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Highly Efficient Charge-to-Spin Interconversion in Graphene Heterostructures
Researchers present a new route for designing a graphene-based active spintronic component KAIST physicists described a route to design the energy-efficient generation, manipulation and detection of spin currents using nonmagnetic two-dimensional materials. The research team, led by Professor Sungjae Cho, observed highly efficient charge-to-spin interconversion via the gate-tunable Rashba-Edelstien effect (REE) in graphene heterostructures. This research paves the way for the application of graphene as an active spintronic component for generating, controlling, and detecting spin current without ferromagnetic electrodes or magnetic fields. Graphene is a promising spintronic component owing to its long spin diffusion length. However, its small spin-orbit coupling limits the potential of graphene in spintronic applications since graphene cannot be used to generate, control, or detect spin current. “We successfully increased the spin-orbit coupling of graphene by stacking graphene on top of 2H-TaS2, which is one of the transition metal dichalcogenide materials with the largest spin-orbit coupling. Graphene now can be used to generate, control, and detect spin current,” Professor Cho said. The Rashba-Edelstein effect is a physical mechanism that enables charge current-to-spin current interconversion by spin-dependent band structure induced by the Rashba effect, a momentum-dependent splitting of spin bands in low-dimensional condensed matter systems. Professor Cho’s group demonstrated the gate-tunable Rashba-Edelstein effect in a multilayer graphene for the first time. The Rahsba-Edelstein effect allows the two-dimensional conduction electrons of graphene to be magnetized by an applied charge current and form a spin current. Furthermore, as the Fermi level of graphene, tuned by gate voltage, moves from the valence to conduction band, the spin current generated by graphene reversed its spin direction. This spin reversal is useful in the design of low-power-consumption transistors utilizing spins in that it provides the carrier “On” state with spin up holes (or spin down electrons) and the "Off" state with zero net spin polarization at so called “charge neutrality point” where numbers of electrons and holes are equal. “Our work is the first demonstration of charge-to-spin interconversion in a metallic TMD (transition-metal dichalcogenides) and graphene heterostructure with a spin polarization state controlled by a gate. We expect that the all-electrical spin-switching effect and the reversal of non-equilibrium spin polarization by the application of gate voltage is applicable for the energy-efficient generation and manipulation of spin currents using nonmagnetic van der Waals materials,” explained Professor Cho. This study (https://pubs.acs.org/doi/10.1021/acsnano.0c01037) was supported by the National Research Foundation of Korea. Publication: Lijun Li, Jin Zhang, Gyuho Myeong, Wongil Shin, Hongsik Lim, Boram Kim, Seungho Kim, Taehyeok Jin, Stuart Cavill, Beom Seo Kim, Changyoung Kim, Johannes Lischner, Aires Ferreira, and Sungjae Cho, Gate-Tunable Reversible Rashba−Edelstein Effect in a Few-Layer Graphene/2H-TaS2 Heterostructure at Room Temperature. ACS Nano 2020. Link to download the paper: https://pubs.acs.org/doi/10.1021/acsnano.0c01037 Profile: Professor Sungjae Cho, PhD firstname.lastname@example.org http://qtak.kaist.ac.kr Department of Physics Korea Advanced Institute of Science and Technology (KAIST) https://www.kaist.ac.kr Daejeon 34141, Korea
‘OSK Rising Stars 30’ Recognizes Four KAISTians
Four KAISTians were selected as star researchers to brighten the future of optics in commemoration of the 30th anniversary of the Optical Society of Korea (OSK). As ‘OSK Rising Stars 30’, the OSK named 27 domestic researchers under the age of 40 who have made significant contributions and will continue contributing to the development of Korea’s optics academia and industry. Professor YongKeun Park from the Department of Physics was selected in recognition of his contributions to the field of biomedical optics. Professor Park focuses on developing novel optical methods for understanding, diagnosing, and treating human diseases, based on light scattering, light manipulation, and interferometry. As a member of numerous international optics societies including the OSA and the SPIE and a co-founder of two start-up companies, Professor Park continues to broaden his boundaries as a leading opticist and entrepreneur. Professor Jonghwa Shin from the Department of Materials Science and Engineering was recognized for blazing a trail in the field of broadband metamaterials. Professor Shin’s research on the broadband enhancement of the electric permittivity and refractive index of metamaterials has great potential in both academia and industry. Professor Hongki Yoo from the Department of Mechanical Engineering is expected to create a significant ripple effect in the diagnosis of cardiovascular disorders through the development of new optical imaging techniques and applications. Finally, Dr. Sejeong Kim, a KAIST graduate and a Chancellor’s postdoctoral research fellow at the University of Technology Sydney (UTS), was acknowledged for her optical device research utilizing two-dimensional materials. Dr. Kim’s research at UTS now focuses on the introduction of micro/nano cavities for new materials. (END)
