SrTiO3 exhibits superconductivity for carrier densities 1019−1021 cm−3. Across this range, the Fermi level traverses a number of vibrational modes in the system, making it ideal for studying dilute superconductivity. We use high-resolution planar-tunneling spectroscopy to probe chemically-doped SrTiO3 across the superconducting dome. The over-doped superconducting boundary aligns, with surprising precision, to the Fermi energy crossing the Debye energy. Superconductivity emerges with decreasing density, maintaining throughout the Bardeen-Cooper-Schrieffer (BCS) gap to transition-temperature ratio, despite being in the anti-adiabatic regime. At lowest superconducting densities, the lone remaining adiabatic phonon van Hove singularity is the soft transverse-optic mode, associated with the ferroelectric instability. We suggest a scenario for pairing mediated by this mode in the presence of spin-orbit coupling, which naturally accounts for the superconducting dome and BCS ratio.
@article{arxiv.2106.10802,
title = {Low-density superconductivity in SrTiO$_3$ bounded by the adiabatic criterion},
author = {Hyeok Yoon and Adrian G. Swartz and Shannon P. Harvey and Hisashi Inoue and Yasuyuki Hikita and Yue Yu and Suk Bum Chung and Srinivas Raghu and Harold Y. Hwang},
journal= {arXiv preprint arXiv:2106.10802},
year = {2021}
}