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The nature of superconductivity in the dilute semiconductor SrTiO₃ has remained an open question for over half a century. Thin film heterostructures provide new opportunities to examine SrTiO₃ superconductivity using a newly developed method for engineering band alignments at oxide interfaces to access the electronic structure of Nb-doped SrTiO₃ using high-resolution tunneling spectroscopy. While we observe strong coupling to the highest-energy longitudinal optic phonon consistent with λ ~ 1, the superconducting gap is found to be in the weak-coupling limit of BCS theory, i.e. λ_BCS~ 0.1. This discrepancy arises in the context of an unusual anti-adiabatic condition for superconductivity in SrTiO₃ (Fermi energy < Debye energy), which we find precisely bounds the superconducting dome.

This work was done in collaboration with Hyeok Yoon, Adrian Swartz, Hisashi Inoue, Yasuyuki Hikita, and Sri Raghu.