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The internal (inter-band) Josephson coupling in superconductors with more than one Fermi surface gives rise to new phase [1] and amplitude [2] collective modes associated with out-of-phase fluctuations of the multiple superconducting (SC) condensates in addition to the in-phase Bogolyubov-Anderson phase and Littlewood-Varma [3] amplitude modes. The counter flow of the interacting superfluids leading to small fluctuations of the relative phase of the condensates while the total electron density is locally conserved was first discussed by Leggett [1]. The Leggett phase mode directly couple into density fluctuation response functions, which can be studied by Raman scattering. In the charge-density-wave (CDW) systems the amplitude modes couple to Raman response via coupling to the CDW phonons [3].
Here we present direct spectroscopic observation of the Leggett's excitation in the MgB2 superconductor containing two pairs of Fermi surfaces resulting from \pi- and \sigma-bands. Our electronic Raman scattering studies have revealed three distinct SC features: (i) a clean threshold of Raman intensity at 4.6 meV consistent with the \pi-band SC gap; (ii) the SC pair breaking coherence peak at 13.5 meV consistent with excitations above the \sigma-band gap; and (iii) the Leggett collective mode at 9.4 meV [4]. The temperature and field dependencies for all three features (i) - (iii) have been established; the effects of magnetic field on the pair cross-tunneling in multiband systems will be discussed [5]. We also discuss observation of in- and out-of-phase amplitude modes in the multiband NbSe2 superconductor-CDW system. The modes in the electronic Raman data for different scattering channels provide input for classification and characterization of the internal Josephson coupling strength and sign. Our calculation of the Raman response function for MgB2 and NbSe2 superconductors based on multiband interaction matrices by first principle computations show good agreement with spectroscopic observations. *In collaboration with A. Mialitsin, M.V. Klein, N.D. Zhigadlo, and J. Karpinski. 1. A.J. Leggett, Progr. Theor. Phys. 36, 901 (1966). 2. W.C. Wu and A. Griffin, Phys. Rev. Lett. 74, 158 (1995). 3. P.B. Littlewood and C.M. Varma, Phys. Rev. Lett. 47, 811(1981). 4. G. Blumberg et al., Phys. Rev. Lett. 99, 227002 (2007). 5. G. Blumberg et al., Physica (Amsterdam) 456C, 75 (2007). |
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