Browsing by Author "Chen, Tong"

Now showing 1 - 9 of 9
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    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 Materials
    Electronic 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.
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    Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system
    (Springer Nature, 2023) Gao, Bin; Chen, Tong; Wu, Xiao-Chuan; Flynn, Michael; Duan, Chunruo; Chen, Lebing; Huang, Chien-Lung; Liebman, Jesse; Li, Shuyi; Ye, Feng; Stone, Matthew B.; Podlesnyak, Andrey; Abernathy, Douglas L.; Adroja, Devashibhai T.; Duc Le, Manh; Huang, Qingzhen; Nevidomskyy, Andriy H.; Morosan, Emilia; Balents, Leon; Dai, Pengcheng
    Magnetic order in most materials occurs when magnetic ions with finite moments arrange in a particular pattern below the ordering temperature. Intriguingly, if the crystal electric field (CEF) effect results in a spin-singlet ground state, a magnetic order can still occur due to the exchange interactions between neighboring ions admixing the excited CEF levels. The magnetic excitations in such a state are spin excitons generally dispersionless in reciprocal space. Here we use neutron scattering to study stoichiometric Ni2Mo3O8, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has magnetic order. Furthermore, CEF spin excitons from the tetrahedral sites form a dispersive diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.
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    Dynamic Spin-Lattice Coupling and Nematic Fluctuations in NaFeAs
    (American Physical Society, 2018) Li, Yu; Yamani, Zahra; Song, Yu; Wang, Weiyi; Zhang, Chenglin; Tam, David W.; Chen, Tong; Hu, Ding; Xu, Zhuang; Chi, Songxue; Xia, Ke; Zhang, Li; Cui, Shifeng; Guo, Wenan; Fang, Ziming; Liu, Yi; Dai, Pengcheng
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    Electronic and Magnetic Anisotropies in FeSe Family of Iron-Based Superconductors
    (Frontiers, 2020) Chen, Tong; Yi, Ming; Dai, Pengcheng
    Most parent compounds of iron-based superconductors (FeSCs) exhibit a tetragonal-to-orthorhombic lattice distortion below Ts associated with an electronic nematic phase that breaks the four-fold (C4) rotational symmetry of the underlying lattice, and then forms collinear antiferromagnetic (AF) below TN (TN ≤ Ts). Optimal superconductivity emerges upon suppression of the nematic and AF phases. FeSe, which also exhibits a nematic phase transition below Ts but becomes superconducting in the nematic phase without AF order, provides a unique platform to study the interplay amongst the nematic phase, AF order, and superconductivity. In this review, we focus on the experiments done on uniaxial pressure detwinned single crystals of FeSe and other FeSCs and highlight the importance of understanding the electronic and magnetic anisotropy in elucidating the nature of unconventional superconductivity.
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    Magnetic field effects in an octupolar quantum spin liquid candidate
    (American Physical Society, 2022) Gao, Bin; Chen, Tong; Yan, Han; Duan, Chunruo; Huang, Chien-Lung; Yao, Xu Ping; Ye, Feng; Balz, Christian; Stewart, J. Ross; Nakajima, Kenji; Ohira-Kawamura, Seiko; Xu, Guangyong; Xu, Xianghan; Cheong, Sang-Wook; Morosan, Emilia; Nevidomskyy, Andriy H.; Chen, Gang; Dai, Pengcheng
    Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to an external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here we use neutron scattering to show that the application of a large external magnetic field to Ce2Zr2O7, an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wave vectors in Ce2Sn2O7 pyrochlore, and its absence in Ce2Zr2O7. Therefore, our results identify Ce2Zr2O7 as a strong candidate for an octupolar U(1) QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.
