Photon absorption edge in superconductors and gapped one-dimensional systems

V. V. Mkhitaryan; E. G. Mishchenko; M. E. Raikh; L. I. Glazman

doi:10.1103/PhysRevB.80.205416

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Photon absorption edge in superconductors and gapped one-dimensional systems

V. V. Mkhitaryan, E. G. Mishchenko, M. E. Raikh, L. I. Glazman

DOI 10.1103/PhysRevB.80.205416 · Physical Review B

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Abstract

Opening of a gap in the low-energy excitations spectrum affects the power-law singularity in the photon absorption spectrum A(Ω). In the normal state, the singularity, A(Ω)∝[D/(Ω−Ωth)]α, is characterized by an interaction-dependent exponent α. On the contrary, in the superconducting state the divergence, A(Ω)∝(D/Δ)α(Ω−Ω̃th)−1/2, is interaction independent, while threshold is shifted, Ω̃th=Ωth+Δ; the “normal-metal” form of A(Ω) resumes at (Ω−Ω̃th)≳Δ exp(1/α). If the core hole is magnetic, it creates in-gap states; these states transform drastically the absorption edge. In addition, processes of scattering off the magnetic core hole involving spin-flip give rise to inelastic absorption with one or several real excited pairs in the final state, yielding a structure of peaks in A(Ω) at multiples of 2Δ above the threshold frequency. The above conclusions apply to a broad class of systems, e.g., Mott insulators, where a gap opens at the Fermi level due to the interactions.