Strongly correlated oxides with a broken symmetry could exhibit various phase transitions, such as superconductivity, magnetism and ferroelectricity. Simultaneous break of time-reversal and space-inversion symmetries may bring about emergent quantum phenomena and functionalities in strongly correlated oxides, such as topologically non-trivial states, magnetoelctric effect, etc. However, most of the oxide materials hold a complicated phase in crystal structure, electronic structure, orbital structure or even spin structures. Those promising quantum states can only occur in very limited materials under very critical conditions, restricting their practical applications. In this presentation, I will share with you the strategy for the symmetry design in correlated oxides. Antiferromagnetic Ruddlesden-Popper Sr2IrO4 and perovskite paraelectric (ferroelectric) SrTiO3 (BaTiO3) are selected to epitaxially fabricate superlattices for symmetry engineering. An emergent magnetoelectric phase transition is achieved in Sr2IrO4/SrTiO3 superlattices with artificially designed ferroelectricity, where an observable interfacial Dzyaloshinskii-Moriya interaction driven by non-equivalent interface is considered as the microscopic origin. By further increasing the polarization namely interfacial Dzyaloshinskii-Moriya interaction via replacing SrTiO3 with BaTiO3 and constructing three-component Sr2IrO4/SrTiO3/BaTiO3 superlattices, the transition temperature can be enhanced from 46 K to 290 K, accompanying a pronounced magnetoelectric coefficient of ~500 mV/cm·Oe. This interfacial engineering of Dzyaloshinskii-Moriya interaction provides a strategy to design quantum phases and orderings in correlated electron systems.

Interest in ferroelectric materials for processing, memory and sensing devices has been spurred by the discovery of nanoscale ferroelectricity in insulating binary oxides based on hafnia and zirconia. However, stabilizing the ferroelectric phase and achieving good ferroelectric performance in these materials is challenging because several non-ferroelectric phases have similar formation energy to the ferroelectric phase. The search for binary ferroelectrics which can be achieved by industry-friendly processes is therefore still ongoing. Here, we report the deposition of epitaxial tungsten trioxide films at temperatures below 400°C using a chemical atmospheric process. In these films, strain imposed by the substrate promotes the formation of the low-temperature polar phase at room temperatures, giving rise to switchable polarization, evidenced by Raman spectroscopy and piezo-response force microscopy, and supported by ab initio calculations. Exploring ferroelectricity in ultrathin perovskite films could provide a new platform for polarization-controlled memory and photo-ferroelectric applications.

Relaxor ferroelectrics and antiferroelectrics exhibit favorable properties for capacitive energy storage, including a large recoverable energy density and a high electrical efficiency. While the optimally performing material compositions for energy storage contain lead, recent research has demonstrated the possibility of utilizing Ag(Nb,Ta)O₃ (ANT) as a lead-free substitute. ANT is a solid solution containing AgNbO₃ and AgTaO₃, whereby the former exhibits antiferroelectricity and the latter exhibits relaxor-like properties. Thus, the B site cation ratio enables the modulation of non-linear behavior in ANT. Studies in polycrystalline powder systems have demonstrated a recoverable energy density of 6.3 J/cm³ and an efficiency of 90% at room temperature for an optimized composition of Ag(Nb₀.₅Ta₀.₅)O₃. ANT is a complex material system, with the A site cation exhibiting significantly higher volatility relative to the B site, which results in Ag deficiency for ANT films grown by pulsed laser deposition (PLD). It has recently been grown by two research groups; however, the chemical composition was not investigated. Here, we present for the first time the optimization of coherent growth for ANT thin films by pulsed laser deposition on oxide substrates with biaxial strain values up to 2.4%. We demonstrate the insights obtained during in situ monitoring of the evolution of the growth mode (high-pressure reflection high-energy electron diffraction, RHEED) and stress (multi-beam optical stress sensor, MOSS). This data is complemented by the ex situ analysis utilizing X-ray diffractometry, electron microscopy and chemical analyses, correlating the real-time information to the structural and chemical properties. We further discuss the role of Ag excess in the target and its influence on the film composition towards achieving stoichiometric ANT thin films with a robust relaxor antiferroelectric response. Finally, we discuss the possibilities for dielectric and ferroelectric properties in strain-engineered ANT films in the ultra-thin regime.