Woo Seok 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 epitax

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

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 epitaxy allow

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:

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

We investigated the magnetic and optical properties of [$({\mathrm{LaMnO}}_{3}){}_{n}$/$({\mathrm{SrTiO}}_{3}){}_{8}$]${}_{20}$ ($n$ $=$ 1, 2, and 8) superlattices grown by pulsed-laser deposition. We found that a weak ferromagnetic and semiconducting state developed in all superlattices. An analysis of the optical conductivity showed that the ${\mathrm{LaMnO}}_{3}$ layers in the superlattices were slightly doped. The amount of doping was almost identical regardless of the ${\mathrm{LaMnO}}_{3}$

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 (SrRuO₃)-nonmagnetic insulator (SrTiO₃) heterostructures. Owing to the unconventional interlayer exchange interaction, w

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

We investigated the effects of temperature and magnetic field on the electronic structure of hexagonal $R{\text{MnO}}_{3}$ ($R=\text{Gd}$, Tb, Dy, and Ho) thin films using optical spectroscopy. As the magnetic ordering of the system was disturbed, a systematic change in the electronic structure was identified in this series. The optical-absorption peak near 1.7 eV showed an unexpectedly large shift of more than 150 meV from 300 to 15 K, accompanied by an anomaly of the shift at the N\'eel temper

Using optical spectroscopy, we investigated the electrodynamic properties of $\text{Nb}:{\text{SrTiO}}_{3}/{\text{SrTiO}}_{3}$ superlattices. In these superlattices, a large enhancement of the Seebeck coefficient $(S)$ has been reported with decreasing $\text{Nb}:{\text{SrTiO}}_{3}$ layer thickness [H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. Wi