Browsing by Author "Guo, Yucheng"

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    Observation of flat bands and Dirac cones in a pyrochlore lattice superconductor
    (Springer Nature, 2024) Huang, Jianwei; Setty, Chandan; Deng, Liangzi; You, Jing-Yang; Liu, Hongxiong; Shao, Sen; Oh, Ji Seop; Guo, Yucheng; Zhang, Yichen; Yue, Ziqin; Yin, Jia-Xin; Hashimoto, Makoto; Lu, Donghui; Gorovikov, Sergey; Dai, Pengcheng; Denlinger, Jonathan D.; Allen, J. W.; Hasan, M. Zahid; Feng, Yuan-Ping; Birgeneau, Robert J.; Shi, Youguo; Chu, Ching-Wu; Chang, Guoqing; Si, Qimiao; Yi, Ming; Rice Center for Quantum Materials
    Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge order, nematicity, and superconductivity. However, the interlayer coupling of the kagome layers disrupts the destructive interference needed to completely quench the kinetic energy. Here we demonstrate that an interwoven kagome network—a pyrochlore lattice—can host a three dimensional (3D) localization of electron wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice guarantees all band crossings at the Brillouin zone X point to be 3D gapless Dirac points, which was predicted theoretically but never yet observed experimentally. Through a combination of angle-resolved photoemission spectroscopy, fundamental lattice model and density functional theory calculations, we investigate the novel electronic structure of a Laves phase superconductor with a pyrochlore sublattice, CeRu2. We observe evidence of flat bands originating from the Ce 4f orbitals as well as flat bands from the 3D destructive interference of the Ru 4d orbitals. We further observe the nonsymmorphic symmetry-protected 3D gapless Dirac cone at the X point. Our work establishes the pyrochlore structure as a promising lattice platform to realize and tune novel emergent phases intertwining topology and many-body interactions.
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    Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet
    (Springer Nature, 2024) Wu, Han; Chen, Lei; Malinowski, Paul; Jang, Bo Gyu; Deng, Qinwen; Scott, Kirsty; Huang, Jianwei; Ruff, Jacob P. C.; He, Yu; Chen, Xiang; Hu, Chaowei; Yue, Ziqin; Oh, Ji Seop; Teng, Xiaokun; Guo, Yucheng; Klemm, Mason; Shi, Chuqiao; Shi, Yue; Setty, Chandan; Werner, Tyler; Hashimoto, Makoto; Lu, Donghui; Yilmaz, Turgut; Vescovo, Elio; Mo, Sung-Kwan; Fedorov, Alexei; Denlinger, Jonathan D.; Xie, Yaofeng; Gao, Bin; Kono, Junichiro; Dai, Pengcheng; Han, Yimo; Xu, Xiaodong; Birgeneau, Robert J.; Zhu, Jian-Xin; da Silva Neto, Eduardo H.; Wu, Liang; Chu, Jiun-Haw; Si, Qimiao; Yi, Ming; Rice Center for Quantum Materials
    Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5−δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.
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    Room-Temperature Topological Phase Transition in Quasi-One-Dimensional Material Bi4I4
    (American Physical Society, 2021) Huang, Jianwei; Li, Sheng; Yoon, Chiho; Oh, Ji Seop; Wu, Han; Liu, Xiaoyuan; Dhale, Nikhil; Zhou, Yan-Feng; Guo, Yucheng; Zhang, Yichen; Hashimoto, Makoto; Lu, Donghui; Denlinger, Jonathan; Wang, Xiqu; Lau, Chun Ning; Birgeneau, Robert J.; Zhang, Fan; Lv, Bing; Yi, Ming
    Quasi-one-dimensional (1D) materials provide a superior platform for characterizing and tuning topological phases for two reasons: (i) existence for multiple cleavable surfaces that enables better experimental identification of topological classification and (ii) stronger response to perturbations such as strain for tuning topological phases compared to higher dimensional crystal structures. In this paper, we present experimental evidence for a room-temperature topological phase transition in the quasi-1D material Bi4I4, mediated via a first-order structural transition between two distinct stacking orders of the weakly coupled chains. Using high-resolution angle-resolved photoemission spectroscopy on the two natural cleavable surfaces, we identify the high-temperature β phase to be the first weak topological insulator with two gapless Dirac cones on the (100) surface and no Dirac crossing on the (001) surface, while in the low-temperature α phase, the topological surface state on the (100) surface opens a gap, consistent with a recent theoretical prediction of a higher-order topological insulator beyond the scope of the established topological materials databases that hosts gapless hinge states. Our results not only identify a rare topological phase transition between first-order and second-order topological insulators but also establish a novel quasi-1D material platform for exploring unprecedented physics.