Scientists Observe the Elusive Kondo Screening Cloud
Scientists ended a 50-year quest by directly observing a quantum phenomenon An international research group of Professor Heung-Sun Sim has ended a 50-year quest by directly observing a quantum phenomenon known as a Kondo screening cloud. This research, published in Nature on March 11, opens a novel way to engineer spin screening and entanglement. According to the research, the cloud can mediate interactions between distant spins confined in quantum dots, which is a necessary protocol for semiconductor spin-based quantum information processing. This spin-spin interaction mediated by the Kondo cloud is unique since both its strength and sign (two spins favor either parallel or anti-parallel configuration) are electrically tunable, while conventional schemes cannot reverse the sign. This phenomenon, which is important for many physical phenomena such as dilute magnetic impurities and spin glasses, is essentially a cloud that masks magnetic impurities in a material. It was known to exist but its spatial extension had never been observed, creating controversy over whether such an extension actually existed. Magnetism arises from a property of electrons known as spin, meaning that they have angular momentum aligned in one of either two directions, conventionally known as up and down. However, due to a phenomenon known as the Kondo effect, the spins of conduction electrons—the electrons that flow freely in a material—become entangled with a localized magnetic impurity, and effectively screen it. The strength of this spin coupling, calibrated as a temperature, is known as the Kondo temperature. The size of the cloud is another important parameter for a material containing multiple magnetic impurities because the spins in the cloud couple with one another and mediate the coupling between magnetic impurities when the clouds overlap. This happens in various materials such as Kondo lattices, spin glasses, and high temperature superconductors. Although the Kondo effect for a single magnetic impurity is now a text-book subject in many-body physics, detection of its key object, the Kondo cloud and its length, has remained elusive despite many attempts during the past five decades. Experiments using nuclear magnetic resonance or scanning tunneling microscopy, two common methods for understanding the structure of matter, have either shown no signature of the cloud, or demonstrated a signature only at a very short distance, less than 1 nanometer, so much shorter than the predicted cloud size, which was in the micron range. In the present study, the authors observed a Kondo screening cloud formed by an impurity defined as a localized electron spin in a quantum dot—a type of “artificial atom”—coupled to quasi-one-dimensional conduction electrons, and then used an interferometer to measure changes in the Kondo temperature, allowing them to investigate the presence of a cloud at the interferometer end. Essentially, they slightly perturbed the conduction electrons at a location away from the quantum dot using an electrostatic gate. The wave of conducting electrons scattered by this perturbation returned back to the quantum dot and interfered with itself. This is similar to how a wave on a water surface being scattered by a wall forms a stripe pattern. The Kondo cloud is a quantum mechanical object which acts to preserve the wave nature of electrons inside the cloud. Even though there is no direct electrostatic influence of the perturbation on the quantum dot, this interference modifies the Kondo signature measured by electron conductance through the quantum dot if the perturbation is present inside the cloud. In the study, the researchers found that the length as well as the shape of the cloud is universally scaled by the inverse of the Kondo temperature, and that the cloud’s size and shape were in good agreement with theoretical calculations. Professor Sim at the Department of Physics proposed the method for detecting the Kondo cloud in the co-research with the RIKEN Center for Emergent Matter Science, the City University of Hong Kong, the University of Tokyo, and Ruhr University Bochum in Germany. Professor Sim said, “The observed spin cloud is a micrometer-size object that has quantum mechanical wave nature and entanglement. This is why the spin cloud has not been observed despite a long search. It is remarkable in a fundamental and technical point of view that such a large quantum object can now be created, controlled, and detected. Dr. Michihisa Yamamoto of the RIKEN Center for Emergent Matter Science also said, “It is very satisfying to have been able to obtain real space image of the Kondo cloud, as it is a real breakthrough for understanding various systems containing multiple magnetic impurities. The size of the Kondo cloud in semiconductors was found to be much larger than the typical size of semiconductor devices.” Publication: Borzenets et al. (2020) Observation of the Kondo screening cloud. Nature, 579. pp.210-213. Available online at https://doi.org/10.1038/s41586-020-2058-6 Profile: Heung-Sun Sim, PhD Professor email@example.com https://qet.kaist.ac.kr/ Quantum Electron Correlation & Transport Theory Group (QECT Lab) https://qc.kaist.ac.kr/index.php/group1/ Center for Quantum Coherence In COndensed Matter Department of Physics https://www.kaist.ac.kr Korea Advanced Institute of Science and Technology (KAIST) Daejeon, Republic of Korea
