Disappearance of Nodal Gap across the Insulator-Superconductor Transition in a Copper-Oxide Superconductor

TL;DR

This study investigates the evolution of electronic structure in a copper-oxide superconductor, revealing a critical doping point where the nodal gap closes and superconductivity emerges, highlighting the link between antiferromagnetism and superconductivity.

Contribution

It provides high-resolution measurements showing the disappearance of the nodal gap at the insulator-superconductor transition in a cuprate, clarifying the transition mechanism.

Findings

Nodal energy gap approaches zero at critical doping ~0.10

Superconductivity emerges as antiferromagnetic order disappears

Electronic structure shows coexistence of coherence peak and broad hump

Abstract

The parent compound of the copper-oxide high temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high temperature superconductors and the superconductivity mechanism. Here we report high resolution angle-resolved photoemission measurement on heavily underdoped Bi2Sr2-xLaxCuO6+d system. The electronic structure of the lightly-doped samples exhibit a number of characteristics: existence of an energy gap along the nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of a coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetism and superconductivity.