arXiv subjects

W. S. Choi

Publications and source records attributed to W. S. Choi.

14 recordsLinked to original sources

arXiv2025

Anisotropic Strain Relaxation-Induced Directional Ultrafast Carrier Dynamics in RuO2 Films

Ultrafast light-matter interactions inspire potential functionalities in picosecond optoelectronic applications. However, achieving directional carrier dynamics in metals remains challenging due to strong carrier scattering within a multiband environment, typically expected to isotropic carrier relaxation. In this study, we demonstrate epitaxial RuO2/TiO2 (110) heterostructures grown by hybrid molecular beam epitaxy to engineer polarization-selectivity of ultrafast light-matter interactions via anisotropic strain engineering. Combining spectroscopic ellipsometry, X-ray absorption spectroscopy, and optical pump-probe spectroscopy, we revealed the strong anisotropic transient optoelectronic response of strain-engineered RuO2/TiO2 (110) heterostructures along both in-plane [001] and [1-10] crystallographic directions. Theoretical analysis identifies strain-induced modifications in band nesting as the underlying mechanism for enhanced anisotropic carrier relaxation. These findings establish epitaxial strain engineering as a powerful tool for tuning anisotropic optoelectronic responses in metallic systems, paving the way for next-generation polarization-sensitive ultrafast optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2010

Control of defect-mediated tunneling barrier heights in ultrathin MgO films

The impact of oxygen vacancies on local tunneling properties across rf-sputtered MgO thin films was investigated by optical absorption spectroscopy and conducting atomic force microscopy. Adding O$_2$ to the Ar plasma during MgO growth alters the oxygen defect populations, leading to improved local tunneling characteristics such as a lower density of current hotspots and a lower tunnel current amplitude. We discuss a defect-based potential landscape across ultrathin MgO barriers.

cond-mat.mtrl-sci↗

arXiv2009

Two-Dimensional Confinement of 3d1 Electrons in LaTiO3/LaAlO3 Multilayers

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n = 1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d^1 state of the Ti ions. In combination with LDA+U calculations, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t_{2g} states leaving only the planar d_xy orbitals occupied. We outline that these multilayers can serve as a model system for the study of the t_{2g} 2D Hubbard model.

cond-mat.str-el↗

arXiv2009

The electronic structure of epitaxially stabilized 5d perovskite Ca_{1-x}Sr_xIrO_3 (x = 0, 0.5, and 1) thin films: the role of strong spin-orbit coupling

We have investigated the electronic structure of meta-stable perovskite Ca1-xSrxIrO3 (x = 0, 0.5, and 1) thin films using transport measurements, optical spectroscopy, and first-principles calculations. We artificially fabricated the perovskite phase of Ca1-xSrxIrO3, which has a hexagonal or post perovskite crystal structure in bulk form, by growing epitaxial thin films on perovskite GdScO3 substrates using epi-stabilization technique. The transport properties of the perovskite Ca1-xSrxIrO3 films systematically changed from nearly insulating (or semi-metallic) for x = 0 to bad metallic for x = 1. Due to the extended wavefunctions, 5d electrons are usually delocalized. However, the strong spin-orbit coupling in Ca1-xSrxIrO3 results in the formation of effective total angular momentum Jeff = 1/2 and 3/2 states, which puts Ca1-xSrxIrO3 in the vicinity of a metal-insulator phase boundary. As a result, the electrical properties of the Ca1-xSrxIrO3 films are found to be sensitive to x and strain.

cond-mat.str-el↗

arXiv2009

Optical spectroscopic investigation on the temperature-dependent electronic structure evolution of the Jeff,1/2 Mott insulator Sr2IrO4

We investigated the temperature-dependent evolution of the electronic structure of the Jeff,1/2 Mott insulator Sr2IrO4 using optical spectroscopy. The optical conductivity spectra $σ(ω)$ of this compound has recently been found to exhibit two d-d transitions associated with the transition between the Jeff,1/2 and Jeff,3/2 bands due to the cooperation of the electron correlation and spin-orbit coupling. As the temperature increases, the two peaks show significant changes resulting in a decrease in the Mott gap. The experimental observations are compared with the results of first-principles calculation in consideration of increasing bandwidth. We discuss the effect of the temperature change on the electronic structure of Sr2IrO4 in terms of local lattice distortion, excitonic effect, electron-phonon coupling, and magnetic ordering.

cond-mat.str-el↗

arXiv2009

Dual Character of Magnetism in Ferropnictides: Insights from Optical Measurements

We investigate the electronic structure of EuFe$_{2}$As$_{2}$ using optical spectroscopy and first-principles calculations. At low temperature we observe the evolution of \textit{two} gap-like features, one having a BCS mean-field behavior and another with strongly non-BCS behavior. Using band structure calculations, we identify the former with a spin-Peierls-like partial gap in $d_{yz}$ bands, and the latter with the transition across the large exchange gap in $d_{xz}/d_{xy}$ bands. Our results demonstrate that the antiferromagnetism in the ferropnictides is neither fully local nor fully itinerant, but contains elements of both.

cond-mat.supr-con↗

arXiv2009

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

We have found that there is more than one type of conducting carriers generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low-density. Since the low-temperature mobilities determined by dc-transport in the LaAlO3/SrTiO3 heterostructures are much higher than those extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.

cond-mat.str-el↗

arXiv2008

Localized electronic states induced by defects and possible origin of ferroelectricity in strontium titanate thin films

