Leggett modes and multiband superconductivity in Sr2RuO4

Sr2RuO4 is a prototypical multiband superconductor with three bands crossing the Fermi level. These bands exhibit distinct dimensional characteristics, with one quasi-2D γ band and two quasi-1D α and β bands. This leads to the expectation that the superconductivity on the γ band may be only weakly Josephson-coupled to that on the other two bands. Based on an explicit microscopic weak coupling calculation appropriate for Sr2RuO4, we study the collective Leggett modes associated with the relative phase oscillations between the bands and show that a relatively soft Leggett mode exists due to the comparatively weaker interband Josephson coupling. These calculations also provide insight into why the superconducting gap magnitudes may be comparable on all three bands, despite the noticeable differences between the γ and α/β bands. The analyses can be readily applied to other multiband superconductors.