Visualizing quantum phenomena at complex oxide interfaces: An atomic view from scanning transmission electron microscopy

Abstract

Complex oxide interfaces have been one of the central focuses in condensed matterphysics and material science. Over the past decade, aberration corrected scanning transmission electron microscopy and spectroscopy has proven to be invaluable to visualize and understand the emerging quantum phenomena at an interface. In this paper, we briefly review some recent progress in the utilization of electron microscopy to probe interfaces. Specifically, we discuss several important challenges for electron microscopy to advance our understanding on interface phenomena, from the perspective of variable temperature, magnetism, electron energy loss spectroscopy analysis, electronic symmetry, and defects probing.

Epitaxial ferroelectric interfacial devices

“…It is evident that the effective piezoelectric constants c eff describing the effect of epitaxial strain (to linear order) on the spontaneous polarization are much larger for BaTiO 3 and PbTiO 3 than for the other three systems. This is mainly due to the larger c 33 in BaTiO 3 =PbTiO 3 but also due to the smaller Poisson ratio of the P4mm structures [28]. The Poisson ratios are comparable for systems with the same structural symmetry.…”

“…143 Finally, we mention here high resolution scanning transmission electron microscopy (HR-STEM), which is a particularly powerful technique for probing in real space the atomic structure of both ferroelectric films and interfaces, and capable also of locally probing some aspects of the electronic structure via electron energy loss spectroscopy (EELS). 28,144 The direction of the ferroelectric polarization can be determined from an accurate determination of the relative atomic displacement between cation and anion species, such that ferroelectric domains and domain walls can be identified; [145][146][147] also from the amplitude of the atomic displacement, estimates of the ferroelectric polarization can be obtained from the relation between effective Born charges and ferroelectric polarization. [147][148][149] In many systems, local electrical contacts to the sample enable one to carry out in situ ferroelectric switching studies, 146 including of ferroelectric domain wall dynamics.…”

“…The study of such phenomena often requires probing the local electronic structure at the interface, and in this context, the recent advances in high resolution TEM have been invaluable. 28,144 One aspect concerns the chemical bonding between the ferroelectric layer and the adjacent conducting channel, which may impose restrictions on the ferroelectric soft-mode across the heterointerface and affect significantly the ferroelectric properties of the system. 52,506 In addition, the asymmetric environment at the interface can induce changes in the electronic state and a concomitant modification of the ferroelectric energy landscape.…”

“…Additional important insights into the electronic and magnetic modifications at manganiteferroelectric interfaces have been gained as well by local atomic characterization using highresolution scanning transmission electron microscopy (HR-STEM), particularly in combination with atomically resolved electron energy loss spectroscopy (EELS), by following the evolution of the energy loss spectra as a function of distance from the interface (although interpretation of such variations in terms of changes in valency is challenging). 28,144,971,972 In addition, other important information, such as local atomic spacing and oxygen octahedra rotations, can be further related to other properties, such as crystalline structure, oxygen stoichiometry, and ferroelectric polarization; 973,974 further, by combining STEM and EELS, the specific interface stacking at the interface from one material to the other can be determined. 162 For example, while XANES measurements demonstrated a change in the valence state in the LSMO/PZT interface, as discussed above, they do not allow one to determine precisely the charge distribution across the interface.…”

Asymmetric interfaces and high-TC ferromagnetic phase in La0.67Ca0.33MnO3/SrRuO3 superlattices

“…Correspondingly, this sample shows a much lower TC. Note that the interfacial charge transfer-induced doping effect can easily span over few nanometers from the heterointerfaces [66][67][68]. Therefore, although our LCMO/SRO SLs consist of both interfacial terminations, all of them exhibit single ferromagnetic transitions with the higher TC only.…”