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Title: Polaronic behavior in a weak-coupling superconductor

Abstract

We report the nature of superconductivity in the dilute semiconductor SrTiO 3 has remained an open question for more than 50 y. The extremely low carrier densities (10 18–10 20 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 SrTiO 3, 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 SrTiO 3 is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron–phonon coupling strength.

Authors:
 [1];  [1];  [2];  [3];  [4]; ORCiD logo [5];  [6];  [1]
  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
  2. Stanford Univ., CA (United States). Department of Applied Physics and Geballe Laboratory for Advanced Materials
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Institute for Materials and Energy Sciences
  4. SLAC National Accelerator Lab., Menlo Park, CA (United States). Stanford Institute for Materials and Energy Sciences; Stanford Univ., CA (United States). Dept. of Physics
  5. 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
  6. Univ. of Tennessee, Knoxville, TN (United States). Department of Physics and Astronomy
Publication Date:
Research Org.:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
OSTI Identifier:
1426490
Grant/Contract Number:
AC02-76SF00515
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Proceedings of the National Academy of Sciences of the United States of America
Additional Journal Information:
Journal Volume: 115; Journal Issue: 7; Journal ID: ISSN 0027-8424
Publisher:
National Academy of Sciences, Washington, DC (United States)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; superconductivity; electronic structure; polaron; oxide interface

Citation Formats

Swartz, Adrian G., Inoue, Hisashi, Merz, Tyler A., Hikita, Yasuyuki, Raghu, Srinivas, Devereaux, Thomas P., Johnston, Steven, and Hwang, Harold Y.. Polaronic behavior in a weak-coupling superconductor. United States: N. p., 2018. Web. doi:10.1073/pnas.1713916115.
Swartz, Adrian G., Inoue, Hisashi, Merz, Tyler A., Hikita, Yasuyuki, Raghu, Srinivas, Devereaux, Thomas P., Johnston, Steven, & Hwang, Harold Y.. Polaronic behavior in a weak-coupling superconductor. United States. doi:10.1073/pnas.1713916115.
Swartz, Adrian G., Inoue, Hisashi, Merz, Tyler A., Hikita, Yasuyuki, Raghu, Srinivas, Devereaux, Thomas P., Johnston, Steven, and Hwang, Harold Y.. Tue . "Polaronic behavior in a weak-coupling superconductor". United States. doi:10.1073/pnas.1713916115.
@article{osti_1426490,
title = {Polaronic behavior in a weak-coupling superconductor},
author = {Swartz, Adrian G. and Inoue, Hisashi and Merz, Tyler A. and Hikita, Yasuyuki and Raghu, Srinivas and Devereaux, Thomas P. and Johnston, Steven and Hwang, Harold Y.},
abstractNote = {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 (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. 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.},
doi = {10.1073/pnas.1713916115},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
number = 7,
volume = 115,
place = {United States},
year = {Tue Jan 30 00:00:00 EST 2018},
month = {Tue Jan 30 00:00:00 EST 2018}
}

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