Browsing by Author "Xie, Yaofeng"

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    Conventional superconductivity in the doped kagome superconductor Cs(V0.86Ta0.14)3Sb5 from vortex lattice studies
    (Springer Nature, 2024) Xie, Yaofeng; Chalus, Nathan; Wang, Zhiwei; Yao, Weiliang; Liu, Jinjin; Yao, Yugui; White, Jonathan S.; DeBeer-Schmitt, Lisa M.; Yin, Jia-Xin; Dai, Pengcheng; Eskildsen, Morten Ring
    A hallmark of unconventional superconductors is a complex electronic phase diagram where intertwined orders of charge-spin-lattice degrees of freedom compete and coexist. While the kagome metals such as CsV3Sb5 also exhibit complex behavior, involving coexisting charge density wave order and superconductivity, much is unclear about the microscopic origin of the superconducting pairing. We study the vortex lattice in the superconducting state of Cs(V0.86Ta0.14)3Sb5, where the Ta-doping suppresses charge order and enhances superconductivity. Using small-angle neutron scattering, a strictly bulk probe, we show that the vortex lattice exhibits a strikingly conventional behavior. This includes a triangular symmetry with a period consistent with 2e-pairing, a field dependent scattering intensity that follows a London model, and a temperature dependence consistent with a uniform superconducting gap. Our results suggest that optimal bulk superconductivity in Cs(V1−xTax)3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer electron-lattice coupling, different from spin fluctuation mediated unconventional copper- and iron-based superconductors.
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    Imaging real-space flat band localization in kagome magnet FeSn
    (Springer Nature, 2023) Multer, Daniel; Yin, Jia-Xin; Hossain, Md Shafayat; Yang, Xian; Sales, Brian C.; Miao, Hu; Meier, William R.; Jiang, Yu-Xiao; Xie, Yaofeng; Dai, Pengcheng; Liu, Jianpeng; Deng, Hanbin; Lei, Hechang; Lian, Biao; Zahid Hasan, M.; Rice Center for Quantum Materials
    Kagome lattices host flat bands due to their frustrated lattice geometry, which leads to destructive quantum interference of electron wave functions. Here, we report imaging of the kagome flat band localization in real-space using scanning tunneling microscopy. We identify both the Fe3Sn kagome lattice layer and the Sn2 honeycomb layer with atomic resolution in kagome antiferromagnet FeSn. On the Fe3Sn lattice, at the flat band energy determined by the angle resolved photoemission spectroscopy, tunneling spectroscopy detects an unusual state localized uniquely at the Fe kagome lattice network. We further show that the vectorial in-plane magnetic field manipulates the spatial anisotropy of the localization state within each kagome unit cell. Our results are consistent with the real-space flat band localization in the magnetic kagome lattice. We further discuss the magnetic tuning of flat band localization under the spin–orbit coupled magnetic kagome lattice model.
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    Neutron scattering studies of Sr(Co1−xNix)2As2, FeSn, CsV3Sb5, and YbMnBi2
    (2024-12-05) Xie, Yaofeng; Dai, Pengcheng
    In this thesis, we present several neutron scattering investigations on the complex magnetic and electronic properties of a series of quantum materials, including helical order in Sr(Co1-xNix)2As2, spin excitations in FeSn and CoSn, electron-phonon coupling in charge-density-wave state of CsV3Sb5, vortex lattice in Ta doped CsV3Sb5, and spin chirality in YbMnBi2. Firstly, we investigate magnetic ordering and spin fluctuations in Sr(Co1-xNix)2As2, a quasi-two-dimensional planar magnet. Neutron scattering studies reveal a c-axis incommensurate helical magnetic structure in Sr(Co1-xNix)2As2, with enhanced quasi-2D ferromagnetic spin fluctuations induced by Ni doping. Band structure calculations suggest that this helical order arises from Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions mediated by itinerant electrons, offering insight into the quantum order-by-disorder mechanism near a quantum critical point. Next, we examine spin excitations in the metallic kagome lattice materials FeSn and CoSn. In these systems, destructive quantum interference of electronic hopping paths produces nearly localized electrons, resulting in flat electronic bands. Our neutron scattering measurements uncover well-defined spin waves in FeSn and paramagnetic scattering in CoSn, highlighting the delicate balance between geometric frustration and magnetic order in kagome systems. Furthermore, we observe anomalous non-dispersive excitations, attributed to the scattering from hydrocarbon contamination. We also investigate the electron-phonon