Order parameter collective modes in Sr2RuO4

Order parameter collective modes coupled to sound propagation are studied for a two-dimensional p-wave superconductor with symmetry d((k) over cap) = (z) over cap((k) over cap(x) +/- i (k) over cap(y)), which has been proposed as a candidate pairing state for Sr-2 RuO4. It is known in the study of F-wave superfluid He-3 that internal degrees of freedom of the order parameter lead to order parameter collective modes, many of which lie below the pair-breaking frequency omega = 2 Delta. We show that the pairing state (z) over cap((k) over cap(x) +/- i (k) over cap(y)) possesses a collective mode which has the same character as the clapping mode in superfluid He-3-A. The eigen frequency is found to be omega = root 2 Delta in the long wavelength limit. It is shown that this collective mode can be excited by sound waves. (C) 2000 Elsevier Science B.V. All rights reserved.