Wooseok Choi

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeO x epitaxial thin f

The electronic structure of a strongly correlated SrRuO₃epitaxial thin film influences the electrocatalytic activity significantly.

We investigated the electronic structure of multiferroic hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$ ($R=\mathrm{Gd}$, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. One of the difficulties in explaining the electronic structures of hexagonal $R\mathrm{Mn}{\mathrm{O}}_{3}$ is that they exist in nature with limited rare earth ions (i.e., $R=\mathrm{Sc}$, Y, and Ho-Lu), so a systematic study in terms of the different $R$ ions has been lacking. Recently,

Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoO(x) (x=2.5-3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to the oxygen content x, are found in the two topotactic phases: i.e., the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First-principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transiti

Abstract Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidation front in single‐crystal Cu thin film is demonstrated to achieve a full‐color spectrum for Cu by precisely controlling its oxide‐layer thickness. Grain‐boundary‐free and atomically flat films prepared by atomic‐sputtering epit

We investigated the dielectric functions $\stackrel{\fontencoding{LECO}\selectfont\char177{}}{\ensuremath{\epsilon}}(\ensuremath{\omega})$ of Ir, Ru, Pt, and ${\mathrm{IrO}}_{2}$, which are commonly used as electrodes in ferroelectric thin-film applications. In particular, we investigated the contributions from bound charges ${\stackrel{\fontencoding{LECO}\selectfont\char177{}}{\ensuremath{\epsilon}}}^{b}(\ensuremath{\omega})$, since these are important scientifically as well as technologically:

Defect engineering has brought about a unique level of control for Si-based semiconductors, leading to the optimization of various opto-electronic properties and devices. With regard to perovskite transition metal oxides, O vacancies have been a key ingredient in defect engineering, as they play a central role in determining the crystal field and consequent electronic structure, leading to important electronic and magnetic phase transitions. Therefore, experimental approaches toward understandin

Transition-metal oxides (TMOs) with brownmillerite (BM) structures possess one-dimensional oxygen vacancy channels (OVCs), which play a key role in realizing high ionic conduction at low temperatures. The controllability of the vacancy channel orientation, thus, possesses a great potential for practical applications and would provide a better visualization of the diffusion pathways of ions in TMOs. In this study, the orientations of the OVCs in BM-SrFeO 2.5 are stabilized along two crystallograp

Atomic layer engineering enables fabrication of a chemically sharp oxide heterointerface. The interface formation and strain evolution during the initial growth of LaAlO(3) /SrTiO(3) heterostructures by pulsed laser deposition are investigated in search of a means for controlling the atomic-sharpness of the interface. This study shows that inserting a monolayer of LaAlO(3) grown at high oxygen pressure dramatically enhances interface abruptness.

Resonant tunnelling is a quantum mechanical process that has long been attracting both scientific and technological attention owing to its intriguing underlying physics and unique applications for high-speed electronics. The materials system exhibiting resonant tunnelling, however, has been largely limited to the conventional semiconductors, partially due to their excellent crystalline quality. Here we show that a deliberately designed transition metal oxide superlattice exhibits a resonant tunn

We investigated the effects of annealing on LaMnO$_3$ epitaxial thin films grown by pulsed laser deposition and propose an efficient method of characterizing their stoichiometry. Structural, magnetic, and optical properties coherently indicate non-stoichiometric ferromagnetic and semiconducting phases for as-grown LaMnO$_3$ films. By annealing in an oxygen-reducing atmosphere, we recovered the antiferromagnetic and insulating phases of bulk-like stoichiometric LaMnO$_3$. We show that non-destruc

Abstract The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO 3 , graphene/LaAlO 3 /SrTiO 3 , graphene/ferroelectric oxide, MoS 2 /SrTiO 3 , and FeSe/SrTiO 3 heterostructures include voltage scaling in field‐effect transistors, charge state coupling across an interface, quantum conductanc

Chiral symmetry breaking of phonons plays an essential role in emergent quantum phenomena owing to its strong coupling to spin degree of freedom. However, direct experimental evidence of the chiral phonon–spin coupling is lacking. In this study, we report a chiral phonon–mediated interlayer exchange interaction in atomically controlled ferromagnetic metal (SrRuO 3 )–nonmagnetic insulator (SrTiO 3 ) heterostructures. Owing to the unconventional interlayer exchange interaction, we have observed ro

The electrodynamic properties of La‐doped SrTiO 3 thin films with controlled elemental vacancies are investigated using optical spectroscopy and thermopower measurement. In particular, a correlation between the polaron formation and thermoelectric properties of the transition metal oxide (TMO) thin films is observed. With decreasing oxygen partial pressure during the film growth ( P (O 2 )), a systematic lattice expansion is observed along with the increased elemental vacancy and carrier density