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5 results on '"Lim, Hyeon Wook"'

1. Advanced interfacial phase change material: structurally confined and interfacially extended superlattice

Author

Lim, Hyeon wook, Kim, Young sam, Jo, Kyu-jin, Seok-Choi, Lee, Chang Woo, Kim, Dasol, Kwon, Ki hyeon, Kwon, Hoe don, Hwang, Soo bin, Choi, Byung-Joon, Yang, Cheol-Woong, Sim, Eun Ji, and Cho, Mann-Ho

Subjects
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Interfacial Phase Change Memory (iPCM) retrench unnecessary power consumption due to wasted heat generated during phase change by reducing unnecessary entropic loss. In this study, an advanced iPCM (GeTe/Ti-Sb2Te3 Superlattice) is synthesized by doping Ti into Sb2Te3. Structural analysis and density functional theory (DFT) calculations confirm that bonding distortion and structurally well-confined layers contribute to improve phase change properties in iPCM. Ti-Sb2Te3 acts as an effective thermal barrier to localize the generated heat inside active region, which leads to reduction of switching energy. Since Ge-Te bonds adjacent to short and strong Ti-Te bonds are more elongated than the bonds near Sb-Te, it is easier for Ge atoms to break the bond with Te due to strengthened Peierls distortions (Rlong/Rshort) during phase change process. Properties of advanced iPCM (cycling endurance, write speed/energy) exceed previous records. Moreover, well-confined multi-level states are obtained with advanced iPCM, showing potential as a neuromorphic memory. Our work paves the way for designing superlattice based PCM by controlling confinement layers., Comment: 5 Figures and 2 table, 5 Supplementary figures, 27 pages There are problems with typos and quality degradation of figures during PDF conversion process, so we resubmit the updated one in which those issues are resolved. Under mutual agreement between our group(Prof.m.h.cho) and Prof. Robert E.Simpson group, each research content is submitted separately to arXiv.org on the same day. (2022. 9/30)

Published
2022

4. Exceptional Spin‐to‐Charge Conversion in Selective Band Topology of Bi/Bi1‐xSbx with Spintronic Singularity.

Author

Rho, Seungwon, Park, Hanbum, Park, Jeehong, Jeong, Kwangsik, Kim, Hyeongmun, Hong, Seok‐Bo, Kim, Jonghoon, Lim, Hyeon Wook, Yi, Yeonjin, Kang, Chul, and Cho, Mann‐Ho

Subjects
EMISSION spectroscopy, TERAHERTZ spectroscopy, TOPOLOGY, SURFACE states, TOPOLOGICAL insulators
Abstract

In this study, spin‐to‐charge conversion (SCC) of various topological materials with ferromagnet is investigated using spintronic terahertz (THz) emission spectroscopy. Compared with other topological materials, significantly large THz emission is observed for topologically nontrivial phases of Bi1‐xSbx (x > 0.2) that predominantly originates from the topological surface state. When Bi is superposed above a certain stoichiometry of Bi1‐xSbx, it plays a crucial role in generating a highly spin‐split state and enhancing the spin‐mixing conductance, resulting in colossal THz emission. This proves that improving the SCC efficiency through interface engineering is a useful strategy to design a powerful spintronic device. Collectively, this study proposes a methodology for systematically analyzing SCC efficiency or spin Hall angle using THz emission spectroscopy and offers an efficient structure for future spintronic devices. [ABSTRACT FROM AUTHOR]

Published
2023
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5. Ferroelectric switching in GeTe through rotation of lone-pair electrons by Electric field-driven phase transition

Author

Lim Hyeon Wook, Eunha Lee, Hyangsook Lee, Mann Ho Cho, Changwoo Lee, Kwangsik Jeong, and Hyoungsub Kim

Subjects
Phase transition, Phase boundary, Materials science, Condensed matter physics, Polarization (waves), Ferroelectricity, symbols.namesake, chemistry.chemical_compound, chemistry, symbols, General Materials Science, Raman spectroscopy, Germanium telluride, Lone pair, Monoclinic crystal system
Abstract

Germanium telluride (GeTe), which has long been used in phase-change devices, has very unique structural characteristics. While atomic movement and related bond-order switching have been believed to determine the ferroelectric characteristics of this compound, structural changes accompanying ferroelectric switching have not been examined. In this study, using transmission electron microscopy and Raman spectroscopy, we directly observed the atomic movement and the related change in the local structure of GeTe during ferroelectric switching under an applied E-field. The crystal symmetry of GeTe could be alternated between two phases (R3m (rhombohedral) and Cm (monoclinic)) by changing the compound's polarization direction, indicating that the ferroelectric switching behavior of GeTe can be controlled. This observation suggests that ferroelectric switching of GeTe may originate from the phase transition between R3m and Cm through the formation of a morphotropic phase boundary (MPB). Since these two phases can generate MPBs with similar energies, the rotation of lone-pair direction during the phase transition can effectively change the polarization direction. The exotic phenomenon of ferroelectric switching, which involves changes in the chemical bonds, can be predicted on the basis of the role of several dopants through DFT calculations.

Published
2021
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