Ultrafast Quantum Motion in a Nanoscale Trap Detected
< Professor Heung-Sun Sim (left) and Co-author Dr. Sungguen Ryu (right) > KAIST researchers have reported the detection of a picosecond electron motion in a silicon transistor. This study has presented a new protocol for measuring ultrafast electronic dynamics in an effective time-resolved fashion of picosecond resolution. The detection was made in collaboration with Nippon Telegraph and Telephone Corp. (NTT) in Japan and National Physical Laboratory (NPL) in the UK and is the first report to the best of our knowledge. When an electron is captured in a nanoscale trap in solids, its quantum mechanical wave function can exhibit spatial oscillation at sub-terahertz frequencies. Time-resolved detection of such picosecond dynamics of quantum waves is important, as the detection provides a way of understanding the quantum behavior of electrons in nano-electronics. It also applies to quantum information technologies such as the ultrafast quantum-bit operation of quantum computing and high-sensitivity electromagnetic-field sensing. However, detecting picosecond dynamics has been a challenge since the sub-terahertz scale is far beyond the latest bandwidth measurement tools. A KAIST team led by Professor Heung-Sun Sim developed a theory of ultrafast electron dynamics in a nanoscale trap, and proposed a scheme for detecting the dynamics, which utilizes a quantum-mechanical resonant state formed beside the trap. The coupling between the electron dynamics and the resonant state is switched on and off at a picosecond so that information on the dynamics is read out on the electric current being generated when the coupling is switched on. NTT realized, together with NPL, the detection scheme and applied it to electron motions in a nanoscale trap formed in a silicon transistor. A single electron was captured in the trap by controlling electrostatic gates, and a resonant state was formed in the potential barrier of the trap. The switching on and off of the coupling between the electron and the resonant state was achieved by aligning the resonance energy with the energy of the electron within a picosecond. An electric current from the trap through the resonant state to an electrode was measured at only a few Kelvin degrees, unveiling the spatial quantum-coherent oscillation of the electron with 250 GHz frequency inside the trap. Professor Sim said, “This work suggests a scheme of detecting picosecond electron motions in submicron scales by utilizing quantum resonance. It will be useful in dynamical control of quantum mechanical electron waves for various purposes in nano-electronics, quantum sensing, and quantum information”. This work was published online at Nature Nanotechnology on November 4. It was partly supported by the Korea National Research Foundation through the SRC Center for Quantum Coherence in Condensed Matter. For more on the NTT news release this article, please visit https://www.ntt.co.jp/news2019/1911e/191105a.html Profile: Prof. Heung-Sun Sim Professor, Department of Physics Director, SRC Center for Quantum Coherence in Condensed Matter Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea firstname.lastname@example.org https://qet.kaist.ac.kr Publication: Gento Yamahata, Sungguen Ryu, Nathan Johnson, H.-S. Sim, Akira Fujiwara, and Masaya Kataoka. 2019. Picosecond coherent electron motion in a silicon single-electron source. Nature Nanotechnology (Online Publication). 6 pages. https://doi.org/10.1038/s41565-019-0563-2 Link to download the full-text paper: https://www.nature.com/articles/s41565-019-0563-2.pdf
Dr. Sejeong Kim Recognized as Excellent Young Scientist
(Dr. Sejeong Kim) Dr. Sejeong Kim, a postdoctoral research associate in the School of Mathematical and Physical Sciences at the University of Technology Sydney was honored to receive the Excellence Award for a Young Scientist by the Korea Federation of Women’s Science & Technology Association (KOFWST). The award ceremony will be held on October 31 in Seoul. KOFWST recognizes ten promising young female scientists and engineers every year who show significant potential, passion, and remarkable achievement in their work. The awardees are selected among those who finished their degree within the previous five years. Dr. Kim earned her Ph.D. in physics at KAIST in 2014 and was selected as the winner in the field of physics in recognition of her outstanding research activities in photonics. Dr. Kim conducted various research activities in the field of photonics and was published in high impact journals including Nano Letters and Advanced materials. In July, she developed the first photonic cavity from van der Waals materials and published the study in Nature Communications titled “Photonic Crystal Cavities from Hexagonal Boron Nitride.” At UTS, she carries out research activities supervised by Professor Igor Aharonovich and has engaged in many science outreach activities.
