Keehyun Ahn

Abstract Applications of correlated vanadium dioxides VO 2 (A) and VO 2 (B) in electrical devices are limited due to the lack of effective methods for tuning their fundamental properties. We find that the resistivity of VO 2 (A) and VO 2 (B) is widely tunable by doping them with tungsten ions. When x < 0.1 in V 1 −x W x O 2 (A), the resistivity decreases drastically by four orders of magnitude with increasing x , while that of V 1 −x W x O 2 (B) shows the opposite behaviour. Using spectroscop

Ferromagnetic insulators have great potential for spintronic applications. For such applications, it is essential to find materials with a robust and controllable ferromagnetic insulating phase. However, because ferromagnetism in functional transition-metal oxides is usually coupled to metallicity, ferromagnetic insulators are very rare and independent control of their magnetic and electrical properties is difficult. In this study, the electrical, magnetic, and optical properties of ${(\mathrm{L

Abstract We report on infrared spectroscopy experiments on the electronic response in (Sr 1− x La x ) 2 IrO 4 ( x = 0, 0.021, and 0.067). Our data show that electron doping induced by La substitution leads to an insulator-to-metal transition. The evolution of the electronic structure across the transition reveals the robustness of the strong electronic correlations against the electron doping. The conductivity data of the metallic compound show the signature of the pseudogap that bears close sim

We report on infrared spectroscopic studies of the electronic response of the (Sr1-xLax)3Ir2O7 system. Our experiments revealed hallmarks of strong electronic correlations in the evolution of the electronic response across the filling-controlled insulator-metal transition. We observed a collapse of the Jeff = 1/2 Mott gap accompanying the transfer of the spectral weight from the high-energy region to the gap region with electron doping. The intraband conductivity at the metallic side of the tran

In this work we investigate single crystal $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$, a promising candidate material for Floquet engineering and magnetic control, using ultrafast optical pump-probe reflectivity and magneto-optical Kerr spectroscopy. $\mathrm{Gd}\mathrm{Ti}{\mathrm{O}}_{3}$ is a Mott-Hubbard insulator with a ferrimagnetic and orbitally ordered ground state (${T}_{C}$ = 32 K). We observe multiple signatures of the magnetic phase transition in the photoinduced reflectivity signal, i

Non-Fermi-liquid behavior of correlated metals is one of the central problems in condensed matter physics. To advance the understanding of the non-Fermi-liquid phase, it is crucial to know the intrinsic electronic response of correlated metals. The authors study the optical response of an archetypal correlated metal SrVO${}_{3}$ in the ultraclean limit. The results reveal that interband transitions can give rise to a significant contribution to the low-energy electronic response, mimicking the n

We report a combined infrared and angle-resolved photoemission study of the electronic response of $\mathrm{S}{\mathrm{r}}_{3}{(\mathrm{I}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x})}_{2}{\mathrm{O}}_{7}$ ($x=0$, 0.22, 0.34). The low-temperature optical conductivities of the three compounds exhibit the characteristic feature of the effective total angular momentum ${J}_{\mathrm{eff}}=1/2$ antiferromagnetic Mott state. As the temperature increases across the antiferromagnetic orderi

The topochemical transformation of single crystals of ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ into ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}{\mathrm{F}}_{2}$ is reported via fluorine insertion. Characterization of the newly formed ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}{\mathrm{F}}_{2}$ phase shows a nearly complete oxidation of ${\mathrm{Ir}}^{4+}$ cations into ${\mathrm{Ir}}^{5+}$ that in turn drives the system from an antiferromagnetic Mott insulator with a

Abstract We report on optical spectroscopic study of the Sr 3 (Ir 1- x Ru x ) 2 O 7 system over a wide doping regime. We find that the changes in the electronic structure occur in the limited range of the concentration of Ru ions where the insulator–metal transition occurs. In the insulating regime, the electronic structure associated with the effective total angular momentum J eff = 1/2 Mott state remains robust against Ru doping, indicating the localization of the doped holes. Upon entering th

Abstract We investigated the doping and temperature evolutions of the optical response of Sr 3 (Ir 1− x Mn x ) 2 O 7 single crystals with 0 ≤ x ≤ 0.36 by utilizing infrared spectroscopy. Substitution of 3 d transition metal Mn ions into Sr 3 Ir 2 O 7 is expected to induce an insulator-to-metal transition via the decrease in the magnitude of the spin–orbit coupling and the hole doping. In sharp contrast, our data reveal the resilience of the spin–orbit coupling and the incoherent character of the

Abstract We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO 2 thin films. For the a b -plane, an optical peak near Ω ≈ 750 cm −1 drives the scattering rate 1/ τ ( ω ) and effective mass m * ( ω ) of the Drude carrier to increase and decrease respectively for ω ≧ Ω. For the c -axis, a longitudinal optical phonon (LO) is present at Ω as evidenced by a peak in the loss function Im[−1/ ε c ( ω )]. Further polarized measurements in different light pr

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Abstract We performed polarized reflection and transmission measurements on the layered conducting oxide PdCoO 2 thin films. For the ab-plane, an optical peak near Ω ≈ 750 cm−1 drives the scattering rate γ ∗ (ω) and effective mass m ∗ (ω) of the Drude carrier to increase and decrease respectively for ω ≧ Ω. For the c-axis, a longitudinal optical phonon (LO) is present at Ω as evidenced by a peak in the loss function Im[−1/ε c (ω)]. Further polarized measurements in different light propagation (q

We performed polarized reflection and transmission measurements on the layered conducting oxide $\rm PdCoO_2$ thin films. For the ab-plane, an optical peak near $Ω$ $\approx$ 750 cm$^{-1}$ drives the scattering rate $γ^{*}(ω)$ and effective mass $m^{*}(ω)$ of the Drude carrier to increase and decrease respectively for $ω$ $\geqq$ $Ω$. For the c-axis, a longitudinal optical phonon (LO) is present at $Ω$ as evidenced by a peak in the loss function Im[$-1/\varepsilon_{c}(ω)$]. Further polarized mea

(연구배경 및 목적) 어린이 교육 및 발달을 촉진하는 장소로서 어린이 전시 공간은 어린이의 감각경험과 전반적인 인지 발달을 위한 경험과 놀이를 제공한다. 특히, 어린이에게 신체적 행위를 동반하는 감각경험은 어린이의 창의성 발달과 공간인지능력의 성장에 지대한 영향을 미칠 수 있다. 이러한 관점에서, 감각체험을 위한 전시공간 디자인은 어린이의 건강한 성장과 발달, 자유로운 사고와 비판적 해결능력을 촉진시키는 주요한 요소로서 디자인에 적극적으로 반영되어야 한다. 이에 본 연구에서는 어린이 발달 단계와 행동에 따른 선행이론들을 고찰하고, 국내 어린이 전시공간의 사례분석을 통해, 어린이 전시공간에 감각체험을 최적화하고 어린이의 전인적 발달을 촉진하기 위한 전시공간 디자인을 제안하는 것을 목적으로 한다. (연구방법) 본 디자인 제안의 방법은 선행이론 고찰을 통해 어린이 인지발달 과정과 특성, 그리고 이에 따른 공간 행위 특징을 분석하고, 감각체험 공간 디자인 요소들을 도출하였다. 도출된 감각체