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The properties of thin films and heterostructures of complex and strongly correlated oxides are one of the most fascinating and demanding topics in today´s solid state physics research. Oxide heterostructures give rise to novel and unexpected physical phenomena such as metallicity in nominally non-metallic materials, magnetism in nominally non-magnetic materials, ferroelectricity in nominally non-ferroelectric materials, or so far unobserved topological effects. Many of these phenomena are affected or even more driven by the nanoscale defect structure established during material synthesis and processing, ultimately limited by the thermodynamics of the system. Tuning the thermodynamic processes by engineering dedicated heterostructures may be used to tailor their defect structure in a desired way, hence, to deplete or accumulate defects at interfaces, to spatially separate electronic and ionic charge carriers, or to confine defects to dedicated regions. In this talk, I discuss the low-dimensional electron transport observed along complex oxide heterointerfaces, such as the one in LaAlO3/SrTiO3 heterostructures. While the formation of these 2-dimensional electron gases is attributed to electronic charge transfer triggered by a built-in electric field, the ionic defect structure at these interfaces is still being discussed controversially. I address the thermodynamic processes associated with built-in electric fields and derive ionic defect concentration profiles established at polar/non-polar oxide interfaces. [1] The specific ionic-electronic defect structure stabilized within such interfacial space charge layers strongly depends on ambient oxygen partial pressure applied during sample fabrication and on the strength of the built-in electric field. As will be discussed, the low-temperature behavior of these novel electron system is unambiguously correlated to the adjacent ionic structure typically set at high temperature, making thermodynamic considerations indispensable in order to understand novel phenomena occurring at cryostatic temperatures. Therefore, a comprehensive study of low temperature physics on the one hand and high temperature thermodynamics on the other hand is essential for a unified understanding of the manifold (and sometimes contradictory) phenomena observed in these low-dimensional electron systems. Here, we explicitly compare the thermodynamic ground states obtained for various oxide heterostructure systems [2] and discuss resulting implications for important measures characterizing the electron gas, such as electron mobility [3] as well as its magnetic signature [4], both controllable by thermodynamic means. The thermodynamic model obtained for oxide heterointerfaces is furthermore linked to kinetic space charge formation occurring at complex oxide surfaces [5, 6]. [1] F. Gunkel et al., “Defect concentration profiles at complex oxide interfaces”, Physical Review B 93, 245431 (2016) [2] F. Gunkel et al., “Thermodynamic ground states of complex oxide heterointerfaces”, ACS Applied Materials & interfaces, ACS Appl. Mater. Interfaces, 9 (1), 1086 (2017) [3] C. Xu et al., “Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures”, Scientific Reports 6, 22410 (2016) [4] F. Gunkel, et al., “Defect-control of anomalous and conventional electron transport in NdGaO3/SrTiO3 heterostructures”, Physical Review X, 6, 031035 (2016) [5] R. Meyer et al., “Dynamics of the metal-insulator transition of donor-doped SrTiO3”, Physical Review B 94, 115408 (2016) [6] M. Andrae et al., “Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3”, APL Materials 5, 056106 (2017)

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Felix Gunkel, FZ Juelich
SR Kernphysik
Contact: Paul van Loosdrecht