Browsing by Author "Chen, Lebing"

Now showing 1 - 7 of 7
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    Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe3
    (Springer Nature, 2022) Chen, Lebing; Mao, Chengjie; Chung, Jae-Ho; Stone, Matthew B.; Kolesnikov, Alexander I.; Wang, Xiaoping; Murai, Naoki; Gao, Bin; Delaire, Olivier; Dai, Pengcheng
    Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and breakdown of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons.
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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 Effect on Topological Spin Excitations in CrI3
    (American Physical Society, 2021) Chen, Lebing; Chung, Jae-Ho; Stone, Matthew B.; Kolesnikov, Alexander I.; Winn, Barry; Garlea, V. Ovidiu; Abernathy, Douglas L.; Gao, Bin; Augustin, Mathias; Santos, Elton J. G.; Dai, Pengcheng
    The search for topological spin excitations in recently discovered two-dimensional (2D) van der Waals (vdW) magnetic materials is important because of their potential applications in dissipationless spintronics. In the 2D vdW ferromagnetic (FM) honeycomb lattice CrI3 (TC=61 K), acoustic and optical spin waves are found to be separated by a gap at the Dirac points. The presence of such a gap is a signature of topological spin excitations if it arises from the next-nearest-neighbor (NNN) Dzyaloshinskii-Moriya (DM) or bond-angle-dependent Kitaev interactions within the Cr honeycomb lattice. Alternatively, the gap is suggested to arise from an electron correlation effect not associated with topological spin excitations. Here, we use inelastic neutron scattering to conclusively demonstrate that the Kitaev interactions and electron correlation effects cannot describe spin waves, Dirac gaps, and their in-plane magnetic field dependence. Our results support the idea that the DM interactions are the microscopic origin of the observed Dirac gap. Moreover, we find that the nearest-neighbor (NN) magnetic exchange interactions along the c axis are antiferromagnetic (AF), and the NNN interactions are FM. Therefore, our results unveil the origin of the observed c-axis AF order in thin layers of CrI3, firmly determine the microscopic spin interactions in bulk CrI3, and provide a new understanding of topology-driven spin excitations in 2D vdW magnets.
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    Spin Excitations in Honeycomb van der Waals Magnetic Insulators
    (2023-04-12) Chen, Lebing; Dai, Pengcheng
    The discovery of two-dimensional magnetism in van der Waals honeycomb magnetic insulators CrI$_3$, CrGeTe$_3$ has opened up a new degree of freedom in the two-dimensional realm of physics and material science. The robustness of the magnetic order of these systems in the monolayer limit shows promising future applications for spintronic devices. On the other hand, honeycomb magnetic insulators are by themselves an interesting topic to study, as it is a magnetic analog of graphene. This makes the magnon dispersion in honeycomb ferromagnets similar to the electron dispersion in graphene, with Dirac cones at the Brillouin zone corners. With proper additional terms in the Hamiltonian such as the Dzyaloshinskii-Moriya (DM) interaction and Kitaev interactions, nontrivial topological magnons can be created in these materials. Furthermore, the interplay between spin, orbital, and lattice degrees of freedom can give rise to richer excitation topology and textures. This thesis is dedicated to presenting the neutron scattering results on several van der Waals magnetic insulators including CrI$_3$, CrCl$_3$, CrGeTe$_3$, and FePSe$_3$, with discussions on results on other similar compounds. After a brief introduction to linear spin wave theory, neutron scattering, and topological magnons in Chapter \ref{ch:Intro}, we show in Chapter 2 that a Dirac gap is observed in the magnon dispersion in CrI$_3$, which partially closes upon applying an in-plane field. The strength of DM and Kitaev interactions is quantitatively estimated after considering the extrinsic origins of the magnon gap. If the topological magnon exists in CrI$_3$, its edge modes will adapt non-dissipative magnon transport and can be utilized in spintronic applications. We also discussed other compounds that can potentially host topological magnons, such as the Dirac gaps in Cr(Si/Ge)Te$_3$, and the anomalous thermal Hall effect in VI$_3$, compared with Dirac magnons in CrBr$_3$ and CrCl$_3$. In Chapter 3, magnon-phonon coupling effects on the magnon spectrum are extensively discussed. We start with a simple theory showing that nontrivial topological properties of magnons and phonons can arise from linear magnon-phonon coupling, then we present Raman scattering results on FePS$_3$ and FePSe$_3$, showing that linear magnon-phonon coupling can be realized in these honeycomb van der Waals systems, and can potentially host topological magnon polarons that can be easily tuned by a magnetic field. Finally, we introduce nonlinear magnon-phonon coupling originating from the Heisenberg interactions changing with atomic displacement and show that these interactions can cause magnon decay and renormalizations in the honeycomb ferromagnet CrGeTe$_3$, making it unsuitable for topological magnon transfer. We conclude the thesis in Chapter 4 with a summary and state the challenge and prospect of this work.
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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.