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1. Bulk transport paths through defects in floating zone and Al flux grown SmB$_6$

Author

Eo, Yun Suk, Rakoski, Alexa, Sinha, Shriya, Mihaliov, Dmitri, Fuhrman, Wesley T., Saha, Shanta R., Rosa, Priscila F. S., Fisk, Zachary, Hatnean, Monica Ciomaga, Balakrishnan, Geetha, Chamorro, Juan R., Koohpayeh, Seyed M., McQueen, Tyrel M., Kang, Boyoun, Song, Myung-suk, Cho, Beongki, Fuhrer, Michael S., Paglione, Johnpierre, and Kurdak, Cagliyan

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

We investigate the roles of disorder on low-temperature transport in SmB$_6$ crystals grown by both the Al flux and floating zone methods. We used the inverted resistance method with Corbino geometry to investigate whether low-temperature variations in the standard resistance plateau arises from a surface or a bulk channel in floating zone samples. The results show significant sample-dependent residual bulk conduction, in contrast to smaller amounts of residual bulk conduction previously observed in Al flux grown samples with Sm vacancies. We consider hopping in an activated impurity band as a possible source for the observed bulk conduction, but it is unlikely that the large residual bulk conduction seen in floating zone samples is solely due to Sm vacancies. We therefore propose that one-dimensional defects, or dislocations, contribute as well. Using chemical etching, we find evidence for dislocations in both flux and floating zone samples, with higher dislocation density in floating zone samples than in Al flux grown samples. In addition to the possibility of transport through one-dimensional dislocations, we also discuss our results in the context of recent theoretical models of SmB$_6$., Comment: 22 pages, 4 figures

Published
2020
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2. Definitive Surface Magnetotransport Study of SmB$_{6}$

Author

Eo, Yun Suk, Wolgast, Steven, Rakoski, Alexa, Mihaliov, Dmitri, Kang, Boyoun, Song, Myung-suk, Cho, Beongki, Hatnean, Monica Ciomaga, Balakrishnan, Geetha, Fisk, Zachary, Saha, Shanta, Wang, Xiangfeng, Paglione, Johnpierre, and Kurdak, Cagliyan

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Strongly Correlated Electrons
Abstract

After the theoretical prediction that SmB$_6$ is a topological Kondo insulator, there has been an explosion of studies on the SmB$_6$ surface. However, there is not yet an agreement on even the most basic quantities such as the surface carrier density and mobility. In this paper, we carefully revisit Corbino disk magnetotransport studies to find those surface transport parameters. We first show that subsurface cracks exist in the SmB$_6$ crystals, arising both from surface preparation and during the crystal growth. We provide evidence that these hidden subsurface cracks are additional conduction channels, and the large disagreement between earlier surface SmB$_6$ studies may originate from previous interpretations not taking this extra conduction path into account. We provide an update of a more reliable magnetotransport data than the previous one (Phys. Rev. B 92, 115110) and find that the orders-of-magnitude large disagreements in carrier density and mobility come from the surface preparation and the transport geometry rather than the intrinsic sample quality. From this magnetotransport study, we find an updated estimate of the carrier density and mobility of 2.71$\times$10$^{13}$ (1/cm$^2$) and 104.5 (cm$^{2}$/V$\cdot$sec), respectively. We compare our results with other studies of the SmB$_6$ surface. By this comparison, we provide insight into the disagreements and agreements of the previously reported angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and magnetotorque quantum oscillations measurements.

