Physical Review B 82,14 (2010)
Using a two-orbital model of the superconducting phase of the pnictides, we compute the spectrum of the Leggett mode-a collective excitation of the phase of the superconducting gap known to exist in multigap superconductors-for different possible symmetries of the superconducting order parameter. Specifically, we identify the small regions of parameter space where the Leggett mode lies below the two-particle continuum, and hence should be visible as a sharp resonance peak. We discuss the possible utility of the Leggett mode in distinguishing different momentum dependencies of the superconducting gap. We argue that the observation of a sharp Leggett mode would be consistent with the presence of strong electron-electron correlations in iron-based superconductors. We also emphasize the importance of the orbital character of the Leggett mode, which can result in an experimental observation of the mode in channels other than A(1g).
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