# In-gap features in superconducting LaAlO$_3$-SrTiO$_3$ interfaces observed by tunneling spectroscopy

## TL;DR

This study uses tunneling spectroscopy to detect in-gap quasiparticle states in superconducting LaAlO₃-SrTiO₃ interfaces, revealing complex behaviors dependent on sample history and external conditions, with potential implications for exotic superconducting states.

## Contribution

It reports the observation of in-gap states in LaAlO₃-SrTiO₃ interfaces and discusses possible unconventional physical mechanisms behind these states.

## Key findings

- In-gap states depend on thermal cycling history.
- States include zero-energy and finite-energy peaks.
- States vanish with increasing temperature and magnetic field.

## Abstract

We identified quasiparticle states at well-defined energies inside the superconducting gap of the electron system at the LaAlO$_3$-SrTiO$_3$ interface using tunneling spectroscopy. The states are found only in a number of samples and depend upon the thermal-cycling history of the samples. The states consist of a peak at zero energy and other peaks at finite energies, symmetrically placed around zero energy. These peaks disappear, together with the superconducting gap, with increasing temperature and magnetic field. We discuss the likelihood of various physical mechanisms that are known to cause in-gap states in superconductors and conclude that none of these mechanisms can easily explain the results. The conceivable scenarios are the formation of Majorana bound states, Andreev bound states, or the presence of an odd-frequency spin triplet component in the superconducting order parameter.