A local maximum in the superconducting transition temperature of Nb-doped strontium titanate under uniaxial compressive stress

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.