Optical Study of Tetragonal Domains in LaAlO₃/SrTiO₃

Scanning superconducting quantum interference device (SQUID) measurements recently revealed enhanced channels of conductivity at the conducting LaAlO₃/SrTiO₃ (LAO/STO) interface (Kalisky et al. Nat. Mater. 12, 1091 2013). The orientation of the channels and their thermal behavior suggest that they originate as a consequence of the STO tetragonal domain formation which sets in below ∼105 K. In this work, we use polarized light microscopy to acquire images of the tetragonal domains in the same group of LAO/STO samples. We looked at the configuration of the domains (orientation and spacing) and followed their behavior as a function of temperature and back-gate voltage. The optical data agrees with the electrical behavior mapped magnetically. This direct and independent study of the domain structure confirms that the channel-like conductivity in LAO/STO is due to the STO tetragonal domain structure and emphasizes the importance of STO physics to the interfacial properties. These results demonstrate how small structural changes in perovskite crystals strongly influence the electronic characteristics of heterostructures.