Ferroelectricity and superconductivity in strained EuxSr1−xTiO3 films

The superconducting transition of ${\mathrm{SrTiO}}_{3}$ can be influenced by tuning its ferroelectric transition, but the underlying reasons remain poorly understood. Here, we investigate compressively strained, Sm-doped films of ${\mathrm{Eu}}_{x}{\mathrm{Sr}}_{1\text{--}x}{\mathrm{TiO}}_{3}$ 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 ${\mathrm{SrTiO}}_{3}$ 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 there is a strong reduction of the upper critical field $({H}_{c2})$, compared to nonalloyed, strained ${\mathrm{SrTiO}}_{3}$ 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.