arXiv subjects

Deok-Yong Cho

Publications and source records attributed to Deok-Yong Cho.

18 recordsLinked to original sources

arXiv2025

Strain Engineering of van Hove Singularity and Coupled Itinerant Ferromagnetism in Quasi-2D Oxide Superlattices

Engineering van Hove singularities (vHss) near the Fermi level, if feasible, offers a powerful route to control exotic quantum phases in electronic and magnetic behaviors. However, conventional approaches, which rely primarily on chemical and electrical doping, focus mainly on local electrical or optical measurements, limiting their applicability to coupled functionalities. In this study, a vHs-induced insulator-metal transition coupled with a ferromagnetic phase transition was empirically achieved in atomically designed quasi-2D SrRuO3 (SRO) superlattices via epitaxial strain engineering, which has not been observed in conventional 3D SRO systems. Theoretical calculations revealed that epitaxial strain effectively modulates the strength and energy positions of vHs of specific Ru orbitals, driving correlated phase transitions in the electronic and magnetic ground states. X-ray absorption spectroscopy confirmed the anisotropic electronic structure of quasi-2D SRO modulated by epitaxial strain. Magneto-optic Kerr effect and electrical transport measurements demonstrated modulated magnetic and electronic phases. Furthermore, magneto-electrical measurements detected significant anomalous Hall effect signals and ferromagnetic magnetoresistance, indicating the presence of magnetically coupled charge carriers in the 2D metallic regime. This study establishes strain engineering as a promising platform for tuning vHss and resultant itinerant ferromagnetism of low-dimensional correlated quantum systems.

cond-mat.mtrl-sci↗

arXiv2025

Chemical States and Local Structure in Cu-Deficient CuInSe2 Thin Films: Insights into Engineering and Bandgap Narrowing

The Cu-deficient CuxInSe2 (x larger than 0.3) phase can be stabilized as a thin film. A uniform Cu-deficient composition with a chalcopyrite structure was obtained by the precision engineering of a two-step synthesis process involving electron-beam evaporation and Se vapor deposition. Detailed structural and chemical analyses were performed employing various X-ray and microscopic techniques to demonstrate that the chemical states and local structure in the Cu-Se-In tetrahedral networks change with the loss of Cu, the In-Se bond becomes shorter, and the In ions become excessively oxidized without phase separation. Moreover, the results indicate that the bandgap narrowing is primarily attributed to the reconstruction of In3+d 5s orbital states. The bandgap narrows from 1.51 eV to 1.4 eV, which is optimal for the photon absorber. Therefore, cation-deficient selenide is promising for stable nontoxic photovoltaics with tunable bandgaps.

cond-mat.mtrl-sci↗

arXiv2024

Dimensionality Engineering of Magnetic Anisotropy from Anomalous Hall Effect in Synthetic SrRuO3 Crystals

Magnetic anisotropy in atomically thin correlated heterostructures is essential for exploring quantum magnetic phases for next-generation spintronics. Whereas previous studies have mostly focused on van der Waals systems, here, we investigate the impact of dimensionality of epitaxially-grown correlated oxides down to the monolayer limit on structural, magnetic, and orbital anisotropies. By designing oxide superlattices with a correlated ferromagnetic SrRuO3 and nonmagnetic SrTiO3 layers, we observed modulated ferromagnetic behavior with the change of the SrRuO3 thickness. Especially, for three-unit-cell-thick layers, we observe a significant 1,500% improvement of coercive field in the anomalous Hall effect, which cannot be solely attributed to the dimensional crossover in ferromagnetism. The atomic-scale heterostructures further reveal the systematic modulation of anisotropy for the lattice structure and orbital hybridization, explaining the enhanced magnetic anisotropy. Our findings provide valuable insights into engineering the anisotropic hybridization of synthetic magnetic crystals, offering a tunable spin order for various applications.

cond-mat.mtrl-sci↗

arXiv2023

An unconventional platform for two-dimensional Kagome flat bands on semiconductor surfaces

