Coulomb Disorder Effects on ARPES and NQR Spectra in Cuprates

TL;DR

This paper investigates how Coulomb disorder from surface defects and dopants affects ARPES and NQR spectra in cuprates, revealing spectral broadening, evolution with doping, and Coulomb gap formation.

Contribution

It introduces a model linking Coulomb disorder to spectral features in cuprates, including ARPES line broadening and NQR spectra, supported by Hartree-Fock simulations and experimental data comparison.

Findings

Surface defects cause Gaussian ARPES line broadening.

Doping alters ARPES lineshape from Gaussian to Lorentzian.

A Coulomb gap of about 10 meV at the surface was identified.

Abstract

The role of Coulomb disorder, either of extrinsic origin or introduced by dopant ions in undoped and lightly-doped cuprates, is studied. We demonstrate that charged surface defects in an insulator lead to a Gaussian broadening of the Angle-Resolved Photoemisson Spectroscopy (ARPES) lines. The effect is due to the long-range nature of the Coulomb interaction. A tiny surface concentration of defects about a fraction of one per cent is sufficient to explain the line broadening observed in Sr$_2$CuO$_2$Cl$_2$, La$_2$CuO$_{4}$, and Ca$_{2}$CuO$_{2}$Cl$_{2}$. Due to the Coulomb screening, the ARPES spectra evolve dramatically with doping, changing their shape from a broad Gaussian form to narrow Lorentzian ones. To understand the screening mechanism and the lineshape evolution in detail, we perform Hartree-Fock simulations with random positions of surface defects and dopant ions. To check validity of the model we calculate the Nuclear Quadrupole Resonance (NQR) lineshapes as a function of doping and reproduce the experimentally observed NQR spectra. Our study also indicates opening of a substantial Coulomb gap at the chemical potential. For a surface CuO$_2$ layer the value of the gap is of the order of 10 meV while in the bulk it is reduced to the value about a few meV.