Browsing by Author "Ye, Feng"

Now showing 1 - 4 of 4
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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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    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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    Spin structure and dynamics of the topological semimetal Co3Sn2-xInxS2
    (Springer Nature, 2022) Neubauer, Kelly J.; Ye, Feng; Shi, Yue; Malinowski, Paul; Gao, Bin; Taddei, Keith M.; Bourges, Philippe; Ivanov, Alexandre; Chu, Jiun-Haw; Dai, Pengcheng
    The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM) metals with broken time-reversal symmetry, depends on electronic and magnetic properties. In Co3Sn2-xInxS2, a giant AHE has been attributed to Berry curvature associated with the FM Weyl semimetal phase, yet recent studies report complicated magnetism. We use neutron scattering to determine the spin dynamics and structures as a function of x and provide a microscopic understanding of the AHE and magnetism interplay. Spin gap and stiffness indicate a contribution from Weyl fermions consistent with the AHE. The magnetic structure evolves from c-axis ferromagnetism at $$x = 0$$to a canted antiferromagnetic (AFM) structure with reduced c-axis moment and in-plane AFM order at $$x = 0.12$$and further reduced c-axis FM moment at $$x = 0.3$$. Since noncollinear spins can induce non-zero Berry curvature in real space acting as a fictitious magnetic field, our results revealed another AHE contribution, establishing the impact of magnetism on transport.
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    Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe
    (American Physical Society, 2024) Wu, Shangfei; Klemm, Mason L.; Shah, Jay; Ritz, Ethan T.; Duan, Chunruo; Teng, Xiaokun; Gao, Bin; Ye, Feng; Matsuda, Masaaki; Li, Fankang; Xu, Xianghan; Yi, Ming; Birol, Turan; Dai, Pengcheng; Blumberg, Girsh
    Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature TN≈400 K and a charge density wave (CDW) transition at TCDW≈110 K, followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10−4 and the associated splitting of the double-degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of increasing the crystalline symmetry upon cooling is unusual; it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter and hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system, and it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.