Enhanced transition temperature due to tetragonal domains in two-dimensional superconducting strontium titanate

Strontium titanate (SrTiO$_3$) is a key component in superconducting heterostructures such as LaAlO$_3$/SrTiO$_3$ and monolayer FeSe on SrTiO$_3$, yet superconductivity in bare SrTiO$_3$ is not fully understood. We used a scanning superconducting quantum interference device susceptometer to image the diamagnetic response as a function of temperature in samples of SrTiO$_3$ containing either a 5.5 nm or a 36.9 nm-thick slab of niobium-doped SrTiO$_3$. We find that stripe-like regions remain superconducting at higher temperatures than the rest of the sample. The shape and orientation of the features, as well as their behavior in a subsequent cooldown, are consistent with an origin in the low-temperature tetragonal domain structure of SrTiO$_3$. These results suggest a variety of mechanisms by which the transition temperature could be enhanced and may help constrain theories of superconductivity in SrTiO$_3$.