Growth and Electronic Structure of Copper Oxide Monolayer Epitaxial Films.

Copper-based high-temperature superconductors share a common feature in their crystal structure, which is the presence of a CuO2 plane, where superconductivity takes place. Therefore, important questions arise as to whether superconductivity can exist in a single layer of the CuO2 plane and, if so, how such superconductivity in a single CuO2 plane differs from that in a bulk cuprate system. To answer these questions, studies of the superconductivity in cuprate monolayers are necessary. In this study, we constructed a heterostructure system with a La2-xSrxCuO4 (LSCO) monolayer containing a single CuO2 plane and measured the resulting electronic structures. Monolayer LSCO has metallic and bulk-like electronic structures. The hole doping ratio of the monolayer LSCO is found to depend on the underlying buffer layer due to the interface effect. Our work will provide a platform for research into ideal two-dimensional cuprate systems.

Extended Oxygen Octahedral Tilt Proximity near Oxide Heterostructures.

The oxide interfaces between materials with different structural symmetries have been actively studied due to their novel physical properties. However, the investigation of intriguing interfacial phenomena caused by the oxygen octahedral tilt (OOT) proximity effect has not been fully exploited, as there is still no clear understanding of what determines the proximity length and what the underlying control mechanism is. Here, we achieved scalability of the OOT proximity effect in SrRuO3 (SRO) by epitaxial strain near the SRO/SrTiO3 heterointerface. We demonstrated that the OOT proximity length scale of SRO is extended from 4 unit cells to 14 unit cells by employing advanced scanning transmission electron microscopy. We also suggest that this variation may originate from changes in phonon dispersions due to electron-phonon coupling in SRO. This study will provide in-depth insights into the structural gradients of correlated systems and facilitate potential device applications.

Defect Engineering in A2 BO4 Thin Films via Surface-Reconstructed LaSrAlO4 Substrates.

Ruddlesden-Popper oxides (A2 BO4 ) have attracted significant attention regarding their potential application in novel electronic and energy devices. However, practical uses of A2 BO4 thin films have been limited by extended defects such as out-of-phase boundaries (OPBs). OPBs disrupt the layered structure of A2 BO4 , which restricts functionality. OPBs are ubiquitous in A2 BO4 thin films but inhomogeneous interfaces make them difficult to suppress. Here, OPBs in A2 BO4 thin films are suppressed using a novel method to control the substrate surface termination. To demonstrate the technique, epitaxial thin films of cuprate superconductor La2- x Srx CuO4 (x = 0.15) are grown on surface-reconstructed LaSrAlO4 substrates, which are terminated with self-limited perovskite double layers. To date, La2- x Srx CuO4 thin films are grown on LaSrAlO4 substrates with mixed-termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La2- x Srx CuO4 thin films grown on surface-reconstructed LaSrAlO4 substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB-suppressed La2- x Srx CuO4 thin films exhibit significantly enhanced superconducting properties compared with OPB-containing La2- x Srx CuO4 thin films. Defect engineering in A2 BO4 thin films will allow for the elimination of various types of defects in other complex oxides and facilitate next-generation quantum device applications.

Overcharge-Induced Phase Heterogeneity and Resultant Twin-Like Layer Deformation in Lithium Cobalt Oxide Cathode for Lithium-Ion Batteries.

Overcharging is expected to be one of the solutions to overcome the current energy density limitation of lithium-ion battery cathodes, which will support the rapid growth of the battery market. However, high-voltage charging often poses a major safety threat including fatal incendiary incidents, limiting further application. Numerous researches are dedicated to the disadvantages of the overcharging process; nonetheless, the urgent demand for addressing failure mechanisms is still unfulfilled. Herein, it is revealed that overcharging induces phase heterogeneity into layered and cobalt oxide phases, and consequent "twin-like deformation" in lithium cobalt oxide. The interplay between the uncommon cobalt(III) oxide and the deformation is investigated by revealing the atomistic formation mechanism. Most importantly, abnormal cracking is discovered in the vicinity of the cobalt oxide where structural instability induces substantial contraction. In addition, surface degradation is widely observed in the crack boundary inside the particle. As unintentional overcharging can occur due to local imbalance in state-of-charge in severe operating conditions such as fast charging, the issues on overcharging should be emphasized to large extent and this study provides fundamental knowledge of overcharge by elucidating the crack development mechanism of layered cathodes, which is expected to broaden the horizon into high voltage operation.