Abstract
The resistance of the electron gas (2DEG) at the interface between the two band insulators LaAlO3 (LAO) and SrTiO3 (STO) typically drops monotonically with temperature and R/T curves during cooling and warm-up look identical for large area structures. Here we show that 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. During cool-down current filaments emerge at the domain walls that form during a structural phase transition of the STO substrate. During warm-up the reverse phase transition can interrupt filaments before the sheet conductivity which dominates at higher temperature is reestablished. Due to the limited number of filaments in a nanostructure this process can result in a complete loss of conductance. As a consequence of these findings the transport physics extracted from experiments in small and large area LAO/STO structures may need to be reconsidered.
Highlights
We have shown that LAO/STO nanostructures show one or two peaks in resistance when warmed up from low temperatures
The peaks occur at the temperatures of structural phase transitions of the STO
We can explain the behavior by the formation of current filaments at the boundaries of the domains which appear during the structural phase transition at TC2 in the STO and which may break during warm-up
Summary
All films used in our experiments are deposited by pulsed laser deposition (PLD) as described previously[13]. During growth the background oxygen pressure is 10−3 mbar. LAO layers are deposited from a single crystal LAO target on TiO2-terminated STO (001) substrates[32, 33]. The substrate temperature during deposition is 850 °C. Laser fluence and pulse frequency are kept at 2 J/cm[2] and 2 Hz, respectively, during the deposition. Reflection high-energy electron diffraction (RHEED) is used to monitor the layer thickness with unit cell resolution during the growth. After deposition of 6 unit cells of LAO the sample is slowly cooled down to room temperature while the oxygen pressure is maintained. As a result we obtain layers with a sheet resistance of 133Ω/□ and a typical mobility of 1.124 × 103 cm2V−1s−1 at 4.2 K
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