Polaronic behavior in a weak-coupling superconductor

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

doi:10.1073/pnas.1713916115

Title: Polaronic behavior in a weak-coupling superconductor

Journal Article · 30 January 2018 · Proceedings of the National Academy of Sciences of the United States of America

DOI: https://doi.org/10.1073/pnas.1713916115 · OSTI ID:1426490

Swartz, Adrian G. ^[1]; Inoue, Hisashi ^[1]; Merz, Tyler A. ^[2]; Hikita, Yasuyuki ^[3]; Raghu, Srinivas ^[4];

^[5]; Johnston, Steven ^[6]; Hwang, Harold Y. ^[1]

Stanford Univ., CA (United States). Department of Applied Physics and Geballe Laboratory for Advanced Materials; SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Institute for Materials and Energy Sciences
Stanford Univ., CA (United States). Department of Applied Physics and Geballe Laboratory for Advanced Materials
SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Institute for Materials and Energy Sciences
SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Institute for Materials and Energy Sciences; Stanford Univ., CA (United States). Dept. of Physics
SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Institute for Materials and Energy Sciences; Stanford Univ., CA (United States). Geballe Laboratory for Advanced Materials
Univ. of Tennessee, Knoxville, TN (United States). Department of Physics and Astronomy

We report 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 (T_c), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling (λ_BCS≈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. Finally, 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.

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Research Organization:: SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division

Grant/Contract Number:: AC02-76SF00515

OSTI ID:: 1426490

Journal Information:: Proceedings of the National Academy of Sciences of the United States of America, Vol. 115, Issue 7; ISSN 0027-8424

Publisher:: National Academy of Sciences, Washington, DC (United States)Copyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 51 works

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