Professor YongKeun Park Wins the 2018 Fumio Okano Award
(Professor Park) Professor YongKeun Park from the Department of Physics won the 2018 Fumio Okano Award in recognition of his contributions to 3D display technology development during the annual conference of the International Society for Optics and Photonics (SPIE) held last month in Orlando, Florida in the US. The Fumio Okano Best 3D Paper Prize is presented annually in memory of Dr. Fumio Okano, a pioneer and innovator of 3D displays who passed away in 2013, for his contributions to the field of 3D TVs and displays. The award is sponsored by NHK-ES. Professor Park and his team are developing novel technology for measuring and visualizing 3D images by applying random light scattering. He has published numerous papers on 3D holographic camera technology and 3000x enhanced performance of 3D holographic displays in renowned international journals such as Nature Photonics, Nature Communications, and Science Advances. His technology has drawn international attention from renowned media outlets including Newsweek and Forbes. He has established two startups to commercialize his technology. Tomocube specializes in 3D imaging microscopes using holotomographic technology and the company exports their products to several countries including the US and Japan. The.Wave.Talk is exploring technology for examining pre-existing bacteria anywhere and anytime. Professor Park’s innovations have already been recognized in and out of KAIST. In February, he was selected as the KAISTian of the Year for his outstanding research, commercialization, and startups. He was also decorated with the National Science Award in April by the Ministry of Science and ICT and the Hong Jin-Ki Innovation Award later in May by the Yumin Cultural Foundation. Professor Park said, “3D holography is emerging as a significant technology with growing potential and positive impacts on our daily lives. However, the current technology lags far behind the levels displayed in SF movies. We will do our utmost to reach this level with more commercialization."
Park Chosen for Principality of Monaco/ITER Postdoctoral Fellowship
(Jaesun Park in the Integrated Master's and Doctoral Degree Program ) Jaesun Park from the Department of Physics, was selected as a Principality of Monaco/ITER Postdoctoral Fellowship recipient. This program was established by the Principality of Monaco and an international organization, ITER, in January 2008 to support postdoctoral researchers who will be working for ITER. It is a relatively competitive program because it chooses only five people every two years. The selected postdoctoral researchers will be working for ITER for two years while conducting research projects with outstanding researchers in the field of nuclear fusion. ITER, one of the most ambitious energy projects, was launched in 1985 with the purpose of carrying out joint research on nuclear fusion energy. Currently, about 800 people are working for this organization. Seven ITER member countries (i.e. Korea, the European Union, the United States, China, Japan, Russia, and India) are sharing the expenses and engaging in mega-scale science projects. Korea shares 9.1% (20 billion Euro) of the total construction costs of ITER experimental devices. Park will begin his duties in early 2019.
