Why the anti-nodal quasiparticle dispersion is so flat in the superconducting cuprates?

TL;DR

This paper explains the flat anti-nodal quasiparticle dispersion in high-Tc cuprates as a result of strong coupling between electrons and diffusive spin fluctuations, challenging the traditional electron-Boson coupling interpretation.

Contribution

It introduces a new understanding that the quasiparticle peak arises from a pole in the Green's function due to electron-spin fluctuation interactions, not from a simple Bogoliubov quasiparticle picture.

Findings

Quasiparticle peak is generated by electron coupling to diffusive spin fluctuations.

Normal self-energy causes level repulsion, reducing dispersion and dissipation.

Peak-dip separation does not indicate pairing glue energy.

Abstract

The emergence of the coherent quasiparticle peak and the development of the peak-dip-hump structure in the anti-nodal region below $T_{c}$ is the most prominent non-BCS signature of the underdoped high-$T_{c}$ cuprates, in which no coherent quasiparticle can be defined in the anti-nodal region above $T_{c}$. The peak-dip-hump structure has been commonly interpreted as the result of the coupling of the electron to some Bosonic mode. However, such an electron-Boson coupling picture does not answer the question of \textit{why the quasiparticle dispersion is so flat in the anti-nodal region}, a behavior totally unexpected for Bogoliubov quasiparticle in a d-wave BCS superconductor. Here we show that the sharp quasiparticle peak in the anti-nodal region should be understood as a new pole in the electron Green's function generated by the strong coupling of the electron to diffusive spin fluctuation around the antiferromagnetic wave vector $\mathbf{Q}=(\pi,\pi)$, rather than a nearly free Bogoliubov quasiparticle in a d-wave BCS superconductor. More specifically, we find that the normal self-energy of the electron from the scattering with the diffusive spin fluctuation manifests itself mainly as a level repulsion effect and is responsible for the reduction of both the quasiparticle dispersion and the quasiparticle dissipation rate in the anti-nodal region. We argue that the peak-dip separation in the anti-nodal spectrum should not be interpreted as the energy of the pairing glue.