Wonshik Kyung

Institute for Basic Science

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Changyoung Kim

Institute for Basic Science

Jonathan D. Denlinger

Lawrence Berkeley National Laboratory

Garam Han

ETH Zurich

Yoshiyuki Yoshida

National Institute of Advanced Industrial Science and Technology

Junyoung Kwon

Pohang University of Science and Technology

Minsoo Kim

Institute for Basic Science

Dongjoon Song

National Institute of Advanced Industrial Science and Technology

Yeongkwan Kim

Korea Advanced Institute of Science and Technology

Soonsang Huh

Seoul National University

Beomyoung Kim

Kootenay Association for Science & Technology

Cooperative Institutions

Seoul National University

Institute for Basic Science

Yonsei University

Lawrence Berkeley National Laboratory

Pohang University of Science and Technology

Sungkyunkwan University

Korea Advanced Institute of Science and Technology

Government of the Republic of Korea

Samsung (South Korea)

The University of Tokyo

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Strong One-Dimensional Characteristics of Hole-Carriers in ReS2 and ReSe2

Scientific Reports (2019)

Beom Seok Kim Wonshik Kyung Jonathan D. Denlinger Changyoung Kim S. R. Park

Each plane of layered ReS2 and ReSe2 materials has 1D chain structure, from which intriguing properties such as 1D character of the exciton states and linearly polarized photoluminescence originate. However, systematic studies on the 1D character of charge carriers have not been done yet. Here, we report on systematic and comparative studies on the energy-momentum dispersion relationships of layered transition metal dichalcogenides ReS2 and ReSe2 by angle resolved photoemission. We found that the valence band maximum or the minimum energy for holes is located at the high symmetric Z-point for both materials. However, the out-of-plane ([Formula: see text]) dispersion for ReSe2 (20 meV) is found to be much smaller than that of ReS2 (150 meV). We observe that the effective mass of the hole carriers along the direction perpendicular to the chain is about 4 times larger than that along the chain direction for both ReS2 and ReSe2. Remarkably, the experimentally measured hole effective mass is about twice heavier than that from first principles calculation for ReS2 although the in-plane anisotropy values from the experiment and calculations are comparable. These observation indicate that bulk ReS2 and ReSe2 are unique semiconducting transition metal dichalcogenides having strong one-dimensional characters.

10.1038/s41598-019-39540-4

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Spin fluctuations in Sr1.8La0.2RuO4

Physical review. B./Physical review. B (2023)

Zheng He Qisi Wang Yu Feng Chul Kim Wonshik Kyung

We use inelastic neutron scattering to study spin fluctuations in Sr$_{1.8}$La$_{0.2}$RuO$_4$, where Lanthanum doping triggers a Lifshitz transition by pushing the van Hove singularity in the $\gamma$ band to the Fermi energy. Strong spin fluctuations emerge at an incommensurate wave vector $\mathbf{Q}_{ic} = (0.3,0.3)$, corresponding to the nesting vector between $\alpha$ and $\beta$ Fermi sheets. The incommensurate antiferromagnetic fluctuations shift toward $(0.25,0.25)$ with increasing energy up to ${\sim}110$ meV. By contrast, scatterings near the ferromagnetic wave vectors $\mathbf{Q} = (1,0)$ and $(1,1)$ remain featureless at all energies. This contradicts the weak-coupling perspective that suggests a sharp enhancement of ferromagnetic susceptibility due to the divergence of density of states in the associated $\gamma$ band. Our findings imply that ferromagnetic fluctuations in Sr$_2$RuO$_4$ and related materials do not fit into the weak-coupling paradigm, but instead are quasi-local fluctuations induced by Hund's coupling. This imposes significant constraints for the pairing mechanism involving spin fluctuations.

10.1103/physrevb.107.l201107

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Electric field induced octahedral rotation and its controllability

Bulletin of the American Physical Society (2017)

Wonshik Kyung Kim Choong-Hyun Yeongkwan Kim Beomyoung Kim Chul Kim

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Possible electric field induced indirect to direct band gap transition in MoSe 2

APS (2017)

Beom Seo Kim Wonshik Kyung Jeongjin Seo Junyoung Kwon Changyoung Kim

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Enhanced superconductivity in surface-electron-doped iron pnictide Ba 2 As 2

Wonshik Kyung Soonsang Huh Youngwoo Koh Kwanseok Choi M. Nakajima

Pnictogen

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Relativistic Quasi-particle Nature Emerging in semi-metallic Sr 2 IrO 4 .

