  Superconducting strontium titanate (STO) is in the spotlight as a low carrier
concentration semiconductor in proximity to polar order. Its superconducting
pairing mechanism poses an open fundamental challenge, which may be resolved by
controlling how strongly the superconducting and polar orders interact. Here,
we show for the first time that non-monotonic superconducting transition
temperature behavior in uniaxially compressed STO can occur, revealing a
reduction followed by a peak in the superconducting transition temperature. The
corresponding displacements in the lattice are extremely small, in the sub
picometer range, indicating that this material approaches a singularity in
electron-phonon coupling, consistent with STO transforming to the polar
(ferroelectric) phase that influences the superconductivity.