Published
2019
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3. Investigation of high-temperature bulk transport characteristics and skew scattering in samarium hexaboride

Author

Rakoski, Alexa, Eo, Yun Suk, Kurdak, Çağlıyan, Kang, Boyoun, Song, Myungsuk, and Cho, Beongki

Subjects
Condensed Matter - Strongly Correlated Electrons
Abstract

A well-known feature in transport data of the topological Kondo insulator SmB$_6$ is the sign change in the Hall coefficient at 65 K. Carriers in SmB$_6$ are known to be negative, but above 65 K, the Hall sign suggests that the carriers are positive. Here, we extend Hall measurements up to 400 K and observe that the Hall coefficient changes back to the correct (negative) sign at about 305 K. We interpret the anomalous sign of the Hall coefficient in the context of skew scattering arising from the strong correlations between the $f$ and $d$ electrons. At energy scales where the gap is closed, the number of $d$ electrons in resonance with the $f$ electrons at the Fermi energy varies. When a large proportion of $d$ and $f$ electrons are in resonance, skew scattering is dominant, leading to the observation of the positive sign, but when fewer are in resonance, conventional scattering mechanisms dominate instead., Comment: 4 pages, 4 figures, submitted to Journal of Superconductivity and Novel Magnetism

Published
2019

5. Current-Induced Field-Free Switching of Co/Pt Multilayer via Modulation of Interlayer Exchange Coupling and Magnetic Anisotropy.

Author

Kim, Byungro, Seo, Dongpyo, Yoon, Seungha, Han, Songhee, Kim, Taeheon, and Cho, Beongki

Abstract

Current-induced field-free magnetic switching using spin–orbit torque has been an important topic for decades due to both academic and industrial interest. Most research has focused on introducing symmetry breakers, such as geometrical and compositional variation, pinned layers, and symmetry-broken crystal structures, which add complexity to the magnetic structure and fabrication process. We designed a relatively simple magnetic structure, composed of a [Co/Pt] multilayer and a Co layer with perpendicular and in-plane magnetic anisotropy, respectively, with a Cu layer between them. Current-induced deterministic magnetic switching was observed in this magnetic system. The system is advantageous due to its easy control of the parameters to achieve the optimal condition for magnetic switching. The balance between magnetic anisotropic strength and interlayer coupling strength is found to provide the optimal condition. This simple design and easy adjustability open various possibilities for magnetic structures in spin-based electronics applications using spin–orbit torque. [ABSTRACT FROM AUTHOR]

Published
2024
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9. Observation of warping effects in the band and angular momentum structures of topological insulator Bi2Te3

Author

Jung, Wonsig, Kim, Yeongkwan, Kim, Beomyoung, Koh, Yoonyoung, Kim, Chul, Matsunami, Masaharu, Kimura, Shin-ichi, Arita, Masashi, Shimada, Kenya, Han, Jung Hoon, Kim, Juyoung, Cho, Beongki, and Kim, Changyoung

Subjects
Condensed Matter - Materials Science
Abstract

We performed angle resolved photoemission (ARPES) experiments on Bi2Te3 with circularly polarized light. ARPES data show very strong circular dichroism, indicating existence of orbital angular momentum (OAM). Moreover, the alignment of OAM is found to have a strong binding energy dependence. Such energy dependence comes from a relatively strong band warping effect in Bi2Te3 compared to Bi2Se3. OAM close to Dirac point has an ideal chiral structure (sin ?) without out-of-plane component. Warping effect comes in as the binding energy decreases and circular dichroism along a constant energy contour can no longer be explained by a simple sin? function but requires a sin3? term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism gains an additional sin6? term. Such behavior is found to be compatible with the theoretically predicted OAM structure.

Published
2011
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15. Highly Optimized Iron Oxide Embedded Poly(Lactic Acid) Nanocomposites for Effective Magnetic Hyperthermia and Biosecurity

Author

Ryu,Chiseon, Lee,Hwangjae, Kim,Hohyeon, Hwang,Seong, Hadadian,Yaser, Mohanty,Ayeskanta, Park,In-Kyu, Cho,Beongki, Yoon,Jungwon, Lee,Jae Young, Ryu,Chiseon, Lee,Hwangjae, Kim,Hohyeon, Hwang,Seong, Hadadian,Yaser, Mohanty,Ayeskanta, Park,In-Kyu, Cho,Beongki, Yoon,Jungwon, and Lee,Jae Young

