Unconventional exciton evolution from the pseudogap to superconducting phases in cuprates.

Electron quasiparticles play a crucial role in simplifying the description of many-body physics in solids with surprising success. Conventional Landau's Fermi-liquid and quasiparticle theories for high-temperature superconducting cuprates have, however, received skepticism from various angles. A path-breaking framework of electron fractionalization has been established to replace the Fermi-liquid theory for systems that show the fractional quantum Hall effect and the Mott insulating phenomena; whether it captures the essential physics of the pseudogap and superconducting phases of cuprates is still an open issue. Here, we show that excitonic excitation of optimally doped Bi₂Sr₂CaCu₂O_8+δ with energy far above the superconducting-gap energy scale, about 1 eV or even higher, is unusually enhanced by the onset of superconductivity. Our finding proves the involvement of such high-energy excitons in superconductivity. Therefore, the observed enhancement in the spectral weight of excitons imposes a crucial constraint on theories for the pseudogap and superconducting mechanisms. A simple two-component fermion model which embodies electron fractionalization in the pseudogap state provides a possible mechanism of this enhancement, pointing toward a novel route for understanding the electronic structure of superconducting cuprates. The nature of the excitations in the pseudogap regime and their relation to superconductivity remain core issues in cuprate high-T_c superconductivity. Here, using resonant inelastic x-ray scattering, the authors find that high-energy excitons in optimally-doped Bi₂Sr₂CaCu₂O_8+δ are enhanced by the onset of superconductivity, an effect possibly explained in terms of electron fractionalization. [ABSTRACT FROM AUTHOR]