Directly visualizing the sign change of d-wave superconducting gap in Bi2Sr2CaCu2O8+{\delta} by phase-referenced quasiparticle interference

TL;DR

This study uses phase-referenced quasiparticle interference in scanning tunneling spectroscopy to directly visualize the sign change of the d-wave superconducting gap in Bi2Sr2CaCu2O8+{eta}, providing a new method to understand unconventional superconductivity.

Contribution

It introduces a novel phase-referenced QPI technique to directly observe the sign change of the d-wave gap in cuprate superconductors.

Findings

Visualized the sign change of the d-wave gap in Bi2Sr2CaCu2O8+{eta}.

Identified seven basic scattering vectors connecting quasiparticle contours.

Demonstrated an effective method for probing the gap structure in unconventional superconductors.

Abstract

The superconducting state is achieved by the condensation of Cooper pairs and is protected by the superconducting gap. The pairing interaction between the two electrons of a Cooper pair determines the superconducting gap function. Thus, it is very pivotal to detect the gap structure for understanding the mechanism of superconductivity. In cuprate superconductors, it has been well established that the superconducting gap may have a d-wave function {\Delta} = {\Delta}_0cos2{\theta}. This gap function has an alternative sign change by every pi/2 in the momentum space when the in-plane azimuthal angle theta is scanned. It is very hard to visualize this sign change. Early experiments for recommending or proving this d-wave gap function were accomplished by the specially designed phase sensitive measurements based on the Josephson effect. Here we report the measurements of scanning tunneling spectroscopy in one of the model cuprate system Bi2Sr2CaCu2O8+{\delta} and conduct the analysis of phase-referenced quasiparticle interference (QPI). Due to the unique quasiparticle excitations in the superconducting state of cuprate, we have seen the seven basic scattering vectors that connect each pair of the terminals of the banana-shaped contour of constant quasiparticle energy (CCE). The phase-referenced QPI clearly visualizes the sign change of the d-wave gap. Our results illustrate a very effective way for determining the sign change of unconventional superconductors.