Abstract

Chiseon Ryu,1 Hwangjae Lee,1 Hohyeon Kim,2 Seong Hwang,1 Yaser Hadadian,2,3 Ayeskanta Mohanty,4 In-Kyu Park,4 Beongki Cho,1 Jungwon Yoon,2,3 Jae Young Lee1 1School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea; 2School of Integrated Technology, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea; 3Research Center for Nanorobotics in Brain, Gwangju Institute of Science and Technology, Gwangju, 61005, Republic of Korea; 4Department of Biomedical Sciences, Chonnam National University Medical School, Hwasun-gun, Jeollanam-do, Republic of KoreaCorrespondence: Jungwon Yoon; Jae Young Lee Tel +82 62 715 5332; +82 62 715 2358Email jyoon@gist.ac.kr; jaeyounglee@gist.ac.krIntroduction: Iron oxide magnetic nanoparticles (IONPs) have attracted considerable attention for various biomedical applications owing to their ease of synthesis, strong magnetic properties, and biocompatibility. In particular, IONPs can generate heat under an alternating magnetic field, the effects of which have been extensively studied for magnetic hyperthermia therapy. However, the development of IONPs with high heating efficiency, biocompatibility, and colloidal stability in physiological environments is still required for their safe and effective application in biomedical fields.Methods: We synthesized magnetic IONP/polymer nanocomposites (MNCs) by embedding IONPs in a poly(L-lactic acid) (PLA) matrix via nanoemulsion. The IONP contents (Fe: 9– 22 [w/w]%) in MNCs were varied to investigate their effects on the magnetic and hyperthermia performances based on their optimal interparticle interactions. Further, we explored the stability, cytocompatibility, biodistribution, and in vivo tissue compatibility of the MNCs.Results: The MNCs showed enhanced heating efficiency with over two-fold increase compared to nonembedded bare IONPs. The relationship between the IONP content and heating performance in MNCs was

Published
2022

17. Metallic chemical potentials in an insulating topological state

Author

Kotta, Erica, primary, Xu, Yishuai, additional, Lee, Jaewoong, additional, Liu, Shouzheng, additional, Miao, Lin, additional, Xu, Yi, additional, Jiang, Denglin, additional, Denlinger, Jonathan, additional, Yilmaz, Turgut, additional, Vescovo, Elio, additional, Cho, Beongki, additional, and Wray, L. Andrew, additional

Published
2022
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21. Bulk transport paths through defects in floating zone and Al flux grown SmB6

Author

Eo, Yun Suk, primary, Rakoski, Alexa, additional, Sinha, Shriya, additional, Mihaliov, Dmitri, additional, Fuhrman, Wesley T., additional, Saha, Shanta R., additional, Rosa, Priscila F. S., additional, Fisk, Zachary, additional, Hatnean, Monica Ciomaga, additional, Balakrishnan, Geetha, additional, Chamorro, Juan R., additional, Phelan, W. Adam, additional, Koohpayeh, Seyed M., additional, McQueen, Tyrel M., additional, Kang, Boyoun, additional, Song, Myung-suk, additional, Cho, Beongki, additional, Fuhrer, Michael S., additional, Paglione, Johnpierre, additional, and Kurdak, Çağlıyan, additional

Published
2021
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24. Cover Image

Author

Nguyen, Thi Huyen, primary, Nguyen, Thi Minh Hien, additional, Kang, Boyoun, additional, Cho, Beongki, additional, Han, Mancheon, additional, Choi, Hyoung Joon, additional, Kong, Mihye, additional, Lee, Yongjae, additional, and Yang, In‐Sang, additional

Published
2019
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28. Correlation between Geometrically Induced Oxygen Octahedral Tilts and Multiferroic Behaviors in BiFeO3 Films

