Condensed-Matter Physics & Materials Science Seminar

The role of dimensionality has been one of the central issues in superconductivity, because many promising superconductors have quasi-two-dimensional electronic states such as MgB2 and cuprates. Although the low-dimensional properties of superconductivity have been studied in metal thin films for decades, they essentially form from 3D electronic states because of their large carrier density. Here, we focus on strontium titanate (SrTiO3), which is the first known superconducting semiconductor with extremely low carrier density (n > 1019 cm-3).
Using a new growth technique for high-mobility SrTiO3 films, we grew heterostructures of 1 at. % Nb-doped SrTiO3 (Nb:SrTiO3) embedded in insulating SrTiO3 by pulsed laser deposition, which exhibits a superconducting transition around 0.3 K. By decreasing the thickness of Nb:SrTiO3, the low-temperature mobility was enhanced by a factor of ~ 3, exceeding 1,000 cm2 V-1 s-1. In addition, analysis of the superconducting upper critical field indicated a crossover from 2D to 3D superconductivity.
Above the upper critical field, we observed Shubnikov de-Haas oscillations which scale with the perpendicular field, indicating a 2D normal state. These results suggest we can access new regimes of 2D superconducting phase transitions in the clean limit.

This work was done in collaboration with Minu Kim, Christopher Bell, Bog G. Kim, Yasuyuki Hikita, and Harold Y. Hwang.