Octahedral Distortions at High‐Temperature Superconducting La₂CuO₄ Interfaces: Visualizing Jahn–Teller Effects

Abstract

properties of complex perovskite oxide structures are strongly influenced by small structural changes of the BO 6 octahedral network. [2][3][4][5] Impressive examples are heterostructure interfaces for which even small alterations of the octahedral network can give rise to striking changes in the electronic structure [6,7] and induce functional properties such as quantum Hall effect, [8] magnetism, [9] superconductivity, [10,11] ferroelectricity, [12] the formation of 2D free electron gases, [13] and hightemperature interfacial superconductivity (HT-IS). [14] During the last years, these interfacial phenomena have attracted more and more attention not only due to potential applications but also due to the fundamental physics and chemistry aspects of these material systems. [15] For example, tailoring and tuning lattice distortions can be achieved by strain states imposed, e.g., by the lattice mismatch between substrate and film in epitaxial systems. [3,[16][17][18][19] Thus, understanding how octahedral distortions are correlated with dopant distribution and how they modify the functionality of complex oxide heterostructures is of great significance.The Jahn-Teller (JT) effect is a spontaneous geometric distortion of a nonlinear system in an electronic state with orbital degeneracy and results in removing the degeneracy via energy state splitting and lowering its symmetry. [20] Transition metal oxides with octahedral coordination, in which the metal ion has high-spin d 4 , low-spin d 7 , and d 9 configurations, show JT distortions, which can be intimately correlated with the physical properties. [21] For instance, the CuO 6 octahedron in the parent La 2 CuO 4 (LCO) is elongated along the "apical direction" by the JT effect [22] and exhibits two long and four short CuO bonds. [22,23] Distortions of CuO 6 in Sr-doped La 2−x Sr x CuO 4 structures (x up to 0.4) and related changes in CuO apical distances were studied via diffraction techniques providing average structural information. [24] It is reported that, in such systems, increasing the dopant concentration determines the compression of the octahedron (i.e., a decrease of the CuO apical distances) [24] defined as anti-Jahn-Teller (AJT) effect. [22] Recently, it was shown that in the presence of interfacial electronic redistribution leading to a decoupled concentration profile between electron holes and dopant ions (striking interface

High-temperature superconductivity at the lanthanum cuprate/lanthanum–strontium nickelate interface

“…As a matter of fact, certain intermixing is often observed, but its width and its effect on the system properties (being arguably dependent on the deposition process) are in many cases still under debate. 31,[45][46][47] Our case is particularly interesting since it exhibits an asymmetric behavior for the two interfaces, whose intermixing width depends on the layer sequence. Such a finding has been highlighted in the case of epitaxial semiconducting structures (cf.…”

Design of Complex Oxide Interfaces by Oxide Molecular Beam Epitaxy

“…In addition to above-mentioned research, we further utilized the advantages of the layer-by-layer MBE technique. Bauitti et al [105] reported a new approach for the design of epitaxially grown HTS of cuprates and showed that when twodimensional doping, i.e., δ-doping, which is the substitution of a selected La-O layer with a Sr-O layer during the epitaxial growth, is applied to the LCO matrix, an asymmetric The La 2 CuO 4 structure forms a pseudo-tetragonal structure when grown on LaSrAlO 4 substrates [92,93] distribution of the dopant is present [105,106]. For such type of doping, the asymmetric dopant distribution profile generates two different types of HTS mechanisms at each side of the interface.…”

“…5c) turns into a conventionally doped LCO system and most of the layers were found to be superconducting. Furthermore, when the local octahedral distortions, i.e., apical O-O interatomic distances, are quantitatively investigated and associated with the dopant distributions, the correlation with local Jahn-Teller distortions is enlightened [93]. While in the case of sharp interfaces (i.e., Ca and Sr doping), the O-O interatomic distances increase sharply after the nominal M-I interface, anti-Jahn-Teller distortions dominates the structure for Ba doping due to the wide Ba distribution leading to a reduction of the apical O-O interatomic distances [109].…”

“…Chemical pressure effects on M-I bilayers[76,93]. a Transport measurements reveal that all samples, i.e., Sr-(green), Ba-(red), and Ca-doped (blue) M-I bilayers exhibit HTS.…”

Precise control of atoms with MBE: from semiconductors to complex oxides

“…Thus, interface engineering is an intriguing yet challenging task and the atomically precise design of complex oxide heterostructures requires atomic-resolution identification of individual layers and interfaces (Figure 3). The prominent role is played by aberration-corrected scanning transition electron microscopy (STEM) with high-resolution imaging and spectroscopy capabilities [14,15]. STEM high-angle annular dark field (HAADF) images of three different La 2 CuO 4 -based heterostructures are presented in Figure 3a-c [6] displaying the ideally arranged crystal structure without any extended defects.…”