Scanning Tunneling Microscopy and spectroscopy on YBa2Cu3O7: new light on the subgap states

Abstract

The mechanism underlying high temperature superconductivity continues to challenge our understanding. Scanning tunneling spectroscopy (STM) has historically been one of the prime techniques to tackle superconductivity in conventional BCS materials, providing key insight into the pairing mechanism. Here we present a new set of tunneling spectra obtained on YBCO single crystals. Our new results are twofold: Firstly, we have observed low energy electronic states that previously have been associated with the vortex core in HTS, to be present everywhere on the sample surface, both inside and outside vortices. This fact suggests that these features have a more general origin and are not specific signatures of the vortex core. Secondly, we have found these low energy states to be modulated in space, with a nematic order