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 $A_{1g}$.

• Received 28 April 2010

DOI:https://doi.org/10.1103/PhysRevB.82.144506