Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates

TL;DR

This study reveals that lightly doped cuprates exhibit a persistent finite gap across the entire Brillouin zone, which closes as doping increases, challenging existing theories of their normal state electronic structure.

Contribution

The paper provides the first comprehensive ARPES measurements showing a fully gapped normal state in lightly doped cuprates, across different compounds and doping levels.

Findings

A finite gap exists over the entire Brillouin zone in lightly doped cuprates.

The gap closes with increased carrier doping.

The results suggest a need to reconsider the normal state electronic structure of cuprates.

Abstract

The low-energy excitations of the lightly doped cuprates were studied by angle-resolved photoemission spectroscopy. A finite gap was measured over the entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to close with increasing carrier doping. We consider various scenarios to explain our results, including the possible effects of chemical disorder, electronic inhomogeneity, and a competing phase.