Author

Lee, Sung Su, primary, Kim, Young‐Min, additional, Lee, Hyun‐Jae, additional, Seo, Okkyun, additional, Jeong, Hu Young, additional, He, Qian, additional, Borisevich, Albina Y., additional, Kang, Boyoun, additional, Kwon, Owoong, additional, Kang, Seunghun, additional, Kim, Yunseok, additional, Koo, Tae Yeong, additional, Rhyee, Jong‐Soo, additional, Noh, Do Young, additional, Cho, Beongki, additional, Seo, Ji Hui, additional, Lee, Jun Hee, additional, and Jo, Ji Young, additional

Published
2018
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30. Raman spectroscopic evidence of impurity‐induced structural distortion in SmB6.

Author

Nguyen, Thi Huyen, Nguyen, Thi Minh Hien, Kang, Boyoun, Cho, Beongki, Han, Mancheon, Choi, Hyoung Joon, Kong, Mihye, Lee, Yongjae, and Yang, In‐Sang

Subjects
RAMAN spectroscopy, SINGLE crystals, PHONONS, RAMAN effect, X-ray diffraction, OCTAHEDRA, ECHO-planar imaging
Abstract

Impurity‐induced structural distortion in SmB6 is suggested by Raman spectroscopy study on impurity‐controlled samples. Significant differences in the Raman spectra of single crystals SmB6(6N), synthesized with 99.9999%‐pure boron, and SmB6(3N), synthesized with 99.9%‐pure boron, are detected. While no noticeable differences are detected in the X‐ray diffraction of SmB6(3N) and SmB6(6N), all the Raman phonon modes of the T2g, Eg, and A1g of SmB6(3N) are clearly broader and shift to higher wavenumbers than those of SmB6(6N), and the T2g and Eg modes of SmB6(3N) show doublet features. Based on the high‐pressure Raman measurements and phonon calculation in uniaxial compression model, we argue that small amount of impurities in SmB6(3N) is enough to induce anisotropic distortion in the B6 octahedra, leading to peculiar behaviors in Raman spectrum of SmB6(3N). Our results may present a clue for understanding the current puzzles about SmB6 of various origins with different impurities. [ABSTRACT FROM AUTHOR]

Published
2019
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33. Correlation between Geometrically Induced Oxygen Octahedral Tilts and Multiferroic Behaviors in BiFeO3 Films.

Author

Lee, Sung Su, Kim, Young‐Min, Lee, Hyun‐Jae, Seo, Okkyun, Jeong, Hu Young, He, Qian, Borisevich, Albina Y., Kang, Boyoun, Kwon, Owoong, Kang, Seunghun, Kim, Yunseok, Koo, Tae Yeong, Rhyee, Jong‐Soo, Noh, Do Young, Cho, Beongki, Seo, Ji Hui, Lee, Jun Hee, and Jo, Ji Young

Subjects
MULTIFERROIC materials, OXYGEN, BISMUTH compounds, METALLIC films, UNIT cell, FERROMAGNETISM
Abstract

Abstract: The equilibrium position of atoms in a unit cell is directly connected to crystal functionalities, e.g., ferroelectricity, ferromagnetism, and piezoelectricity. The artificial tuning of the energy landscape can involve repositioning atoms as well as manipulating the functionalities of perovskites (ABO3), which are good model systems to test this legacy. Mechanical energy from external sources accommodating various clamping substrates is utilized to perturb the energy state of perovskite materials fabricated on the substrates and consequently change their functionalities; however, this approach yields undesired complex behaviors of perovskite crystals, such as lattice distortion, displacement of B atoms, and/or tilting of oxygen octahedra. Owing to complimentary collaborations between experimental and theoretical studies, the effects of both lattice distortion and displacement of B atoms are well understood so far, which leaves us a simple question: Can we exclusively control the positions of oxygen atoms in perovskites for functionality manipulation? Here the artificial manipulation of oxygen octahedral tilt angles within multiferroic BiFeO3 thin films using strong oxygen octahedral coupling with bottom SrRuO3 layers is reported, which opens up new possibilities of oxygen octahedral engineering. [ABSTRACT FROM AUTHOR]