Bulletin of the American Physical Society (2019)

Jeong Woo Han Sun Woo Kim Wonshik Kyung Changyoung Kim Gang Cao

Particle (ecology)

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Observation of Spin-Dependent Dual Ferromagnetism in Perovskite Ruthenates

Physical Review Letters (2021)

Sungsoo Hahn Byungmin Sohn Minjae Kim Jeong Rae Kim Soonsang Huh

We performed in situ angle-resolved photoemission spectroscopy (ARPES) and spin-resolved ARPES (SARPES) experiments to investigate the relationship between electronic band structures and ferromagnetism in SrRuO_{3} (SRO) thin films. Our high quality ARPES and SARPES results show clear spin-lifted band structures. The spin polarization is strongly dependent on momentum around the Fermi level, whereas it becomes less dependent at high-binding energies. This experimental observation matches our dynamical mean-field theory results very well. As temperature increases from low to the Curie temperature, spin-splitting gap decreases and band dispersions become incoherent. Based on the ARPES study and theoretical calculation results, we found that SRO possesses spin-dependent electron correlations in which majority and minority spins are localized and itinerant, respectively. Our finding explains how ferromagnetism and electronic structure are connected, which has been under debate for decades in SRO.

10.1103/physrevlett.127.256401

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Microscopic mechanism for asymmetric charge distribution in Rashba-type surface states and the origin of the energy splitting scale

Physical Review B (2013)

Beomyoung Kim Panjin Kim Wonsig Jung Yeongkwan Kim Yoonyoung Koh

The microscopic mechanism for Rashba-type band splitting is examined in detail. We show how an asymmetric charge distribution is formed when the local orbital angular momentum (OAM) and crystal momentum get interlocked due to surface effects. An electrostatic energy term in the Hamiltonian appears when such an OAM- and crystal-momentum-dependent asymmetric charge distribution is placed in an electric field produced by inversion-symmetry breaking. Analysis by using an effective Hamiltonian shows that, as the atomic spin-orbit coupling (SOC) strength increases from weak to strong, the originally OAM-quenched states evolve into well-defined chiral OAM states and then to states of total angular momentum $J$. In addition, the energy scale of the band splitting changes from the atomic SOC energy to electrostatic energy. To confirm the validity of the model, we study OAM and spin structures of the Au(111) system by using an effective Hamiltonian for the $d$-orbital case. As for the strong-SOC regime, we choose Bi${}_{2}$Te${}_{2}$Se as a prototype system. We performed circular dichroism angle-resolved photoemission spectroscopy experiments as well as first-principles calculations. We find that the effective model can explain various aspects of the spin and OAM structures of the system.

Point reflection

Hamiltonian (control theory)

Rashba effect

10.1103/physrevb.88.205408

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Electronic band structure of (111) SrRuO3 thin films: An angle-resolved photoemission spectroscopy study

Physical review. B./Physical review. B (2020)

Hanyoung Ryu Y. Ishida Bongju Kim Jeong Rae Kim Woo Jin Kim

We studied the electronic band structure of pulsed laser deposition (PLD) grown (111)-oriented ${\mathrm{SrRuO}}_{3}$ thin films using in situ angle-resolved photoemission spectroscopy technique. We observed light bands with a renormalized quasiparticle effective mass of about $0.8{m}_{e}$. The electron-phonon coupling underlying this mass renormalization yields a characteristic ``kink'' in the band dispersion. The self-energy analysis using the Einstein model suggests five optical phonon modes covering an energy range of 44--90 meV contribute to the coupling. In addition, we show that the quasiparticle spectral intensity at the Fermi level is considerably suppressed, and two prominent peaks appear in the valance band spectrum at binding energies of 0.8 and 1.4 eV, respectively. We discuss the possible implications of these observations. Overall, our work demonstrates that high-quality thin films of oxides with large spin-orbit coupling can be grown along the polar (111) orientation by the PLD technique, enabling in situ electronic band structure study. This could allow for characterizing the thickness-dependent evolution of band structure of (111) heterostructures---a prerequisite for exploring possible topological quantum states in the bilayer limit.

10.1103/physrevb.102.041102

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Unveiling unconventional magnetism at the surface of Sr 2 RuO 4 .

R. Fittipaldi Richard Hartmann Maria Teresa Mercaldo Sachio Komori Anders V. Bjørlig

Magnetism

Rock magnetism

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