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Thursday, 22 September 2022

Hae Young Kee
Title: Symmetry Analysis & Strategy to extract the Kitaev Interaction

After a short review on microscopic mechanism for the Kitaev interaction in higher spin systems[1], I will present strategies to extract the Kitaev interaction out of full Hamiltonian[2].

[1] Phys. Rev. Lett. 123, 037203 (2019)
[2] Commun. Phys. 5, 119 (2022); arXiv:2208.13807

Indra Dasgupta
Title: Density Functional Theory Study of CrI3 and Implications for Neutrons

In this talk, we shall describe the electronic structure of the layered ferromagnet CrI3 where strong SOC from the heavy ligand iodine atoms and the graphene like honeycomb network that is formed by the magnetic Cr atoms holds the potential to give rise to topological electronic and magnetic properties. In particular, we shall obtain the microscopic Hamiltonian for CrI3 and clarify the role of ligand induced spin-orbit coupling on the calculated parameters. We shall argue whether the Kitaev term is important in modelling this system and show that high resolution neutron scattering may be suitable to probe the Kitaev exchange at large B.

(Work done in collaboration with Subhadeep Bandyopadhyay, Finn Lasse Buessen, Ritwik Das, Franz G. Utermohlen, Nandini Trivedi and Arun Paramekanti)

[1] Subhadeep Bandyopadhyay Finn Lasse Buessen,2, Ritwik Das, Franz G. Utermohlen, Nandini Trivedi and Arun Paramekanti, and Indra Dasgupta Phys Rev B 105, 184430 (2022)

Inhee Lee
Title: Ligand Spin-Orbit Coupling Dominantly Coordinating Fundamental Spin Interactions in Chromium Trihalides

We use ferromagnetic resonance (FMR) spectroscopy to determine the values of spin interactions for Chromium trihalides (CrX3, X = Cl, Br, I) such as Heisenberg (J), Kitaev (K), and off-diagonal symmetric (Γ) exchange interactions in the Hamiltonian defined by crystal symmetries and investigate their correlations with spin-orbit coupling (SOC) strength of p orbital of ligand atom X. Three CrX3 compounds provide an excellent material platform for exploring and realizing exotic two-dimensional (2D) spin orders since they have the same crystal structure and have the same Hamiltonian, but different p orbital of ligand X provides the different SOC strength. We find that most magnetic interactions and properties, such as K and Γ, their resulting two magnon band gaps, and the magnetocrystalline anisotropy and Curie temperature T_C, exhibit an exclusive correlation with the single-factor SOC strength of the ligand p orbital, in contrast to Cr-Cr bond distance and Cr-X-Cr path angle, which can also significantly affect them. This shows that the SOC of X plays a central role in the 2D magnetism of CrX3, overwhelming the other key crystalline structural factors. We also calculate and predict the spin wave dispersions with our determined spin interaction constants for three CrX3 and compare them.

R. Valdes Aguilar
Title: Magneto-Raman signatures of spin-flip phase transitions in few-layered CrI3

Here, we report a magneto-Raman spectroscopy study on few-layered CrI3, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field.

Michael Sentef
Title: Direct Optical Probe of Magnon Topology in Two-dimensional Quantum Magnets

Controlling edge states of topological magnon insulators is a promising route to stable spintronics devices. However, to experimentally ascertain the topology of magnon bands is a challenging task. Here we derive a fundamental relation between the light-matter coupling and the quantum geometry of magnon states. This allows to establish the two-magnon Raman circular dichroism as an optical probe of magnon topology in honeycomb magnets, in particular of the Chern number and the topological gap. Our results pave the way for interfacing light and topological magnons in functional quantum devices.

Emil Viñas Boström, Tahereh Sadat Parvini, James W. McIver, Angel Rubio, Silvia Viola Kusminskiy, Michael A. Sentef