Many-body selection rule for quasiparticle pair creations in centrosymmetric superconductors
arXiv:2108.04846
Abstract
When metal becomes superconducting, new optical excitation channels are created by particle-hole mixing. These excitation channels contribute negligibly to optical responses in most superconductors, but they can be relevant in ultra-strong-coupling superconductors that are close to the Bose-Einstein condensate regime. Recently, selection rules for these excitations have been formulated based on single-particle anti-unitary symmetries in the mean-field theory. While being potentially useful for studying optical properties of ultra-strong-coupling superconductors, they had fundamental limitations because significant quantum fluctuations invalidate mean-field approaches. Here, we use many-body states to formulate an optical selection rule that does not rely on the mean-field approximation. In this approach, the physical meaning of the previous selection rules becomes clearer as they are simply recast as the selection rule for many-body inversion eigenstates, not involving anti-unitary symmetries. This selection rule applies not only to the Bogoliubov quasiparticles of Fermi liquids but also to non-Fermi-liquid quasiparticles and electrically charged bosonic excitations. We also study the Bogoliubov Fermi surfaces, whose topological stability is closely related to the selection rule. We provide a many-body formulation of their topological charges and show that the low-energy optical conductivity of the Bogoliubov Fermi surfaces depends crucially on their secondary topological charge. Finally, we discuss the implications of our results to the stability of the superconducting state.
arXiv comment10 pages, 2 figures
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