Published
2018
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37. Warping effects in the band and angular-momentum structures of the topological insulator Bi2Te3

Author

Jung, Wonsig, Kim, Yeongkwan, Kim, Beomyoung, Koh, Yoonyoung, Kim, Chul, Matsunami, Masaharu, Kimura, Shin-ichi, Arita, Masashi, Shimada, Kenya, Han, Jung Hoon, Kim, Juyoung, Cho, Beongki, Kim, Changyoung, Jung, Wonsig, Kim, Yeongkwan, Kim, Beomyoung, Koh, Yoonyoung, Kim, Chul, Matsunami, Masaharu, Kimura, Shin-ichi, Arita, Masashi, Shimada, Kenya, Han, Jung Hoon, Kim, Juyoung, Cho, Beongki, and Kim, Changyoung

Abstract

We performed angle-resolved photoemission (ARPES) experiments on Bi2Te3 with circularly polarized light. ARPES data show very strong circular dichroism, indicating the existence of orbital angular momentum (OAM). Moreover, the alignment of OAM is found to have a strong binding energy dependence. Such energy dependence comes from a relatively strong band warping effect in Bi2Te3 compared to Bi2Se3. OAM close to the Dirac point has an ideal chiral structure (sinθ) without an out-of-plane component. The warping effect comes in as the binding energy decreases, and circular dichroism along a constant energy contour can no longer be explained by a simple sinθ function but requires a sin3θ term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism gains an additional sin6θ term. Such behavior is found to be compatible with the theoretically predicted OAM structure.

Published
2011

39. Warping effects in the band and angular-momentum structures of the topological insulator Bi2Te3

Author

Jung, Wonsig, primary, Kim, Yeongkwan, additional, Kim, Beomyoung, additional, Koh, Yoonyoung, additional, Kim, Chul, additional, Matsunami, Masaharu, additional, Kimura, Shin-ichi, additional, Arita, Masashi, additional, Shimada, Kenya, additional, Han, Jung Hoon, additional, Kim, Juyoung, additional, Cho, Beongki, additional, and Kim, Changyoung, additional

Published
2011
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45. Raman spectroscopic evidence of impurity‐induced structural distortion in SmB6.

Author

Nguyen, Thi Huyen, Nguyen, Thi Minh Hien, Kang, Boyoun, Cho, Beongki, Han, Mancheon, Choi, Hyoung Joon, Kong, Mihye, Lee, Yongjae, and Yang, In‐Sang

Subjects
*RAMAN spectroscopy, *SINGLE crystals, *PHONONS, *RAMAN effect, *X-ray diffraction, *OCTAHEDRA, *ECHO-planar imaging
Abstract

Impurity‐induced structural distortion in SmB6 is suggested by Raman spectroscopy study on impurity‐controlled samples. Significant differences in the Raman spectra of single crystals SmB6(6N), synthesized with 99.9999%‐pure boron, and SmB6(3N), synthesized with 99.9%‐pure boron, are detected. While no noticeable differences are detected in the X‐ray diffraction of SmB6(3N) and SmB6(6N), all the Raman phonon modes of the T2g, Eg, and A1g of SmB6(3N) are clearly broader and shift to higher wavenumbers than those of SmB6(6N), and the T2g and Eg modes of SmB6(3N) show doublet features. Based on the high‐pressure Raman measurements and phonon calculation in uniaxial compression model, we argue that small amount of impurities in SmB6(3N) is enough to induce anisotropic distortion in the B6 octahedra, leading to peculiar behaviors in Raman spectrum of SmB6(3N). Our results may present a clue for understanding the current puzzles about SmB6 of various origins with different impurities. [ABSTRACT FROM AUTHOR]

Published
2019
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