"Balancing Act: Evidence for a Strong Subdominant d-Wave Pairing Channel in Ba0.6K0.4Fe2As2"

T. Böhm: A. F. Kemper, B. Moritz, F. Kretzschmar, B. Muschler, H.-M. Eiter, R. Hackl, T. P. Devereaux, D. J. Scalapino, and Hai-Hu Wen; Physical Review X, 12/18/14.

Additional Authors: A. F. Kemper, B. Moritz, F. Kretzschmar, B. Muschler, H.-M. Eiter, R. Hackl, T. P. Devereaux, D. J. Scalapino, and Hai-Hu Wen

Abstract:

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.