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Title: Balancing act: Evidence for a strong subdominant d-wave pairing channel in Ba0.6K0.4Fe2As2

We present detailed measurements of the temperature-dependent Raman spectra of optimally doped Ba0.6K0.4Fe2As2 and analyze the low-temperature spectra based on local-density-approximation band-structure calculations and the subsequent estimation of effective Raman vertices. Experimentally, a narrow, emergent mode appears in the B1g (dx2-y2) Raman spectra only below Tc, well into the superconducting state and at an energy below twice the energy gap on the electron Fermi-surface sheets. The Raman spectra can be reproduced quantitatively with estimates for the magnitude and momentum-space structure of an A1g (s-wave) pairing gap on different Fermi-surface sheets, as well as the identification of the emergent sharp feature as a Bardasis-Schrieffer exciton. Formed as a Cooper-pair bound state in a subdominant dx2-y2 channel, the binding energy of the exciton relative to the gap edge shows that the coupling strength in the subdominant channel is as strong as 60% of that in the dominant s-wave channel. This result suggests that dx2-y2 may be the dominant pairing symmetry in Fe-based superconductors that lack central hole bands.
 [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [4] ;  [6] ;  [7] ;  [8] ;  [9]
  1. Bayerishe Akademie der Wissenshaften (Germany); Technishe Univ. München (Germany); SLAC National Accelerator Lab., Menlo Park, CA (United States)
  2. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  4. Bayerishe Akademie der Wissenshaften (Germany); Technishe Univ., München (Germany)
  5. Bayerishe Akademie der Wissenshaften (Germany); Technishe Univ. München (Germany)
  6. Bayerishe Akademie der Wissenshaften (Germany)
  7. SLAC National Accelerator Lab., Menlo Park, CA (United States); Stanford Univ., CA (United States). Geballe Lab. for Advanced Materials.
  8. Univ. of California, Santa Barbara, CA (United States)
  9. Nanjing Univ. (China). National Lab. of Solid State Microstructures.
Publication Date:
OSTI Identifier:
Grant/Contract Number:
HA 2071/7; A5; AC02-76SF00515
Accepted Manuscript
Journal Name:
Physical Review. X
Additional Journal Information:
Journal Volume: 4; Journal Issue: 4; Journal ID: ISSN 2160-3308
American Physical Society
Research Org:
SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC)
Country of Publication:
United States