KAIST Professors Selected as Y-KAST Members
Professor YongKeun Park, Professor Bumjoon Kim, Professor Keon Jae Lee, and Professor Young Seok Ju were selected as the newest members of the Young Korean Academy of Science and Technology (Y-KAST). The Korean Academy of Science and Technology, an academic institution of professional experts, selected 26 promising scientists under the age of 43 to join Y-KAST. and four KAIST professors were included in the list. The newest members were conferred on February 26. Research Field Name Natural Sciences YongKeun Park (Dept. of Physics) Engineering Bumjoon Kim (Dept. of Chemical and Biomolecular Engineering) Agricultural & Fishery Sciences Keon Jae Lee (Dept. of Materials Science and Engineering) Medical Sciences Young Seok Ju (Graduate School of Medical Science and Engineering)
KAIST Students Invited to the BNL
Siheon Ryee and Taek Jung Kim, combined Masters and PhD students from the Department of Physics, have been invited to be visiting researchers at the Brookhaven National Laboratory (BNL). The BNL, located in Long Island, New York, is one of the most esteemed institutes in the United States. Ryee and Kim received the invitation from the Center for Computational Design of Functional Strongly Correlated Materials and Theoretical Spectroscopy. This center was established by scholars who have been leading this field in the United States. The two students will be participating in developing a methodology and code for calculating strongly correlated electronic materials, and a grant of 40,000 USD will be provided to each student. This amount of support is not often awarded to researchers outside of postdoctoral programs. Moreover, they are guaranteed to continue their combined Masters and PhD program and write their dissertations under the supervision of their advisor, Professor Myung Joon Han from the Department of Physics. Professor Han said, “I was impressed by how well-known scholars established the center in order to cooperate with each other to solve challenging problems. Also, I was surprised and happy that my students were invited to this outstanding institute.” “I believe that doing research with leaders in their field will give valuable experience to the students. At the same time, my students will be a great help to the scholars of the institute,” he added.
Meet the KAISTian of 2017, Professor YongKeun Park
Professor YongKeun Park from the Department of Physics is one of the star professors in KAIST. Rising to the academic stardom, Professor Park’s daily schedule is filled with series of business meetings in addition to lab meetings and lectures. The year 2017 must have been special for him. During the year, he published numerous papers in international journals, such as Nature Photonics, Nature Communications and Science Advances. These high performances drew international attention from renowned media, including Newsweek and Forbes. Moreover, recognizing his research performance, he was elected as a fellow member of the Optical Society (OSA) in his mid-30s. Noting that the members’ age ranges from late 50s to early 60s, Professor Park’s case considered to be quite exceptional. Adding to his academic achievement, he has launched two startups powered of his own technologies. One is called Tomocube, a company specialized in 3-D imaging microscope using holotomography technology. His company is currently exporting the products to multiple countries, including the United States and Japan. The other one is The.Wave.Talk which has technologies for examining pre-existing bacteria anywhere and anytime. His research career and entrepreneurship are well deserved recipient of many honors. At the 2018 kick-off ceremony, Professor Park was awarded the KAISTian of 2017 in recognition of his developing holographic measure and control technology as well as founding a new field for technology application. KAISTian of the Year, first presented in 2001, is an award to recognize the achievements and exemplary contribution of KAIST member who has put significant effort nationally and internationally, enhancing the value of KAIST. While receiving the award, he thanked his colleagues and his students who have achieved this far together. He said, “I would like to thank KAIST for providing environment for young professors like me so that we can engage themselves in research. Also, I would like to mention that I am an idea seeder and my students do the most of the research. So, I appreciate my students for their hard works, and it is very pleasure to have them. Lastly, I thank the professors for teaching these outstanding students. I feel great responsibility over this title. I will dedicate myself to make further progress in commercializing technology in KAIST.” Expecting his successful startup cases as a model and great inspiration to students as well as professors, KAIST interviewed Professor Park. Q What made you decide to found your startups? A I believed that my research areas could be further used. As a professor, I believe that it is a university’s role to create added value through commercializing technology and creating startups. Q You have co-founded two startups. What is your role in each company? A So, basically I have two full-time jobs, professor in KAIST and CTO in Tomocube. After transferring the technology, I hold the position of advisor in The.Wave.Talk. (Holographic images captured by the product