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    Neutron scattering studies on FeSe, Ce2Zr2O7, BaNi2(AsO4)2, and Ni2Mo3O8
    (2021-11-22) Chen, Tong; Dai, Pengcheng
    Neutron scattering is a powerful and versatile technique to probe spin-spin cor- relations in condensed matter physics. In this thesis, we present our work using neutron scattering to investigate the interplay of magnetism and superconductivity in the iron-based superconductor FeSe, the ground state of the quantum spin liquid candidate Ce2Zr2O7, topological magnons in BaNi2(AsO4)2, and the spin dynamics in the singlet-ground-state compound Ni2Mo3O8. In FeSe, superconductivity emerges from a nematic phase that breaks the in- plane four-fold (C4) rotational symmetry without magnetic ordering. Experimental and theoretical studies of this nematic phase attribute it to orbital ordering, spin fluctuations, or hidden magnetic quadrupolar order. We performed inelastic neutron scattering (INS) on an assembly of detwinned single crystals of FeSe to demonstrate that spin fluctuations are most intense at the antiferromagnetic wavevector QAF = (±1, 0) in the normal state, and the strong nematic anisotropy is also reflected in the spin resonance at QAF in the superconducting state. The electronic anisotropy of the nematic phase supports a highly anisotropic superconducting gap driven by spin fluctuation. Besides, in twinned FeSe, we use INS to study the effect of a magnetic field on spin resonance. Magnetic fields aligned along the c-axis broaden and suppress the spin resonance more efficiently than fields in the in-plane directions. Our results are consistent with the anisotropic effect of magnetic fields on the superfluid den- sity calculated in heat capacity measurements, which suggests that the resonance in FeSe is associated with the superconducting electrons arising from orbital-selective quasiparticle excitations between the hole and electron Fermi surfaces. Among spin systems where unpaired electrons form local spins, the quantum spin liquid (QSL) is an exotic one that does not magnetically order at all temperatures. The agreement of experiments and theories in one-dimensional (1D) systems proved the capability of neutron scattering to detect fractionalized quasiparticles. We carried out INS on single crystals of Ce2Zr2O7and revealed the presence of the hallmark of a QSL - a continuum of magnetic excitations - corresponding to fractionalized spinons at 35 mK. We combined heat capacity, muon spin relaxation, and AC susceptibil- ity measurements to demonstrate that Ce2Zr2O7is three-dimensional (3D) QSL with minimal magnetic and non-magnetic disorder. The bosonic analogs of topological fermionic quasiparticles arising from spin waves were reported in two-dimensional (2D) honeycomb lattice ferro/antiferromagnets and 3D antiferromagnets. We performed INS to study spin waves of the S = 1 honeycomb lattice antiferromagnet BaNi2(AsO4)2, which develops a zig-zag antiferromagnetic (AF) order below TN = 6 K, and determine the interactions with a Heisenberg model up to third-nearest neighbor exchange. The Dirac-like band crossing protected by the glide mirror symmetry was observed in the spectra. Singlet-ground-state systems, in which the crystal field ground state of the mag- netic ion is a singlet, are of great interest because the magnetic properties are sensitive to the ratio of magnetic exchange to single-ion anisotropy. We used thermodynamic and neutron scattering experiments to demonstrate that Ni2Mo3O8, a bipartite hon- eycomb lattice comprised of tetrahedral and octahedral Ni2+ environment, is a singlet- ground-state system. We found ferro- and antiferromagnetic excitonic flat bands in the neutron scattering spectra.
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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.
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    Spin waves and Dirac magnons in a honeycomb-lattice zigzag antiferromagnet BaNi2(AsO4)2
    (American Physical Society, 2021) Gao, Bin; Chen, Tong; Wang, Chong; Chen, Lebing; Zhong, Ruidan; Abernathy, Douglas L.; Xiao, Di; Dai, Pengcheng
    The topological properties of massive and massless fermionic quasiparticles have been intensively investigated over the past decade in topological materials without magnetism. Recently, the bosonic analogs of such quasiparticles arising from spin waves have been reported in a two-dimensional (2D) honeycomb-lattice ferromagnet/antiferromagnet and a 3D antiferromagnet. Here, we use time-of-flight inelastic neutron scattering to study spin waves of the S=1 honeycomb-lattice antiferromagnet BaNi2(AsO4)2, which has a zigzag antiferromagnetic (AFM) ground state identical to that of the Kitaev quantum spin liquid candidate α−RuCl3. We determine the magnetic exchange interactions in the zigzag AFM ordered phase, and show that spin waves in BaNi2(AsO4)2 have symmetry-protected Dirac points inside the Brillouin zone boundary. These results provide a microscopic understanding of the zigzag AFM order and associated Dirac magnons in honeycomb-lattice magnets, and are also important for establishing the magnetic interactions in Kitaev quantum spin liquid candidates.
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    Topological Spin Excitations in Honeycomb Ferromagnet CrI3
    (American Physical Society, 2018) Chen, Lebing; Chung, Jae-Ho; Gao, Bin; Chen, Tong; Stone, Matthew B.; Kolesnikov, Alexander I.; Huang, Qingzhen; Dai, Pengcheng
    In two-dimensional honeycomb ferromagnets, bosonic magnon quasiparticles (spin waves) may either behave as massless Dirac fermions or form topologically protected edge states. The key ingredient defining their nature is the next-nearest-neighbor Dzyaloshinskii-Moriya interaction that breaks the inversion symmetry of the lattice and discriminates chirality of the associated spin-wave excitations. Using inelastic neutron scattering, we find that spin waves of the insulating honeycomb ferromagnet CrI3 (TC=61 K) have two distinctive bands of ferromagnetic excitations separated by a ∼4 meV gap at the Dirac points. These results can only be understood by considering a Heisenberg Hamiltonian with Dzyaloshinskii-Moriya interaction, thus providing experimental evidence that spin waves in CrI3 can have robust topological properties potentially useful for dissipationless spintronic applications.