Observation of Leggett's collective mode in the multi-band MgB2 superconductor

The problem of collective modes in superconductors is almost as old as the microscopic theory of superconductivity. Bogolyubov and Anderson first discovered that density oscillations can couple to oscillations of the phase of the superconducting (SC) order parameter (OP) via the pairing interaction. In a neutral system these are sound-like oscillations, however, for a charged system the frequency of these modes is pushed up to the plasma frequency. The collective oscillations of the amplitude of the SC OP have a gap, which was first observed by Raman spectroscopy in NbSe2, and which plays a role equivalent to the Higgs particle in the electro-weak theory. Besides these two commonly known modes several other collective excitations have been proposed including an unusual exciton corresponding to fluctuations of the relative phase of coupled SC condensates first predicted by Leggett but it has not been experimentally confirmed. Here we report the observation of Leggett's collective mode in a multi-band MgB2 superconductor with Tc = 39 K. The novel mode is observed at 9.4 meV, consistent with theoretical considerations.