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Anomalous Thermal Responses of Magnons and Phonons in Two-imensional Magnetic Layers

Sheikhi, Bahman | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54770 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Langari, Abdollah; Kargarian, Mehdi
  7. Abstract:
  8. The concept of topology has underlined the discovery of new phases of matter since the eighties. Meanwhile the material growth, synthesis, and measurements have enabled us to examine the theoretical predictions. Amongst all, the anomalous thermal Hall and Nernst effects result from the non-trivial topological properties of lowenergy carriers. In this thesis our objection is to study some of these phases in two-dimensional magnetic layers such as honeycomb and Kagome lattices and their heterostructures.First, we consider heterostructures of a pyrochlore lattice along the [111] directions: triangular-kagome (TK), triangular-kagome-triangular (TKT), and kagometriangular-kagome (KTK) lattices. The symmetry of the lattices allows for the Dzyaloshinskii-Moriya exchange interaction in the magnetic Hamiltonian resulting in nontrivial band topology for magnons. We obtain the latter using the linear spinwave theory. We found that the thermal Hall conductivity in Tk and TKT lattices have the same magnitude, while it is one order of magnitude larger in the KTK lattice. The reasonings are attributed to the Chern numbers of the lowest bands and theirthermal occupations thereof. We then study the Nernst response of these magnetic layers and found that the Nernst coefficient increases with Dzyaloshinskii-Moriya interaction.Second, we consider the magnon-phonon hybrid modes in the ferromagnetic honeycomb and Kagome lattices. Both carriers are neutral, yet they carry heat and energy and topological properties are printed in thermal Hall response. We would like to understand how the topological magnon may affect the trivial phonons. For that, the in-plane Dzyaloshinskii-Moriya interaction couples the magnons to the phonons and new hybrid modes, the magneto-polarons, are formed near the anticrossing points through the momentum space. We establish that the Berry curvature enhances for the magneto-polaron modes. As such, it leads to increase the thermal Hall conductivity of the system. In order to understand the underlying physics we developed an effective model describing the hybridization between magnon and phonons. Such hybridization redistributes the Berry curvature among the low-energy bands giving rise to enhancement of Hall response of order of 10−12W/K at lower temperatures. Interestingly enough, we found that even for hybrid trivial modes the produced Berry curvature yields a nonzero thermal Hall response. We also study the same phenomena in regular and twisted Kagome lattices and investigate the topological property arising from the magnon-phonon hybridization.
  9. Keywords:
  10. Topological Insulator ; Thin Films ; Magnon ; Hall Effect ; Nernst Effect ; Topological Properties

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