Polaronic behavior in a weak-coupling superconductor

Adrian G. Swartz, Hisashi Inoue, Tyler A. Merz, Yasuyuki Hikita, Srinivas Raghu, Thomas P. Devereaux, Steven Johnston, Harold Y. Hwang

Research output: Contribution to journalArticlepeer-review

29 Citations (Scopus)

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 cm3) 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.

Original languageEnglish
Pages (from-to)1475-1480
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number7
DOIs
Publication statusPublished - 2018 Feb 13

Keywords

  • Electronic structure
  • Oxide interface
  • Polaron
  • Superconductivity

ASJC Scopus subject areas

  • General

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