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9. Energy-efficient superparamagnetic Ising machine and its application to traveling salesman problems

Author

Si, Jia, Yang, Shuhan, Cen, Yunuo, Chen, Jiaer, Yao, Zhaoyang, Kim, Dong-Jun, Cai, Kaiming, Yoo, Jerald, Fong, Xuanyao, and Yang, Hyunsoo

Subjects
Computer Science - Emerging Technologies, Condensed Matter - Other Condensed Matter, Physics - Applied Physics
Abstract

The growth of artificial intelligence and IoT has created a significant computational load for solving non-deterministic polynomial-time (NP)-hard problems, which are difficult to solve using conventional computers. The Ising computer, based on the Ising model and annealing process, has been highly sought for finding approximate solutions to NP-hard problems by observing the convergence of dynamic spin states. However, it faces several challenges, including high power consumption due to artificial spins and randomness emulated by complex circuits, as well as low scalability caused by the rapidly growing connectivity when considering large-scale problems. Here, we present an experimental Ising annealing computer based on superparamagnetic tunnel junctions (SMTJs) with all-to-all connections, which successfully solves a 70-city travelling salesman problem (4761-node Ising problem). By taking advantage of the intrinsic randomness of SMTJs, implementing a proper global annealing scheme, and using an efficient algorithm, our SMTJ-based Ising annealer shows superior performance in terms of power consumption and energy efficiency compared to other Ising schemes. Additionally, our approach provides a promising way to solve complex problems with limited hardware resources. Moreover, we propose a crossbar array architecture for scalable integration using conventional magnetic random access memories. Our results demonstrate that the SMTJ-based Ising annealing computer with high energy efficiency, speed, and scalability is a strong candidate for future unconventional computing schemes., Comment: 5 figures

Published
2023

29. Observation of transverse spin Nernst magnetoresistance induced by thermal spin current in ferromagnet/non-magnet bilayers

Author

Kim, Dong-Jun, Jeon, Chul-Yeon, Choi, Jong-Guk, Lee, Jae Wook, Surabhi, Srivathsava, Jeong, Jong-Ryul, Lee, Kyung-Jin, and Park, Byong-Guk

Subjects
Condensed Matter - Materials Science
Abstract

Electric generation of spin current via spin Hall effect is of great interest as it allows an efficient manipulation of magnetization in spintronic devices. Theoretically, spin current can be also created by a temperature gradient, which is known as spin Nernst effect. Here, we report spin Nernst effect-induced transverse magnetoresistance in ferromagnet (FM)/non-magnetic heavy metal (HM) bilayers. We observe that the magnitude of transverse magnetoresistance (i.e., planar Nernst signal) in FM/HM bilayers is significantly modified by HM and its thickness. This strong dependence of transverse magnetoresistance on HM evidences the spin Nernst effect in HM; the generation of thermally-induced spin current in HM and its subsequent reflection at the FM/HM interface. Our analysis of transverse magnetoresistance shows that the spin Nernst angles of W and Pt have the opposite sign to their spin Hall angles. Moreover, our estimate implies that the magnitude of the spin Nernst angle would be comparable to that of the spin Hall angle, suggesting an efficient generation of spin current by the spin Nernst effect.

Published
2017
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30. Nonlocal dual-phase-lag thermoelastic damping in small-sized circular cross-sectional ring resonators.

Author

Hai, Liang and Kim, Dong Jun

Subjects
*STRAIN energy, *ENERGY dissipation, *THERMAL strain, *HEAT equation, *HEAT conduction
Abstract

The purpose of current research is to establish a theoretical size-dependent framework for estimating the amount of thermoelastic damping (TED) in miniaturized rings with circular cross section via the nonlocal dual-phase-lag (NDPL) generalized thermoelasticity theory, as one of the most exhaustive non-Fourier heat conduction models. The method used to achieve this goal is based on the definition of TED in the energy dissipation (ED) approach. To do so, after deriving heat equation in the context of NDPL model, the distribution of temperature all over the volume of the ring is specified. Then, the relations of dissipated thermal energy and strain energy in the ring are obtained. Lastly, by employing ED approach, a formula comprising the nonclassical parameters of NDPL model is rendered to determine TED value in small-scaled toroidal rings. By presenting various numerical examples, the dependence of TED on influential parameters including nonclassical thermal constants in NDPL model, ring geometry, vibrational mode number and ring material is surveyed. The Findings enlighten that the inclusion of nonlocal parameter into the model can have conspicuous impacts on TED value, especially in smaller rings or higher vibrational mode numbers. The results also reveal that among the investigated materials (i.e. silicon, copper, silver, and lead), rings made of lead and silicon exhibit the maximum and minimum TED values, respectively. [ABSTRACT FROM AUTHOR]

Published
2024
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31. Tetramerized Small‐Molecule Acceptor for Organic Solar Cells with Enhanced Efficiency, Stability, and Mechanical Robustness: Impact of Chain Length and Dispersity Effects.

Author

Lim, Chulhee, Lee, Jin‐Woo, Kim, Dong Jun, Han, Daehee, Phan, Tan Ngoc‐Lan, Lee, Seungjin, Kim, Taek‐Soo, and Kim, Bumjoon J.

