Leggett modes and multi-band superconductivity in Sr2RuO4

Abstract

Sr2RuO4 is a prototypical multi-band superconductor with three bands crossing the Fermi level. These bands exhibit distinct dimensional characteristics, with one quasi-2D γ-band and two quasi-1D αand 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 inter-band 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 multi-band superconductors.

Amplitude mode oscillations in pump-probe photoemission spectra from a d -wave superconductor

“…The stage is set to take advantage of pump-probe techniques such as trARPES not only to detect the Higgs mode, but also to study the rich assortment of collective modes which have been predicted in the unconventional superconductors. Examples include the Bardasis-Schrieffer modes in systems with competition for superconducting ground states with different pairing symmetries [40], Leggett modes in multi-band superconductors [41,42], multiple Higgs modes in channels corresponding to different irreducible representations of the lattice [26], and the various collective modes arising in gauge theories [43]. Our work serves as a starting point for studying these modes within a framework that includes inelastic scattering and retarded interactions, ingredients which are needed to accurately simulate the amplitude mode in a superconductor out of equilibrium during a pumpprobe experiment.…”