In condensed matter physics, the Kagome lattice and its inherent flat bands have attracted considerable attention for their potential to host a variety of exotic physical phenomena. Despite extensive efforts to fabricate thin films of Kagome materials aimed at modulating the flat bands through electrostatic gating or strain manipulation, progress has been limited. Here, we report the observation of a novel $d$-orbital hybridized Kagome-derived flat band in Ag/Si(111) $\sqrt{3}\times\sqrt{3}$ as revealed by angle-resolved photoemission spectroscopy. Our findings indicate that silver atoms on a silicon substrate form a Kagome-like structure, where a delicate balance in the hopping parameters of the in-plane $d$-orbitals leads to destructive interference, resulting in a flat band. These results not only introduce a new platform for Kagome physics but also illuminate the potential for integrating metal-semiconductor interfaces into Kagome-related research, thereby opening a new avenue for exploring ideal two-dimensional Kagome systems.

cond-mat.mtrl-sci↗

arXiv2023

Suppression of Antiferromagnetic Order by Strain in Honeycomb Cobaltate: Implication for Quantum Spin Liquid

Recently, layered honeycomb cobaltates have been predicted as a new promising system for realizing the Kitaev quantum spin liquid, a many-body quantum entangled ground state characterized by fractional excitations. However, these cobaltates, similar to other candidate materials, exhibit classical antiferromagnetic ordering at low temperatures, which impedes the formation of the expected quantum state. Here, we demonstrate that the control of the trigonal crystal field of Co ions is crucial to suppress classical antiferromagnetic ordering and to locate its ground state in closer vicinity to quantum spin liquid in layered honeycomb cobaltates. By utilizing heterostructure engineering on Cu3Co2SbO6 thin films, we adjust the trigonal distortion of CoO6 octahedra and the associated trigonal crystal field. The original Néel temperature of 16 K in bulk Cu3Co2SbO6 decreases (increases) to 7.8 K (22.7 K) in strained Cu3Co2SbO6 films by decreasing (increasing) the magnitude of the trigonal crystal fields. Our experimental finding substantiates the potential of layered honeycomb cobaltate heterostructures and strain engineering to accomplish the extremely elusive quantum phase of matter.

cond-mat.str-el↗

arXiv2023

Atomic and Electronic Structures of Correlated SrRuO3/SrTiO3 Superlattices

Atomic-scale precision epitaxy of perovskite oxide superlattices provides unique opportunities for controlling the correlated electronic structures, activating effective control knobs for intriguing functionalities including electromagnetic, thermoelectric, and electrocatalytic behaviors. In this study, we investigated the close interplay between the atomic and electronic structures of correlated superlattices synthesized by atomic-scale precision epitaxy. In particular, we employ superlattices composed of correlated magnetic SrRuO3 (SRO) and quantum paraelectric SrTiO3 (STO) layers. In those superlattices, RuO6 octahedral distortion is systematically controlled from 167 to 175 degrees depending on the thickness of the STO layers, also affecting the TiO6 octahedral distortion within the STO layer. Customized octahedral distortion within SRO/STO superlattices in turn modifies the electronic structures of both the Ti and Ru compounds, observed by X-ray absorption spectroscopy. Our results identify the close correlation between atomic lattice and electronic structures enabled by the facile controllability of atomic-scale epitaxy, which would be useful for designing future correlated oxide devices.

cond-mat.mtrl-sci↗

arXiv2021

Direct evidence of electronic interaction at the atomic-layer-deposited MoS2 monolayer/SiO2 interface

The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab-initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.

cond-mat.mtrl-sci↗

arXiv2021

Doping effects on the valence bond solid of Li$_{2}$RuO$_{3}$ with Mn substitution

