Revisiting the vortex-core tunnelling spectroscopy in YBa$_2$Cu$_3$O$_{7-\delta}$

TL;DR

This study uses scanning tunnelling spectroscopy to investigate vortex core states in YBa₂Cu₃O₇−δ, revealing that certain subgap features are pervasive and unrelated to vortices, suggesting they are due to an incoherent tunnelling channel.

Contribution

It demonstrates that the observed subgap states are not vortex-specific but are present throughout the surface, indicating a different origin than previously theorized.

Findings

Subgap features are observed everywhere, not just in vortex cores.

These states are unpaired and persist without magnetic field.

They originate from an incoherent tunnelling channel.

Abstract

The observation by scanning tunnelling spectroscopy (STS) of Abrikosov vortex cores in the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ (Y123) has revealed a robust pair of electron-hole symmetric states at finite subgap energy. Their interpretation remains an open question because theory predicts a different signature in the vortex cores, characterised by a strong zero-bias conductance peak. We present STS data on very homogeneous Y123 at 0.4 K revealing that the subgap features do not belong to vortices: they are actually observed everywhere along the surface with high spatial and energy reproducibility, even in the absence of magnetic field. Detailed analysis and modelling show that these states remain unpaired in the superconducting phase and belong to an incoherent channel which contributes to the tunnelling signal in parallel with the superconducting density of states.