Temperature Dependent Giant Resistance Anomaly in LaAlO₃/SrTiO₃ Nanostructures

The electron gas at the interface between LaAlO₃ and SrTiO₃ has been investigated since more than a decade. Its resistance typically drops monotonically with temperature and R/T curves during cooling and warm-up look identical for large area structures. We have been able to fabricate nanostructures which show a surprising temperature dependence. If the LAO/STO is laterally restricted by nanopatterning the resistance exhibits a temperature anomaly. Warming up nanostructures from low temperatures leads to one or two pronounced resistance peaks between 50 and 100 K not observed for larger dimensions. This result can be explained by the structural phase transitions which take place in the STO substrate during cool-down and warm-up.During cool-down current filaments emerge at the domain walls that form during the phase transition. This effect had already been observed by scanning SQUID microscopy. During warm-up, however, the reverse phase transition can interrupt filaments before the sheet conductivity which dominates at higher temperature is reestablished. It appears that the area between the domain walls becomes completely insulating and due to the limited number of filaments in a nanostructure this process can result in a complete loss of conductance.