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Spectroscopic signatures of collective modes in superconductors

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The formation of the superconducting state leads to the appearance of two collective modes associated with the amplitude and phase fluctuations of the SC order parameter. A direct observation of these two modes is usually elusive, since in a standard BCS superconductor they couple weakly to the spectroscopic observables, and they are overdamped by quasiparticle excitations. However, whenever these two conditions are modified by additional perturbations new remarkable effects can show up. Here I discuss two paradigmatic cases suggested by recent experiments in conventional superconductors. The first example is the visibility of phase fluctuations in a strongly disordered superconductor: we show that phase modes become optical active in the presence of disorder, and their contribution piles up below the optical gap due to quasiparticle excitations, as probed recent by microwave spectroscopy in conventional superconductors near the superconductor-to-insulator transition.The second example is the visibility of Higgs (amplitude) mode in the superconducting state formed after a charge-density-wave (CDW) transition. We show that the pre-existing CDW gap pushes the quasiparticle continuum away from the Higgs pole, making it a true, undamped excitation. This reflects in a strong resonance in the Raman response, where the Higgs visibility is provided by the soft CDW phonon, as suggested long ago for NbSe2.

This talk is part of the Theory of Condensed Matter series.

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