Transport Measurements on (111) Oriented (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)O$_3$/SrTiO$_3$ Heterostructures

Varada Bal

doi:https://doi.org/10.21985/N2B782

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Transport Measurements on (111) Oriented (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)O$_3$/SrTiO$_3$ Heterostructures

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At the interface of two dissimilar entities, something novel can emerge. This idea has driven a vast amount of fruitful work on semiconductor interfaces, and given us the digital revolution. In the past decade, remarkable progress has been made in the synthesis and understanding of interfaces between oxides, opening up new avenues to explore fundamental emergent physics at the interface, as well as to provide viable candidates to augment or even surpass the performance of conventional semiconductor electronics. The two-dimensional conducting interface in the (001) LaAlO$_3$/SrTiO$_3$ system is the most studied example of oxide interfaces, showing a wide range of coexisting and competing phenomena such as superconductivity, superconductor-insulator transitions, magnetism, and spin-orbit interactions. The (111) oriented LaAlO$_3$/SrTiO$_3$ system has been recently found to have a few surprises of its own, with intriguing anisotropies in many transport properties along different in-plane crystal directions. This thesis presents the first results on a different SrTiO$_3$ based system: the conducting interface between (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$) and (111) oriented SrTiO$_3$, which has a smaller strain as compared to the LaAlO$_3$/SrTiO$_3$ system. Electrical transport measurements at cryogenic temperatures reveal that no systematic anisotropy is seen in transport properties, unlike in the case of (111) oriented LaAlO$_3$/SrTiO$_3$. High-field magnetotransport shows the presence of high-mobility carriers at high electron densities, and exhibits multiband behavior, tunable \textit{in situ} using an electrical back-gate, similar to the LaAlO$_3$/SrTiO$_3$ system. The data allow us to draw specific conclusions about band ordering in the system, and point to possible differences between the band ordering in (111) (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)O$_3$/SrTiO$_3$ and (001) as well as (111) oriented LaAlO$_3$/SrTiO$_3$. Low-field magnetotransport reveals that a strong spin-orbit interaction emerges in the regime of low electron density, when the high-mobility carriers are depleted from the system, a trend which is opposite to that observed in (001) oriented LaAlO$_3$/SrTiO$_3$. The most striking feature is that at millikelvin temperatures, in the regime of low electron densities, concomitant with the development of strong spin-orbit interaction, magnetic order emerges

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