In-plane tunnel spectroscopy of cuprate superconductors with nano-scale Josephson junctions

Superconducting tunnel spectroscopy has been used for decades to gather valuable information about the electronic density of states and the energy gap in superconductors. For thin film materials, this has, for the most part, only been possible in directions perpendicular to the substrate. For anisotropic materials like the cuprate superconductors, this has hindered knowledge about the density of states in the a–b plane of the film where superconductivity is strongest. The advent of helium ion beam fabricated planar Josephson junctions has fundamentally changed how we can perform tunnel spectroscopy. In this work, we utilize nanoscale Josephson tunnel junctions orientated along different crystalline directions and report the anisotropy of the superconducting energy gap of cuprates at both the micro- and nanoscale. The smaller nanoelectrodes exhibit more variation that roughly correlates with the granularity of the film. We also observe that the gap energy is inversely proportional to the material's conductivity in the voltage state.