Physical Review Letters ( IF 8.385 ) Pub Date :
G. He; D. Li; D. Jost; A. Baum; P. P. Shen; X. L. Dong; Z. X. Zhao; R. Hackl

We studied the electronic Raman spectra of as a function of light polarization and temperature. In the spectra alone we observe the redistribution of spectral weight expected for a superconductor and two well resolved peaks below . The nearly resolution-limited peak at 110,cm${}^{-1}$ (13.6,meV) is identified as a collective mode. The peak at 190,cm${}^{-1}$ (23.6,meV) is presumably another collective mode since the line is symmetric and its energy is significantly below the gap energy observed by single-particle spectroscopies. Given the experimental band structure of the most plausible explanations include conventional spin-fluctuation pairing between the electron bands and the incipient hole band and pairing between the hybridized electron bands. The absence of gap features in and symmetry favors the second case. Thus, in spite of various differences between the pnictides and chalcogenides this study demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.

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