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| 1 | Stiefel–Whitney classes and topological phases in band theory显示文摘We review the recent progress in the study of topological phases in systems with space–time inversion symmetry IST.ISTis an anti-unitary symmetry which is local in momentum space and satisfies I2 ST= 1 such as PT in two dimensions(2 D)and three dimensions(3 D) without spin–orbit coupling and C2 T in 2 D with or without spin–orbit coupling, where P, T, C2 indicate the inversion, time-reversal, and two-fold rotation symmetries, respectively. Under IST, the Hamiltonian and the periodic part of the Bloch wave function can be constrained to be real-valued, which makes the Berry curvature and the Chern number vanish. In this class of systems, gapped band structures of real wave functions can be topologically distinguished by the Stiefel–Whitney numbers instead. The first and second Stiefel–Whitney numbers w1 and w2, respectively,are the corresponding invariants in 1 D and 2 D, which are equivalent to the quantized Berry phase and the Z2 monopole charge, respectively. We first describe the topological phases characterized by the first Stiefel–Whitney number, including1 D topological insulators with quantized charge polarization, 2 D Dirac semimetals, and 3 D nodal line semimetals. Next we review how the second Stiefel–Whitney class characterizes the 3 D nodal line semimetals carrying a Z2 monopole charge.In particular, we explain how the second Stiefel–Whitney number w2, the Z2 monopole charge, and the linking number between nodal lines are related. Finally, we review the properties of 2 D and 3 D topological insulators characterized by the nontrivial second Stiefel Whitney class. | Junyeong Ahn Sungjoon Park Dongwook Kim Youngkuk Kim Bohm-Jung Yang | 2019 | Chinese Physics B2019，28，11： | 2 |
| 2 | Chain Extension Study of Aqueous Polyurethane Dispersions显示文摘 | Jhon Youngkuk Cheong Inwoo Kim Junghyun | 2001 | Colloids and Surfaces A: Physicochemical and Engineering Aspects2001，197，： | 1 |
| 3 | Selective emitter using porous silicon for crystalline silicon solar cells显示文摘 | Inyong Moon Kyunghae Kim M. Thamilselvan Youngkuk Kim Kyumin Han Doheon Kyeong Taeyoung Kwon Dao Vinh Ai Jeongchul Lee Minkyu Ju Kyungsoo Lee Junsin Yi | 2008 | Solar Energy Materials and Solar Cells2008，，6： | 1 |
| 4 | Design of 2D massless Dirac fermion systems and quantum spin Hall insulators based on sp-sp^(2) carbon sheets显示文摘Grapheneis a massless Dirac fermion system,featuring Dirac points in momentum space.It was also first identified as a quantum spin Hall(QSH)insulator when considering spin–orbit coupling(SOC),which opens a band gap at the Dirac points.This discovery has initiated new research efforts to study the QSH effect,towards its application for quantum computing and spintronics.Although the QSH effect has been observed in HgTe quantum wells,the SOC strength of graphene is too small(~1μeV)to induce the topological insulator phase in an experimentally achievable temperature regime.Here,we perform a systematic atomistic simulation to design two-dimensional sp–sp^(2) hybrid carbon sheets to discover new Dirac systems,hosting the QSH phase.21 out of 31 newly discovered carbon sheets are identified as Dirac fermion systems without SOC,distinct from graphene in the number,shape,and position of the Dirac cones occurring in the Brillouin zone.Moreover,we find 19 out of the 21 new Dirac fermion systems become QSH insulators with a sizable SOC gap enhanced up to an order of meV,thus allowing for the QSH effect at experimentally accessible temperatures.In addition,based on the 26 Dirac fermion systems,we make a connection between the number of Dirac points without SOC and the resultant QSH phase in the presence of SOC.Our findings present new prospects for the design of topological materials with desired properties. | Minwoo Park Youngkuk Kim Hoonkyung Lee | 2018 | npj Computational Materials2018，，1： | 1 |
| 5 | Preparation and characterization of p-type hydrogenated amorphous silicon oxidefilm and its application to solar cell 显示文摘 | Yoon Kichan Kim Youngkuk Park JinJoo | 2011 | Journal of Non-Crystalline Solids2011，357，15： | 1 |
| 6 | Low defect interface study of intrinsic layer for c-Si surface passivation in a-Si:H/c-Si heterojunction solar cells显示文摘 | Sangho Kim Vinh Ai Dao Chonghoon Shin Jaehyun Cho Youngseok Lee Nagarajan Balaji Shihyun Ahn Youngkuk Kim Junsin Yi | 2012 | Thin Solid Films2012，，： | 1 |
| 7 | Theeffect of rear surface polishing to the performance of thincrystalline silicon solar cells 显示文摘 | Kim Youngkuk Jung Sungwook Ju Minkyu | 2011 | Solar Energy2011，85，： | 1 |
| 8 | Replica higher-order topology of Hofstadter butterflies in twisted bilayer graphene显示文摘The Hofstadter energy spectrum of twisted bilayer graphene(TBG)is found to have recursive higher-order topological properties.We demonstrate that higher-order topological insulator(HOTI)phases,characterized by localized corner states,occur as replicas of the original HOTIs to fulfill the self-similarity of the Hofstadter spectrum.We show the existence of exact flux translational symmetry in TBG at all commensurate angles.Based on this result,we identify that the original HOTI phase at zero flux is re-entrant at a half-flux periodicity,where the effective twofold rotation is preserved.In addition,numerous replicas of the original HOTIs are found for fluxes without protecting symmetries.Like the original HOTIs,replica HOTIs feature both localized corner states and edge-localized real-space topological markers.The replica HOTIs originate from the different interaction scales,namely,intralayer and interlayer couplings,in TBG.The topological aspect of Hofstadter butterflies revealed in our results highlights symmetry-protected topology in quantum fractals. | Sun-Woo Kim Sunam Jeon Moon Jip Park Youngkuk Kim | 2023 | npj Computational Materials2023，，1： | 0 |
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