Subjects
SOLAR cell efficiency, MOLECULAR size, SOLAR cells, IMPACT (Mechanics), DIFFUSION kinetics, ELECTROPHILES
Abstract

Discrete dimer or multimer acceptors have enhanced the stability of organic solar cells (OSCs) due to their slow diffusion kinetics resulting from their large molecular sizes. However, development of multimer acceptors with chain length longer than trimers has been challenging, which often require multistep reactions with low synthetic yield. In this study, a new discrete tetramer acceptor (TetA) using one‐pot reaction and subsequent purification processes is developed. During the purification, dimer (DA) and trimer acceptor (TA) are also obtained. The OSCs for TetA demonstrate a higher power conversion efficiency (PCE) of 16.14% than those for the discrete acceptors with shorter chain lengths, such as monomer acceptor (MA, 12.85%), DA (14.31%), and TA (15.10%). Additionally, despite having a similar number‐average molecular weight, TetA‐based OSCs exhibit a significantly higher PCE (16.14%) compared to OSCs based on a mixture of the acceptors (MixA) with dispersity (10.72%). Furthermore, the TetA‐based OSCs have the highest photostability and mechanical robustness among the series. For example, TetA‐based OSCs demonstrate superior photostability (t70% lifetime = 2180 h under 1‐sun illumination) and mechanical robustness (crack‐onset strain (COS) = 8%) compared to those based on MA (t70% = 220 h, and COS = 2%), or MixA (t70% = 745 h, and COS = 6%). [ABSTRACT FROM AUTHOR]

Published
2024
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35. Socioeconomic inequalities in the prevalence, non-awareness, non-treatment, and non-control of diabetes among South Korean adults in 2021.

Author

Kim, Seongju, Kim, Dong Jun, and Lee, Hooyeon

Subjects
*HEALTH & Nutrition Examination Survey, *KOREANS, *OLDER men, *HEALTH equity, *ABSOLUTE value
Abstract

The purpose of this study was to investigate socioeconomic inequalities in diabetes prevalence, non-awareness, non-treatment, and non-control among South Korean adults in 2021. This cross-sectional study used data from the 2021 Korean National Health and Nutrition Examination Survey. Relative concentration indices (RCIs) and relative concentration curves stratified by sex and age were used to investigate socioeconomic inequalities in the prevalence, non-awareness, non-treatment, and non-control of diabetes. The prevalence, non-awareness, lack of treatment, and non-control rates in adults aged 30 years and older in 2021 were 15.9%, 29.5%, 33.3%, and 76.1%, respectively. Diabetes was more prevalent in participants under the age of 65 years than those aged 65 years and older for both men (RCI: -0.081, RCI: -0.158, respectively) and women (RCI: -0.203, RCI: -0.292, respectively). The larger the absolute value of the RCI in non-awareness and non-treatment of diabetes in women, the greater the level of socioeconomic inequalities (RCI: 0.182, RCI: 0.154). Socioeconomic inequalities existed in the prevalence of diabetes among both men and women aged under 65 years. In women, socioeconomic inequalities of non-awareness and non-treatment of diabetes were greater than those in men. Thus, preventive care and monitoring are required, particularly among women and individuals under the age of 65 years. [ABSTRACT FROM AUTHOR]

Published
2024
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45. Large-area transfer of nanometre-thin C60 films

Author

Hogan, Jack, primary, Liu, Chengyi, additional, Zhang, Hui, additional, Salisu, Aliyu, additional, Villamanca, Dan, additional, Zheng, Jianghui, additional, Martin, Jacob, additional, Page, Alister, additional, Ho-Baillie, Anita, additional, Kim, Dong Jun, additional, and Chen, Sam, additional

Published
2024
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50. Large-Area Transfer of Nanometer-Thin C60Films

Author

Hogan, Jack, Liu, Chengyi, Zhang, Hui, Salisu, Aliyu, Villamanca, Dan, Zheng, Jianghui, Martin, Jacob W., Page, Alister J., Ho-Baillie, Anita W. Y., Kim, Dong Jun, and Chen, Sam

Abstract

Fullerenes, with well-defined molecular structures and high scalability, hold promise as fundamental building blocks for creating a variety of carbon materials. The fabrication and transfer of large-area films with precisely controlled thicknesses and morphologies on desired surfaces are crucial for designing and developing fullerene-based materials and devices. In this work, we present strategies for solid-state transferring C60molecular nanometer-thin films, with dimensions of centimeters in lateral size and thicknesses controlled in the range of 1–20 nm, onto various substrates. Furthermore, we have successfully fabricated centimeter-wide graphene/C60/graphene heterostructures through layer-by-layer stacking of C60and graphene films. This transfer methodology is versatile, allowing for the complete transfer of chemically modified C60films, including oxygenated C60films and C60Pdnorganometallic polymer films. Additionally, direct solid-state transfer of C60and C60Pdnfilms onto electrode surfaces has enabled their electrocatalytic performance for the hydrogen evolution reaction to be probed directly. This thin-film transfer strategy allows precise manipulation of large-area, ultrathin C60films on various substrates, providing a platform for fullerene chemistry and the experimental synthesis of artificial carbon structures.

Published
2025
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