coupling in CsV3Sb5, a kagome lattice material exhibiting intertwined CDW and superconductivity. Neutron scattering experiments demonstrate that the CDW in CsV3Sb5 is associated with a static lattice distortion and a sudden hardening of a longitudinal optical phonon mode. This finding underscores the critical role of wave vector-dependent electron-phonon interactions in the CDW order, contributing to our understanding of its coupling with superconductivity in kagome metals. The fourth study focuses on the superconductivity in Ta-doped CsV3Sb5, which exhibits enhanced superconductivity upon suppression of CDW order. Through Small-Angle Neutron Scattering (SANS), we probe the vortex lattice structure and its evolution in the superconducting state of Cs(V0.86Ta0.14)3Sb5. Our results show that the vortex lattice exhibits a strikingly conventional behavior, including a triangular symmetry, conventional 2e pairing, and a field dependent scattering intensity that follows a London model. Our results suggest that optimal bulk superconductivity in Cs(V0.86Ta0.14)3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer electron-lattice coupling. Finally, we investigate the giant anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in the canted antiferromagnet YbMnBi2. The ab-plane spin canting in YbMnBi2 is believed to break time-reversal symmetry, generating a non-zero Berry curvature that gives rise to the giant ANE and AHE. However, direct evidence for this mechanism has remained elusive, as earlier unpolarized neutron measurements excluded significant moment canting. By leveraging the unique advantages of polarized neutron scattering, which can differentiate magnetic scattering from nuclear scattering, we have uncovered clear evidence of spin chirality persisting at temperatures well above room temperature. Additionally, further neutron scattering measurements have revealed inversion-symmetry breaking and anisotropic spin fluctuations, indicating the presence of Dzyaloshinsky-Moriya interactions that likely drive the observed spin chirality, which in turn underlies the ANE and AHE. Our findings provide a detailed mechanism that directly explains the origins of the giant ANE and AHE in YbMnBi2. Overall, the combination of these works advances the understanding of quantum materials by revealing new insights into the magnetic, electronic, and lattice dynamics of these complex systems. The results presented herein pave the way for future studies on quantum magnetism, unconventional superconductivity, and the development of new materials with novel electronic and magnetic properties.
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    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 Institute
    Magnetic 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.
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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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    Spin excitations in metallic kagome lattice FeSn and CoSn
    (Springer Nature, 2021) Xie, Yaofeng; Chen, Lebing; Chen, Tong; Wang, Qi; Yin, Qiangwei; Stewart, J. Ross; Stone, Matthew B.; Daemen, Luke L.; Feng, Erxi; Cao, Huibo; Lei, Hechang; Yin, Zhiping; MacDonald, Allan H.; Dai, Pengcheng
    In two-dimensional (2D) metallic kagome lattice materials, destructive interference of electronic hopping pathways around the kagome bracket can produce nearly localized electrons, and thus electronic bands that are flat in momentum space. When ferromagnetic order breaks the degeneracy of the electronic bands and splits them into the spin-up majority and spin-down minority electronic bands, quasiparticle excitations between the spin-up and spin-down flat bands should form a narrow localized spin-excitation Stoner continuum coexisting with well-defined spin waves in the long wavelengths. Here we report inelastic neutron scattering studies of spin excitations in 2D metallic kagome lattice antiferromagnetic FeSn and paramagnetic CoSn, where angle resolved photoemission spectroscopy experiments found spin-polarized and nonpolarized flat bands, respectively, below the Fermi level. Our measurements on FeSn and CoSn reveal well-defined spin waves extending above 140 meV and correlated paramagnetic scattering around Γ point below 90 meV, respectively. In addition, we observed non-dispersive excitations at ~170 meV and ~360 meV arising mostly from hydrocarbon scattering of the CYTOP-M used to glue the samples to aluminum holder. Therefore, our results established the evolution of spin excitations in FeSn and CoSn, and identified anomalous flat modes overlooked by the neutron scattering community for many years.