Stripes of enhanced transition temperature in superconducting strontium titanate

Strontium titanate (SrTiO$_3$) is used widely in heterostructures that are the subject of intense research, such as the LaAlO$_3$/SrTiO$_3$ interface and FeSe grown on SrTiO$_3$, yet the nature and mechanism of superconductivity in SrTiO$_3$ itself are not fully understood. We used a scanning superconducting quantum interference device susceptometer to map the superfluid density as a function of temperature in a 5.5 nm-thick slab of niobium-doped SrTiO$_3$ embedded in undoped SrTiO$_3$. We find that stripe-like regions of the sample remain superconducting to temperatures typically $\sim$40 mK higher than the transition temperature of featureless regions. We associate the stripes with tetragonal domains in SrTiO$_3$, showing that the orientation of the tetragonal c-axis may be important for tuning the critical temperature. These data may be useful for distinguishing models of superconductivity in SrTiO$_3$.