$Li_{2}RuO_{3}$ with a honeycomb structure undergoes a drastic transition from a regular honeycomb lattice with the $C2/m$ space group to a valence bond solid state of the $P2_{1}/m$ space group with an extremely strong dimerization at 550 K. We synthesized $Li_{2}Ru_{1-x}Mn_{x}O_{3}$ with a full solid solution and investigated doping effects on the valence bond solid state as a function of Mn content. The valence bond solid state is found to be stable up to $x = 0.2$, based on our extensive experiments: structural studies, resistivity, and magnetic susceptibility. On the other hand, the extended x-ray absorption fine structure analyses show that the dimer local structure remains robust even above $x = 0.2$ with a minimal effect on the dimer bond length. This indicates that the locally-disordered dimer structure survives well into the Mn-rich phase even though the thermodynamically stable average structure has the $C2/m$ space group. Our results prove that the dimer formation in $Li_{2}RuO_{3}$ is predominantly a local phenomenon driven by the formation of orbitally-assisted metal-metal bonds and that these dimers are relatively robust against doping-induced disorder.

cond-mat.str-el↗

arXiv2021

Pressure-induced transition from Jeff=1/2 to S=1/2 states in CuAl2O4

The spin-orbit entangled (SOE) Jeff-state has been a fertile ground to study novel quantum phenomena. Contrary to the conventional weakly correlated Jeff=1/2 state of 4d and 5d transition metal compounds, the ground state of CuAl2O4 hosts a Jeff=1/2 state with a strong correlation of Coulomb U. Here, we report that surprisingly Cu2+ ions of CuAl2O4 overcome the otherwise usually strong Jahn-Teller distortion and instead stabilize the SOE state, although the cuprate has relatively small spin-orbit coupling. From the x-ray absorption spectroscopy and high-pressure x-ray diffraction studies, we obtained definite evidence of the Jeff=1/2 state with a cubic lattice at ambient pressure. We also found the pressure-induced structural transition to a compressed tetragonal lattice consisting of the spin-only S=1/2 state for pressure higher than Pc=8 GPa. This phase transition from the Mott insulating Jeff=1/2 to the S=1/2 states is a unique phenomenon and has not been reported before. Our study offers a rare example of the SOE Jeff-state under strong electron correlation and its pressure-induced transition to the S=1/2 state.

cond-mat.str-el↗

arXiv2020

Propagation control of octahedral tilt in SrRuO3 via artificial heterostructuring

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilting of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, we propose a method to selectively engineer the octahedral bonding geometries, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering has been developed using atomically designed SrRuO3/SrTiO3 superlattices. In particular, the propagation of RuO6 octahedral tilting within the SrRuO3 layers having identical thicknesses was systematically controlled by varying the thickness of adjacent SrTiO3 layers. This led to a substantial modification in the electromagnetic properties of the SrRuO3 layer, significantly enhancing the magnetic moment of Ru. Our approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties.

cond-mat.str-el↗

arXiv2018

Ferroelectric polarization rotation in order-disorder-type LiNbO3 thin films

The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure. Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in order-disorder type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction.

physics.app-ph↗

arXiv2017

Self-Assembling Oxide Catalyst for Electrochemical Water Splitting

Renewable energy conversion and storage, and greenhouse gas emission-free technologies are within the primary tasks and challenges for the society. Hydrogen fuel, produced by alkaline water electrolysis is fulfilling all these demands, however the technology is economically feeble, limited by the slow rate of oxygen evolution reaction. Complex metal oxides were suggested to overcome this problem being low-cost efficient catalysts. However, the insufficient long-term stability, degradation of structure and electrocatalytic activity are restricting their utilization. Here we report on a new perovskite-based self-assembling material BaCo0.98Ti0.02O3-$δ$:Co3O4 with superior performance, showing outstanding properties compared to current state-of-the-art materials without degeneration of its properties even at 353 K. By chemical and structural analysis the degradation mechanism was identified and modified by selective doping. Short-range order and chemical composition rather than long-range order are factors determining the outstanding performance. The derived general design rules can be used for further development of oxide-based electrocatalytic materials.

cond-mat.mtrl-sci↗

arXiv2016

Robust singlet dimers with fragile ordering in two-dimensional honeycomb lattice of Li$_2$RuO$_3$

