Balancing Act: Evidence for a Strong Subdominant d-Wave Pairing Channel in Ba_{0.6}K_{0.4}Fe_{2}As_{2}

Abstract

We present detailed measurements of the temperature-dependent Raman spectra of optimally doped Ba_{0.6}K_{0.4}Fe_{2}As_{2} 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 B_{1g} (d_{x^{2}-y^{2}}) Raman spectra only below T_{c}, 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 A_{1g} (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 d_{x^{2}-y^{2}} 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 d_{x^{2}-y^{2}} may be the dominant pairing symmetry in Fe-based superconductors that lack central hole bands.