Ubiquitous antinodal quasiparticles and deviation from simple d-wave form in underdoped Bi-2212

TL;DR

This study demonstrates that in underdoped Bi-2212, the superconducting gap deviates from a simple d-wave form near the antinode, with persistent quasiparticle peaks, challenging previous claims of uniform d-wave behavior.

Contribution

It provides direct evidence that the superconducting gap in underdoped Bi-2212 deviates from simple d-wave symmetry near the antinode, using samples with oxygen doping control.

Findings

Superconducting gap deviates from d-wave near the antinode in underdoped Bi-2212.

Quasiparticle peaks are observed at the antinode down to doping level p=0.08.

Deviation from simple d-wave form is not an artifact of data analysis.

Abstract

The momentum dependence of the superconducting gap in the cuprates has been debated, with most experiments reporting a deviation from a simple $d_{x^2-y^2}$ form in the underdoped regime and a few experiments claiming that a simple $d_{x^2-y^2}$ form persists down to the lowest dopings. We affirm that the superconducting gap function in sufficiently underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) deviates from a simple \textit{d}-wave form near the antinode. This is observed in samples where doping is controlled only by oxygen annealing, in contrast to claims that this effect is only seen in cation-substituted samples. Moreover, a quasiparticle peak is present at the antinode down to p$=$0.08, refuting claims that a deviation from a simple \textit{d}-wave form is a data analysis artifact stemming from difficulty in assessing a gap in the absence of a quasiparticle.