Condensed-Matter Physics & Materials Science Seminar

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

Presented by Girsh Blumberg, Bell Laboratories

Monday, July 2, 2007, 1:00 pm — Small Seminar Room, Bldg. 510

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

Girsh Blumberg, Bell Labs

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 could couple to oscillations of the phase of the superconducting order parameter via the pairing interaction. In a neutral system these are the Goldstone sound-like oscillations that accompany the spontaneous gauge-symmetry breaking, however, for a charged system the frequency of these modes is pushed up to the plasma frequency by the Anderson-Higgs mechanism and the Goldstone mode does not exist. The collective oscillations of the amplitude of the superconducting order parameter 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. For a multi-band superconductor Leggett predicted an unusual exciton corresponding to fluctuations of the relative phase of coupled condensates. This exciton has relativistic dispersion and is only weakly gapped almost maintaining properties of the Goldstone excitation. Here we report the observation of Leggett's collective mode in a multi-band MgB2 superconductor with Tc =39K. Leggett's mode is observed in Raman response at 9.4 meV, consistent with the theoretical considerations based on the first principal computations.

ΒΆ Work done in collaboration with A. Mialitsin, B. S. Dennis, M.V. Klein, N.D. Zhigadlo, and J. Karpinski.

Hosted by: John P. Hill

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