Byungmin Sohn

Sungkyunkwan University

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Changyoung Kim

Institute for Basic Science

Tae Won Noh

Seoul National University

Donghan Kim

Seoul National University

Bongju Kim

Seoul National University Dental Hospital

Sungsoo Hahn

Institute for Basic Science

Minsoo Kim

Institute for Basic Science

Youngdo Kim

Samsung (South Korea)

Wonshik Kyung

Institute for Basic Science

Younsik Kim

Hanyang University

Choong H. Kim

Institute for Basic Science

Cooperative Institutions

Seoul National University

Institute for Basic Science

Yonsei University

Korea Advanced Institute of Science and Technology

Yale University

Pohang University of Science and Technology

Lawrence Berkeley National Laboratory

Korea Basic Science Institute

Sungkyunkwan University

The University of Tokyo

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Direct evidence of orbital-selective confinement effect of Ru 4d orbitals in SrRuO 3 ultrathin film

APS (2019)

Soonmin Kang Yi Tseng Beom Hyun Kim Seokhwan Yun Byungmin Sohn

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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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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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Intrinsic tunable anomalous Hall effect induced by momentum-space Berry curvature in SrRuO 3

Bulletin of the American Physical Society (2020)

Byungmin Sohn Eunwoo Lee Ji Seop Oh Wonshik Kyung Hanyoung Yoo

Momentum (technical analysis)

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Observation of metallic electronic structure in a single-atomic-layer oxide

Byungmin Sohn Jeong Rae Kim Choong H. Kim Sangmin Lee Sungsoo Hahn

Correlated electrons in transition metal oxides (TMOs) exhibit a variety of emergent phases. When TMOs are confined to a single-atomic-layer thickness, experiments so far have shown that they usually lose diverse properties and become insulators. In an attempt to extend the range of electronic phases of the single-atomic-layer oxide, we search for a metallic phase in a monolayer-thick epitaxial SrRuO$_3$ film. Combining atomic-scale epitaxy and angle-resolved photoemission measurements, we show that the monolayer SrRuO$_3$ is a strongly correlated metal. Systematic investigation reveals that the interplay between dimensionality and electronic correlation makes the monolayer SrRuO$_3$ an incoherent metal with orbital-selective correlation. Furthermore, the unique electronic phase of the monolayer SrRuO$_3$ is found to be highly tunable, as charge modulation demonstrates an incoherent-to-coherent crossover of the two-dimensional metal. Our work emphasizes the potentially rich phases of single-atomic-layer oxides and provides a guide to the manipulation of their two-dimensional correlated electron systems.

Atomic units

Electronic correlation

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Electronic characteristics of ultrathin SrRuO$_3$ films and their relationship with the metal$-$insulator transition

arXiv (Cornell University) (2017)

Subeen Pang Yoonkoo Kim Yeong Jae Shin Byungmin Sohn SeungYong Lee

SrRuO$_3$ (SRO) films are known to exhibit insulating behavior as their thickness approaches four unit cells. We employ electron energy$-$loss (EEL) spectroscopy to probe the spatially resolved electronic structures of both insulating and conducting SRO to correlate them with the metal$-$insulator transition (MIT). Importantly, the central layer of the ultrathin insulating film exhibits distinct features from the metallic SRO. Moreover, EEL near edge spectra adjacent to the SrTiO$_3$ (STO) substrate or to the capping layer are remarkably similar to those of STO. The site$-$projected density of states based on density functional theory (DFT) partially reflects the characteristics of the spectra of these layers. These results may provide important information on the possible influence of STO on the electronic states of ultrathin SRO.

Metal–insulator transition

10.48550/arxiv.1712.05137

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Topological Hall Effect in Ultra-thin SrRuO 3 film

Bulletin of the American Physical Society (2018)

Byungmin Sohn Bongju Kim Young-Jae Choi Tae Won Noh Changyoung Kim

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Sign-tunable anomalous Hall effect induced by two-dimensional symmetry-protected nodal structures in ferromagnetic perovskite oxide thin films

arXiv (Cornell University) (2019)

Byungmin Sohn Eunwoo Lee Se Young Park Wonshik Kyung Jinwoong Hwang

Magnetism and spin-orbit coupling (SOC) are two quintessential ingredients underlying novel topological transport phenomena in itinerant ferromagnets. When spin-polarized bands support nodal points/lines with band degeneracy that can be lifted by SOC, the nodal structures become a source of Berry curvature; this leads to a large anomalous Hall effect (AHE). Contrary to three-dimensional systems that naturally host nodal points/lines, two-dimensional (2D) systems can possess stable nodal structures only when proper crystalline symmetry exists. Here we show that 2D spin-polarized band structures of perovskite oxides generally support symmetry-protected nodal lines and points that govern both the sign and the magnitude of the AHE. To demonstrate this, we performed angle-resolved photoemission studies of ultrathin films of SrRuO$_3$, a representative metallic ferromagnet with SOC. We show that the sign-changing AHE upon variation in the film thickness, magnetization, and chemical potential can be well explained by theoretical models. Our study is the first to directly characterize the topological band structure of 2D spin-polarized bands and the corresponding AHE, which could facilitate new switchable devices based on ferromagnetic ultrathin films.

Magnetism

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Capping and gate control of anomalous Hall effect and hump structure in ultra-thin SrRuO3 films

Applied Physics Letters (2021)

Donghan Kim Byungmin Sohn Minsoo Kim Sungsoo Hahn Youngdo Kim

Ferromagnetism and exotic topological structures in SrRuO$_3$ (SRO) induce sign-changing anomalous Hall effect (AHE). Recently, hump structures have been reported in the Hall resistivity of SRO thin films, especially in the ultra-thin regime. We investigate the AHE and hump structure in the Hall resistivity of SRO ultra-thin films with an SrTiO$_3$ (STO) capping layer and ionic liquid gating. STO capping results in sign changes in the AHE and modulation of the hump structure. In particular, the hump structure in the Hall resistivity is strongly modulated and even vanishes in STO-capped 4 unit cell (uc) films. In addition, the conductivity of STO-capped SRO ultra-thin films is greatly enhanced with restored ferromagnetism. We also performed ionic liquid gating to modulate the electric field at SRO/STO interface. Drastic changes in the AHE and hump structure are observed with different gate voltages. Our study shows that the hump structure as well as the AHE can be controlled by tuning inversion symmetry and the electric field at the interface.

10.1063/5.0050163

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Deep learning-based statistical noise reduction for multidimensional spectral data

Review of Scientific Instruments (2021)

Younsik Kim Dongjin Oh Soonsang Huh Dongjoon Song Sunbeom Jeong

In spectroscopic experiments, data acquisition in multi-dimensional phase space may require long acquisition time, owing to the large phase space volume to be covered. In such a case, the limited time available for data acquisition can be a serious constraint for experiments in which multidimensional spectral data are acquired. Here, taking angle-resolved photoemission spectroscopy (ARPES) as an example, we demonstrate a denoising method that utilizes deep learning as an intelligent way to overcome the constraint. With readily available ARPES data and random generation of training datasets, we successfully trained the denoising neural network without overfitting. The denoising neural network can remove the noise in the data while preserving its intrinsic information. We show that the denoising neural network allows us to perform a similar level of second-derivative and line shape analysis on data taken with two orders of magnitude less acquisition time. The importance of our method lies in its applicability to any multidimensional spectral data that are susceptible to statistical noise.

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10.1063/5.0054920

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Citations (32)