Exciton dynamics uncovering electron fractionalization in superconducting cuprates

TL;DR

This paper demonstrates that high-energy excitonic excitations are enhanced in optimally doped cuprates during superconductivity, providing evidence for electron fractionalization and constraining theories of high-temperature superconductivity.

Contribution

It provides experimental evidence linking high-energy excitons to superconductivity in cuprates and supports electron fractionalization as a key concept in their electronic structure.

Findings

High-energy excitons are enhanced by superconductivity onset.

The enhancement constrains existing theories of cuprate superconductivity.

A two-component fermion model explains the observed excitonic behavior.

Abstract

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 Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ 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 well explains the change, pointing toward a novel route for understanding the electronic structure of superconducting cuprates.