Polaronic behavior in a weak-coupling superconductor

Significance The perovskite oxide SrTiO3 is an n-type semiconductor, which exhibits superconductivity at low temperatures even though there are extraordinarily few electrons. In most materials, with so few electrons it becomes difficult to pair them into a macroscopic superconducting ground state. This regime is also unusual in that most theories of superconductivity rely on electrons that are much faster than their coupling (typically lattice vibrations or phonons). By using techniques in which tunneling junctions are engineered with atomic recision, we are able to probe the electronic structure of this material. The experiments indicate a large discrepancy between the electron–phonon coupling strength and the superconducting properties. This places SrTiO3, a host material for a variety of exotic quantum phases (i.e., in FeSe/SrTiO3, LaAlO3/SrTiO3), in a unique regime of superconductivity. The nature of superconductivity in the dilute semiconductor SrTiO3 has remained an open question for more than 50 y. The extremely low carrier densities (1018–1020 cm−3) at which superconductivity occurs suggest an unconventional origin of superconductivity outside of the adiabatic limit on which the Bardeen–Cooper–Schrieffer (BCS) and Migdal–Eliashberg (ME) theories are based. We take advantage of a newly developed method for engineering band alignments at oxide interfaces and access the electronic structure of Nb-doped SrTiO3, using high-resolution tunneling spectroscopy. We observe strong coupling to the highest-energy longitudinal optic (LO) phonon branch and estimate the doping evolution of the dimensionless electron–phonon interaction strength (𝝀). Upon cooling below the superconducting transition temperature (𝑻𝐜), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling (𝝀𝐁𝐂𝐒≈0.1). These results suggest that despite the strong normal state interaction with electrons, the highest LO phonon does not provide a dominant contribution to pairing. They further demonstrate that SrTiO3 is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron–phonon coupling strength.