Browsing by Author "Li, Shuyi"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Dynamic Compressive Spectrum Sensing for Cognitive Radio Networks
    (2011-01) Yin, Wotao; Wen, Zaiwen; Li, Shuyi; Meng, Jia (Jasmine); Han, Zhu
    In the recently proposed collaborative compressive sensing, the cognitive radios (CRs) sense the occupied spectrum channels by measuring linear combinations of channel powers, instead of sweeping a set of channels sequentially. The measurements are reported to the fusion center, where the occupied channels are recovered by compressive sensing algorithms. In this paper, we study a method of dynamic compressive sensing, which continuously measures channel powers and recovers the occupied channels in a dynamic environment. While standard compressive sensing algorithms must recover multiple occupied channels, a dynamic algorithm only needs to recover the recent change, which is either a newly occupied channel or a released one. On the other hand, the dynamic algorithm must recover the change just in time. Therefore, we propose a least-squared based algorithm, which is equivalent to l0 minimization. We demonstrate its fast speed and robustness to noise. Simulation results demonstrate effectiveness of the proposed scheme.
  • Thumbnail Image
    Item
    Magnetic Phases and Topological Excitations in Frustrated Magnetic Systems on Honeycomb Lattices
    (2022-12-02) Li, Shuyi; Nevidomskyy, Andriy
    In recent years, there has been an explosion of interest in frustrated magnetic systems on the honeycomb lattice, a geometry well known to be associated with graphene. In this class of spin systems, different types of spin interactions such as Heisenberg exchange, bond-dependent anisotropic magnetic exchanges, and antisymmetric spin interactions may appear at the same time. The varying values of these parameters can greatly affect the magnetic phases and their properties. Understanding the role played by these interactions is significant and valuable for explaining the various experimental phenomena and for providing routes to potentially new honeycomb magnetic materials. In this thesis, I will present our study of three novel phenomena in spin systems on honeycomb lattices. In the first part, I will present our work on explaining the mechanism of an unusual noncollinear magnetic order of Ni$^{2+}$ $S=1$ moments with a nontrivial angle between adjacent spins appearing in a non-centrosymmetric honeycomb nickelate Ni$_2$Mo$_3$O$_8$. With the help of first principles electronic structure calculations and crystal field analysis, we construct an effective spin-1 bilinear-biquadratic model with estimated exchange parameters and single ion anisotropy. By performing the variational mean-field and linear spin-wave theory calculations, we find that the crucial key to explaining the observed noncollinear spin structure is the inclusion of the Dzyaloshinskii-Moriya interaction between the neighboring spins. In the second part, I will present our study of the topological properties and magnon Hall effect in a three-dimensional ferromagnet CrI$_3$ that crystallizes in the ABC stacked honeycomb layers. We find that the magnon band structure and Chern numbers of the magnon branches are significantly affected by the interlayer coupling $J_c$. Intriguingly, we find several Weyl magnon phases separating the non-equivalent Chern insulating phases, tuned by the ratio of the interlayer coupling $J_c$ and the third-neighbor Heisenberg interaction $J_3$. We further show that the topological character of the magnon bands results in non-zero thermal Hall conductivity, whose sign and magnitude depend on $J_c$ and the intra-layer couplings. Since the interlayer coupling strength $J_c$ can be easily tuned by applying pressure to the quasi-2D material, this provides a potential route to tuning the magnon thermal Hall effect in an experiment. In the last part, I will show our study of the possible magnon thermal Hall effect in a Kitaev model candidate material $\alpha$-RuCl$_3$. Because the model parameters of this system are uncertain, we investigate the behavior of the magnon thermal Hall conductivity $\kappa_{xy}/T$ in a wide parameter regime. Through the minimization of the classical energy and linear spin-wave theory, we determine the magnetic phases and compute $\kappa_{xy}/T$ in each set of parameters. We further compare the temperature and magnetic field dependence of $\kappa_{xy}/T$ to the experimental results, which shows that the magnon thermal Hall effect may not be enough to reproduce the experimental data.