When an electronic system has strong correlations and a large spin-orbit interaction, it often exhibits a plethora of mutually competing quantum phases. How a particular quantum ground state is selected out of several possibilities is a very interesting question. However, equally fascinating is how such a quantum entangled state breaks up due to perturbation. This important question has relevance in very diverse fields of science from strongly correlated electron physics to quantum information. Here we report that a quantum entangled dimerized state or valence bond crystal (VBC) phase of Li2RuO3 shows nontrivial doping dependence as we perturb the Ru honeycomb lattice by replacing Ru with Li. Through extensive experimental studies, we demonstrate that the VBC phase melts into a valence bond liquid phase of the RVB (resonating valence bond) type. This system offers an interesting playground where one can test and refine our current understanding of the quantum competing phases in a single compound.

cond-mat.str-el↗

arXiv2014

Dimensionality Control of d-orbital Occupation in Oxide Superlattices

Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t2g and eg multiplets in quasi-twodimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO3 and a band insulator LaAlO3. As the LaCoO3 sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t2g and eg orbitals are partially filled, to a band insulator by completely filling (emptying) the t2g (eg) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter.

cond-mat.str-el↗

arXiv2014

Phonon-assisted optical excitation in the narrow bandgap Mott insulator Sr3Ir2O7

We examined the temperature (T) evolution of the optical conductivity spectra of Sr$_3$Ir$_2$O$_7$ over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of $\sim$0.1 eV. The low-energy features of the optical d-d excitation (${\hbar}ω$ $<$ 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge X-ray absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead, presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involve, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr$_3$Ir$_2$O$_7$.

cond-mat.str-el↗

arXiv2013

Large in-plane deformation of RuO6 octahedron and ferromagnetism of bulk SrRuO3

SrRuO3 is a ferromagnetic metal with several unusual physical properties such as zero thermal expansion below Tc, so-called Invar behavior. Another anomalous feature is that the a-axis lattice constant is larger than the b-axis lattice constant, a clear deviation from the predictions of the Glazer structural description with rigid RuO6 octahedron motion. Using high resolution neutron diffraction techniques, we show how these two structural anomalies arise from the irregular in-plane deformation, i.e. plastic behavior of the RuO6 octahedron, a weak band Jahn-Teller distortion. We further demonstrate that the ferromagnetic instability of SrRuO3 is related to the temperature-induced localization of Ru 4d bands.

cond-mat.str-el↗

arXiv2013

Temperature Evolution of Itinerant Ferromagnetism in SrRuO3 Probed by Optical Spectroscopy

The temperature ($T$) dependence of the optical conductivity spectra $σ(ω)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$ range from 5 to 450 K. We observed significant $T$ dependence of the spectral weights of the charge transfer and interband $d$-$d$ transitions across the ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. $T$ scans of $σ(Ω, T)$ at fixed frequencies $Ω$ reveal a clear $T^2$ dependence below $T_c$, demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, $σ(Ω, T)$ continues to evolve at temperatures above $T_c$, indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO$_3$ is an intriguing oxide system with itinerant ferromagnetism.

cond-mat.str-el↗

arXiv2007

Influence of oxygen vacancy on the electronic structure of HfO$_2$ film

We investigated the unoccupied part of the electronic structure of the oxygen-deficient hafnium oxide (HfO$_{\sim1.8}$) using soft x-ray absorption spectroscopy at O $K$ and Hf $N_3$ edges. Band-tail states beneath the unoccupied Hf 5$d$ band are observed in the O $K$-edge spectra; combined with ultraviolet photoemission spectrum, this indicates the non-negligible occupation of Hf 5$d$ state. However, Hf $N_3$-edge magnetic circular dichroism spectrum reveals the absence of a long-range ferromagnetic spin order in the oxide. Thus the small amount of $d$ electron gained by the vacancy formation does not show inter-site correlation, contrary to a recent report [M. Venkatesan {\it et al.}, Nature {\bf 430}, 630 (2004)].

cond-mat.mtrl-sci↗