependence of the superconducting and ferroelectric transitions in strained, epitaxial films of SrTiO3 films is studied. Both superconductivity and ferroelectricity are absent in thinner films (25 nm and below). We discuss the possible origins of the thickness dependence of both phenomena.

The superconducting transition of SrTiO3 can be influenced by tuning its ferroelectric transition, but the underlying reasons remain poorly understood. Here, we investigate compressively strained, Sm-doped films of EuxSr1-xTiO3 that were grown by molecular beam epitaxy to determine the effect of alloying with Eu on both superconductivity and ferroelectricity, both of which are present in strained SrTiO3 films. Remarkably, superconductivity survives up to x = 0.14. Films at the lowest alloy concentration studied here, x = 0.09, exhibit no suppression of their superconducting transition temperature, but a strong reduction of the upper critical field (Hc2), compared to non-alloyed, strained SrTiO3 films. In addition, these films lack the sharp ferroelectric transition that appears in films without Eu in second harmonic generation measurements. We postulate that Eu-alloying causes a crossover from a globally ordered ferroelectric state to one with only short-range polar order. We discuss the connection between the loss of global polar order and the change in the superconducting properties.