@article{80a08b321e3b4742aba71bfa959c44b6,
title = "Polaronic behavior in a weak-coupling superconductor",
abstract = "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.",
keywords = "Electronic structure, Oxide interface, Polaron, Superconductivity",
author = "Swartz, {Adrian G.} and Hisashi Inoue and Merz, {Tyler A.} and Yasuyuki Hikita and Srinivas Raghu and Devereaux, {Thomas P.} and Steven Johnston and Hwang, {Harold Y.}",
note = "Funding Information: ACKNOWLEDGMENTS. We acknowledge S. A. Kivelson, P. A. Lee, P. B. Littlewood, D. J. Scalapino, A. V. Maharaj, A. Edelman, A. J. Millis, L. Rademaker, Z.-X. Shen, and Y. Wang for useful discussions. This work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract DE-AC02-76SF00515; and by the Gordon and Betty Moore Foundation{\textquoteright}s Emergent Phenomena in Quantum Systems Initiative through Grant GBMF4415 (dilution refrigerator experiments). S.J. acknowledges support from the University of Tennessee{\textquoteright}s Science Alliance Joint Directed Research and Development program, a collaboration with Oak Ridge National Laboratory. Funding Information: We acknowledge S. A. Kivelson, P. A. Lee, P. B. Littlewood, D. J. Scalapino, A. V. Maharaj, A. Edelman, A. J. Millis, L. Rademaker, Z.-X. Shen, and Y. Wang for useful discussions. This work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Contract DE-AC02-76SF00515; and by the Gordon and Betty Moore Foundation{\textquoteright}s Emergent Phenomena in Quantum Systems Initiative through Grant GBMF4415 (dilution refrigerator experiments). S.J. acknowledges support from the University of Tennessee{\textquoteright}s Science Alliance Joint Directed Research and Development program, a collaboration with Oak Ridge National Laboratory. Publisher Copyright: {\textcopyright} 2018 National Academy of Sciences. All Rights Reserved.",
year = "2018",
month = feb,
day = "13",
doi = "10.1073/pnas.1713916115",
language = "English",
volume = "115",
pages = "1475--1480",
journal = "Proceedings of the National Academy of Sciences of the United States of America",
issn = "0027-8424",
publisher = "National Academy of Sciences",
number = "7",
}