Hyoungjeen Jeen

Transition metal oxides exhibit a variety of physical properties that are tuned by external stimuli such as electric and magnetic fields, mechanical stress, and the formation of heterointerfaces. Oxide superlattices provide a useful system for amplifying the interfacial effects. These effects often become more pronounced with decreasing thickness of the individual layer and increasing number of interfaces. In this study, we investigated the changes in the magnetic properties of cation‐modulated manganite superlattices fabricated from a single manganite target by varying the oxygen partial pressure, resulting in the modulation of cation stoichiometry. Using superconducting quantum interference device (SQUID) magnetometry, we found that the magnetization in superlattices of cation‐modulated manganites increased by up to 36.8 ± 0.4% relative to the volume‐weighted average of the corresponding single‐layer films. Polarized neutron reflectometry further revealed a larger average magnetic response in the shorter‐period superlattice, consistent with the trend observed by SQUID magnetometry. This observation is consistent with the interface‐related magnetic interactions between the layers. These findings point to an interface‐driven route to enhance magnetization in cation‐modulated manganite superlattices and show that the effect is tunable by altering the number of interfaces.

We report the kinetics of the topotactic brownmillerite–perovskite phase transition in epitaxial strontium cobaltite (SrCoO _x ) thin films. Our findings reveal asymmetric switching kinetics between the oxygen‐deficient SrCoO _2.5 and oxygen‐rich SrCoO ₃ phases, driven by the epitaxial strain state. Using phase‐field modeling, we demonstrate that nucleation and growth dynamics are governed by local oxygen concentration‐dependent diffusivity and surface oxygen flux, both of which are strain‐dependent. The perovskite phase exhibits higher strain tunability of these parameters compared to the brownmillerite phase, explaining the divergent kinetic pathways upon changes in epitaxial strain. Specifically, transitions on (LaAlO ₃ ) _0.3 (Sr ₂ TaAlO ₆ ) _0.7 (001) substrates are abrupt and nucleation‐dominated, characteristic of order–disorder transformations. Conversely, transitions on SrTiO ₃ (001) substrates are spatially uniform, with phase propagation originating from the film interface, characteristic of diffuse transformations. These results provide a framework for controlling topotactic transformation rates through strain engineering in functional oxides.

Active control of heat transport in solids is a long-standing challenge for thermal management and phononic technologies. Materials capable of reversibly altering their crystal structures through oxygen exchange provide a promising route to dynamically tune multiple physical properties. Here, we demonstrate electrochemically driven switching of optical, electrical, and thermal properties in epitaxial films of the oxygen-sponge cobaltite PrBaCo2O5+x. This topotactic oxide accommodates large and reversible changes in oxygen content, enabling structural transformations between reduced and oxidized states. X-ray absorption and optical spectroscopy reveal that the Co valence state can be reversibly tuned between +2/+3 and +3/+4 via thermal and electrochemical redox reactions. These transformations induce dramatic changes in physical properties: the electrical resistivity varies by 6 orders of magnitude, from ∼104 Ω cm in the reduced state to ∼10–2 Ω cm in the oxidized state, while the thermal conductivity switches from ∼0.8 to ∼2.7 W m–1 K–1, corresponding to a modulation ratio of ∼3.4. Because the electronic contribution to heat transport is negligible, the thermal switching originates from oxygen-driven structural transformations that modify phonon transport. Our results establish oxygen-sponge cobaltites as a material platform for electrochemically programmable control of heat, charge, and light, opening an opportunity for solid-state thermal switches and adaptive thermal-management technologies.

Even though epitaxy is synonymous with order, its practical realization often depends on the availability of surface sites that locally break ideal flatness, such as steps and kinks, where adatoms can be efficiently incorporated. This challenge is well-known for homoepitaxial growth of thermodynamically stable monoclinic gallium oxide (β-Ga2O3) on (100) substrates, which in the as-cleaved exhibit large-area, atomically flat terraces with a low density of step edges. To address this problem, we propose a self-organized process triggered by annealing that subdivides large-area terraces into narrower domains, with substeps corresponding to approximately half the unit cell height. Using such substep-templated substrates, we achieved a high growth rate of ∼2.83 μm h–1, exceeding values reported for (100) β-Ga2O3 homoepitaxy in the literature, while maintaining reasonable structural quality and electronic functionality. By placing this substep-templating approach in the context of other nucleation-enhancing strategies for β-Ga2O3 homoepitaxy, we conclude that it provides a viable route to mitigate limitations associated with higher surface-energy planes or off-axis miscuts. Altogether, these results demonstrate a practical strategy for improving β-Ga2O3 homoepitaxy for advanced device applications.

Oxygen-vacancy engineering in transition metal oxides enables programmable functionalities by modulating the valence states and local coordination of constituents. Here, we report the selective reduction of cobalt ions in epitaxial SrFe_0.5Co_0.5O_2.5 thin films under reducing gas environments, while iron ions remain unchanged. X-ray absorption spectroscopy reveals an absorption edge shift of 1.65 eV in the Co L-edge upon reduction, and multiplet simulations estimate a decrease in the average Co valence from Co^2.91+ to Co^2.00+. This site- and element-specific reduction leads to the formation of a structurally distinct oxygen-deficient phase stabilized by oxygen vacancies at tetrahedral sites, as confirmed by density functional theory. Optical spectroscopy reveals an increase in the bandgap from 2.47 eV to 3.04 eV, accompanied by enhanced transparency. Furthermore, simultaneous in situ diffraction and transport measurements confirm fully reversible redox-driven transitions among three phases: reduced defective perovskite, brownmillerite, and oxygen-rich perovskite phases. These findings demonstrate that selective redox control in multi-cation oxides enables the realization of chemically and functionally distinct oxygen-deficient phases.

All-insulator heterostructures with an emerging metallicity are at the forefront of material science, which typically contain at least one band insulator while it is not necessary to be. Here we show emergent phenomena in a series of all-correlated-insulator heterostructures that composed of insulating CaIrO₃ and insulating La_0.67Sr_0.33MnO₃. We observed an intriguing insulator-to-metal transition, that depends delicately on the thickness of the iridate component. The simultaneous enhancements of magnetization, electric conductivity, and magnetoresistance effect indicate a percolation-type nature of the insulator-to-metal transition, with the percolation threshold can be reached at an exceptionally low volume fraction of the iridate. Such a drastic transition is induced by an interfacial charge transfer, which interestingly alters the electronic and crystalline structures of the bulk region rather than the limited ultrathin interface. We further showcased the central role of effective correlation in modulating the insulator-to-metal transition, by demonstrating that the critical thickness of iridate for triggering the metallic state can be systematically reduced down to a single unit-cell layer.