Browsing by Author "Wu, Han"
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Item Correlation-driven electronic reconstruction in FeTe1−xSex(Springer Nature, 2022) Huang, Jianwei; Yu, Rong; Xu, Zhijun; Zhu, Jian-Xin; Oh, Ji Seop; Jiang, Qianni; Wang, Meng; Wu, Han; Chen, Tong; Denlinger, Jonathan D.; Mo, Sung-Kwan; Hashimoto, Makoto; Michiardi, Matteo; Pedersen, Tor M.; Gorovikov, Sergey; Zhdanovich, Sergey; Damascelli, Andrea; Gu, Genda; Dai, Pengcheng; Chu, Jiun-Haw; Lu, Donghui; Si, Qimiao; Birgeneau, Robert J.; Yi, Ming; Rice Center for Quantum MaterialsElectronic correlation is of fundamental importance to high temperature superconductivity. While the low energy electronic states in cuprates are dominantly affected by correlation effects across the phase diagram, observation of correlation-driven changes in fermiology amongst the iron-based superconductors remains rare. Here we present experimental evidence for a correlation-driven reconstruction of the Fermi surface tuned independently by two orthogonal axes of temperature and Se/Te ratio in the iron chalcogenide family FeTe1−xSex. We demonstrate that this reconstruction is driven by the de-hybridization of a strongly renormalized dxy orbital with the remaining itinerant iron 3d orbitals in the emergence of an orbital-selective Mott phase. Our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase into an orbital-selective Mott phase in dxy as Se concentration is reduced.Item Nonsymmorphic symmetry-protected band crossings in a square-net metal PtPb4(Springer Nature, 2022) Wu, Han; Hallas, Alannah M.; Cai, Xiaochan; Huang, Jianwei; Oh, Ji Seop; Loganathan, Vaideesh; Weiland, Ashley; McCandless, Gregory T.; Chan, Julia Y.; Mo, Sung-Kwan; Lu, Donghui; Hashimoto, Makoto; Denlinger, Jonathan; Birgeneau, Robert J.; Nevidomskyy, Andriy H.; Li, Gang; Morosan, Emilia; Yi, Ming; Rice Center for Quantum MaterialsTopological semimetals with symmetry-protected band crossings have emerged as a rich landscape to explore intriguing electronic phenomena. Nonsymmorphic symmetries in particular have been shown to play an important role in protecting the crossings along a line (rather than a point) in momentum space. Here we report experimental and theoretical evidence for Dirac nodal line crossings along the Brillouin zone boundaries in PtPb4, arising from the nonsymmorphic symmetry of its crystal structure. Interestingly, while the nodal lines would remain gapless in the absence of spin–orbit coupling (SOC), the SOC, in this case, plays a detrimental role to topology by lifting the band degeneracy everywhere except at a set of isolated points. Nevertheless, the nodal line is observed to have a bandwidth much smaller than that found in density functional theory (DFT). Our findings reveal PtPb4 to be a material system with narrow crossings approximately protected by nonsymmorphic crystalline symmetries.Item Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film(Springer Nature, 2024) Ren, Zheng; Huang, Jianwei; Tan, Hengxin; Biswas, Ananya; Pulkkinen, Aki; Zhang, Yichen; Xie, Yaofeng; Yue, Ziqin; Chen, Lei; Xie, Fang; Allen, Kevin; Wu, Han; Ren, Qirui; Rajapitamahuni, Anil; Kundu, Asish K.; Vescovo, Elio; Kono, Junichiro; Morosan, Emilia; Dai, Pengcheng; Zhu, Jian-Xin; Si, Qimiao; Minár, Ján; Yan, Binghai; Yi, Ming; Smalley-Curl InstituteMagnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X = Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe $${{{{\rm{d}}}}}_{{xy}}+{{{{\rm{d}}}}}_{{x}^{2}-{y}^{2}}$$spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.Item 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 MaterialsNon-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.Item Rice convection model simulations of the centrifugal interchange instability in the magnetospheres of Jupiter and Saturn(2009) Wu, Han; Hill, Thomas W.Radial plasma transport in Jupiter's and Saturn's magnetospheres is driven by the centrifugal interchange instability. Simulations with the Rice Convection Model (RCM) of the injection-dispersion phenomenon in Saturn's magnetosphere produce interchange convection cells stretching outward from the outer edge of the plasma torus centered near the orbit of Enceladus. We first use an idealized cold plasma torus near L = 4 (the orbit of Enceladus) as an initial distribution. Small perturbations in the torus region grow into interchange convection cells with alternating sectors of inflow and outflow. We show that the velocity-dependent Coriolis effects can be included by an effective Hall conductance, which is first implemented here using the grid based RCM. The simulation shows the following effects introduced by the Coriolis force: (1) bending of the convection cells in the retrograde direction, (2) slowing of their growth, and (3) broadening the out-moving fingers. Our simulation results support and distinguish the predictions made by Vasyliunas [GRL, 21, 401, 1994] and Pontius [GRL, 24, 2961, 1997]. Then we address two curious properties revealed by Cassini observations. (1) Several magnetospheric properties, including electron density, are modulated by the SKR longitude of the spacecraft [e.g., Gurnett et al., Science, 316 , 442, 2007]. A two-cell corotating convection model has been invoked by Gurnett et al. and by Goldreich and Farmer [JGR, 112, A05225, 2007] to explain this modulation. We have simulated this system by imposing an initially asymmetric plasma distribution in the Enceladus torus region, and find that the initial distribution must be at least ∼40% asymmetric in order to give a factor ∼2 asymmetry at 5 Saturn radii as reported. (2) The inflow sectors ("injection events") are much narrower in longitude than the interspersed outflow sectors. There is no obvious explanation for this in linear instability theory, nor in previous RCM simulations without an active plasma source. Our inclusion of an extended active continuous plasma source, combined with the Coriolis effects, provides a possible explanation. Finally, the initial results of inclusion of the pickup currents are reported.Item 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, MingQuasi-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.Item Three-dimensional covalent organic frameworks with pto and mhq-z topologies based on Tri- and tetratopic linkers(Springer Nature, 2023) Zhu, Dongyang; Zhu, Yifan; Chen, Yu; Yan, Qianqian; Wu, Han; Liu, Chun-Yen; Wang, Xu; Alemany, Lawrence B.; Gao, Guanhui; Senftle, Thomas P.; Peng, Yongwu; Wu, Xiaowei; Verduzco, RafaelThree-dimensional (3D) covalent organic frameworks (COFs) possess higher surface areas, more abundant pore channels, and lower density compared to their two-dimensional counterparts which makes the development of 3D COFs interesting from a fundamental and practical point of view. However, the construction of highly crystalline 3D COF remains challenging. At the same time, the choice of topologies in 3D COFs is limited by the crystallization problem, the lack of availability of suitable building blocks with appropriate reactivity and symmetries, and the difficulties in crystalline structure determination. Herein, we report two highly crystalline 3D COFs with pto and mhq-z topologies designed by rationally selecting rectangular-planar and trigonal-planar building blocks with appropriate conformational strains. The pto 3D COFs show a large pore size of 46 Å with an extremely low calculated density. The mhq-z net topology is solely constructed from totally face-enclosed organic polyhedra displaying a precise uniform micropore size of 1.0 nm. The 3D COFs show a high CO2 adsorption capacity at room temperature and can potentially serve as promising carbon capture adsorbents. This work expands the choice of accessible 3D COF topologies, enriching the structural versatility of COFs.