Several defect configurations including oxygen vacancies have been investigated as possible origins of the reported room-temperature ferroelectricity of strontium titanate (STO) thin films [Appl. Phys. Letts. 91, 042908 (2007)]. First-principles calculations revealed that the Sr-O-O vacancy complexes create deep localized states in the band gap of SrTiO3 without affecting its insulating property. These results are in agreement with electronic structural changes determined from optical transmission and X-ray absorption measurements. This work opens the way to exploiting oxygen vacancies and their complexes as a source of ferroelectricity in perovskite oxide thin films, including STO.

cond-mat.mtrl-sci↗

arXiv2008

Bandwidth-Controlled Insulator-Metal Transition and Correlated Metallic State in 5$d$ Transition Metal Oxides Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$)

We investigated the electronic structures of the 5$d$ Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$) using optical spectroscopy and first-principles calculations. As 5$d$ orbitals are spatially more extended than 3$d$ or 4$d$ orbitals, it has been widely accepted that correlation effects are minimal in 5$d$ compounds. However, we observed a bandwidth-controlled transition from a Mott insulator to a metal as we increased $n$. In addition, the artificially synthesized perovskite SrIrO$_{3}$ showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.

cond-mat.str-el↗

arXiv2008

Optical Study of the Free Carrier Response of LaTiO3/SrTiO3 Superlattices

We used infrared spectroscopic ellipsometry to investigate the electronic properties of LaTiO3/SrTiO3 superlattices (SLs). Our results indicated that, independent of the SL periodicity and individual layer-thickness, the SLs exhibited a Drude metallic response with sheet carrier density per interface ~3x10^14 cm^-2. This is probably due to the leakage of d-electrons at interfaces from the Mott insulator LaTiO3 to the band insulator SrTiO3. We observed a carrier relaxation time ~ 35 fs and mobility ~ 35 cm^2V^-1s^-1 at 10 K, and an unusual temperature dependence of carrier density that was attributed to the dielectric screening of quantum paraelectric SrTiO3.

cond-mat.str-el↗

arXiv2007

Composition-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(Zr_{x}Ti_{1-x})O_{3} thin films

We investigated the time-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1-x)O3 thin films with various Zr concentrations. We could explain all the polarization switching behaviors well by assuming Lorentzian distributions in the logarithmic polarization switching time [Refer to J. Y. Jo et al., Phys. Rev. Lett. (in press)]. Based on this analysis, we found that the Zr ion-substitution for Ti ions would induce broad distributions in the local field due to defect dipoles, which makes the ferroelectric domain switching occur more easily.

cond-mat.mtrl-sci↗

arXiv2007

Electronic structures of hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films

We investigated the electronic structure of multiferroic hexagonal RMnO3 (R = Gd, Tb, Dy, and Ho) thin films using both optical spectroscopy and first-principles calculations. Using artificially stabilized hexagonal RMnO3, we extended the optical spectroscopic studies on the hexagonal multiferroic manganite system. We observed two optical transitions located near 1.7 eV and 2.3 eV, in addition to the predominant absorption above 5 eV. With the help of first-principles calculations, we attribute the low-lying optical absorption peaks to inter-site transitions from the oxygen states hybridized strongly with different Mn orbital symmetries to the Mn 3d3z2-r2 state. As the ionic radius of the rare earth ion increased, the lowest peak showed a systematic increase in its peak position. We explained this systematic change in terms of a flattening of the MnO5 triangular bipyramid.

cond-mat.str-el↗

arXiv2007

Orbital-Driven Electronic Structure Changes and the Resulting Optical Anisotropy of the Quasi-Two-Dimensional Spin Gap Compound La4Ru2O10

We investigated the electronic response of the quasi-two-dimensional spin gap compound La4Ru2O10 using optical spectroscopy. We observed drastic changes in the optical spectra as the temperature decreased, resulting in anisotropy in the electronic structure of the spin-singlet ground state. Using the orbital-dependent hopping analysis, we found that orbital ordering plays a crucial role in forming the spin gap state in the non-one-dimensional material.

cond-mat.str-el↗

arXiv2006

Dielectric constants of Ir, Ru, Pt, and IrO2: Contributions from bound charges

We investigated the dielectric functions $ε$($ω$) of Ir, Ru, Pt, and IrO$_2$, which are commonly used as electrodes in ferroelectric thin film applications. In particular, we investigated the contributions from bound charges $ε^{b}$($ω$), since these are important scientifically as well as technologically: the $ε_1^{b}$(0) of a metal electrode is one of the major factors determining the depolarization field inside a ferroelectric capacitor. To obtain $ε_1^{b}$(0), we measured reflectivity spectra of sputtered Pt, Ir, Ru, and IrO2 films in a wide photon energy range between 3.7 meV and 20 eV. We used a Kramers-Kronig transformation to obtain real and imaginary dielectric functions, and then used Drude-Lorentz oscillator fittings to extract $ε_1^{b}$(0) values. Ir, Ru, Pt, and IrO$_2$ produced experimental $ε_1^{b}$(0) values of 48$\pm$10, 82$\pm$10, 58$\pm$10, and 29$\pm$5, respectively, which are in good agreement with values obtained using first-principles calculations. These values are much higher than those for noble metals such as Cu, Ag, and Au because transition metals and IrO$_2$ have such strong d-d transitions below 2.0 eV. High $ε_1^{b}$(0) values will reduce the depolarization field in ferroelectric capacitors, making these materials good candidates for use as electrodes in ferroelectric applications.

cond-mat.str-el↗