Professor Park developed) Q Do your students also participate in your companies or can they? A No, the school and companies are separate spaces; in other words, they are not participating in my companies. They have trained my employees when transferring the technologies, but they are not directly working for the companies. However, they can participate if they want to. If there’s a need to develop a certain technology, an industry-academia contract can be made. According to the agreement, students can work for the companies. Q Were there any hardships when preparing the startups? A At the initial stage, I did not have a financial problem, thanks to support from Startup KAIST. Yet, inviting capital is the beginning, and I think every step I made to operate, generate revenue, and so on is not easy. Q Do you believe KAIST is startup-friendly? A Yes, there’s no school like KAIST in Korea and any other country. Besides various programs to support startup activities, Startup KAIST has many professors equipped with a great deal of experience. Therefore, I believe that KAIST provides an excellent environment for both students and professors to create startups. Q Do you have any suggestion to KAIST institutionally? A Well, I would like to make a comment to students and professors in KAIST. I strongly recommend them to challenge themselves by launching startups if they have good ideas. Many students wish to begin their jobs in government-funded research institutes or major corporates, but I believe that engaging in a startup company will also give them valuable and very productive experience. Unlike before, startup institutions are well established, so attracting good capital is not so hard. There are various activities offered by Startup KAIST, so it’s worthwhile giving it a try. Q What is your goal for 2018 as a professor and entrepreneur? A I don’t have a grand plan, but I will work harder to produce good students with new topics in KAIST while adding power to my companies to grow bigger. By Se Yi Kim from the PR Office
Professor YongKeun Park Elected as a Fellow of the Optical Society
Professor YongKeun Park, from the Department of Physics at KAIST, was elected as a fellow member of the Optical Society (OSA) in Washington, D.C. on September 12. Fellow membership is given to members who have made a significant contribution to the advancement of optics and photonics. Professor Park was recognized for his research on digital holography and wavefront control technology. Professor Park has been producing outstanding research outcomes in the field of holographic technology and light scattering control since joining KAIST in 2010. In particular, he developed and commercialized technology for a holographic telescope. He applied it to various medical and biological research projects, leading the field worldwide. In the past, cells needed to be dyed with fluorescent materials to capture a 3-D image. However, Professor Park’s holotomography (HT) technology can capture 3-D images of living cells and tissues in real time without color dyeing. This technology allows diversified research in the biological and medical field. Professor Park established a company, Tomocube, Inc. in 2015 to commercialize the technology. In 2016, he received funding from SoftBank Ventures and Hanmi Pharmaceutical. Currently, major institutes, including MIT, the University of Pittsburgh, the German Cancer Research Center, and Seoul National University Hospital are using his equipment. Recently, Professor Park and his team developed technology based on light scattering measurements. With this technology, they established a company called The Wave Talk and received funding from various organizations, such as NAVER. Its first product is about to be released. Professor Park said, “I am glad to become a fellow member based on the research outcomes I produced since I was appointed as a professor at KAIST. I would like to thank the excellent researchers as well as the school for its support. I will devote myself to continuously producing novel outcomes in both basic and applied fields.” Professor Park has published nearly 100 papers in renowned journals including Nature Photonics, Nature Communications, Science Advances, and Physical Review Letters.
A KAIST Alumnus Receives the Marie Sklodowska-Curie Individual Fellowships
Dr. Je-Kyung Ryu, a graduate of the Physics Department at KAIST in 2014, received the 2017 Marie Sklodowska-Curie Individual Fellowship. Established in 1996, the Marie Sklodowska-Curie Individual Fellowships support young scientists in or outside Europe to help them grow as independent researchers. The recipients are recognized to have the highest potential to make a difference in science and technology and work on research and innovation. Dr. Ryu is currently working as a postdoctoral researcher at the Cees Dekker Lab in the Department of Bionanoscience at the Kavli Institute of Nanoscience at Delft University of Technology (TU Delft), Netherlands. He was among six international researchers at TU Delft who were awarded this research grant. The grant of 177,000 euros will be offered for two years from March 2017 to February 2019 to cover his salary and research expenses. For a news article published by TU Delft on the award, please click below: QN and BN Successfully Attract Young Scientific Talent February 1, 2017
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