The 6th QLC young colloquium
Date & Time : Tuesday, February 15, 2022. 13：30～14：30
Yoshihiro OKAMURA（University of Tokyo)
Hakuto SUZUKI（Tohoku University)
Place：online using “Zoom”
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*Zoom meeting ID information will be sent by the day of this colloquium to those who registered.
Speaker：Yoshiihro OKAMURA (University of Tokyo)
Title：Magneto-optical spectroscopy on topological magnets
In recent years, a number of material systems with topological electronic structures, such as topological insulators and Weyl semimetals, have been proposed and developed, attracting much attention. These novel electronic phases have been found to exhibit unusual magnetoresistance and giant anomalous Hall effect. On the other hand, other electromagnetic responses have not been studied experimentally. To explore the electromagnetic response unique to the topological electronic states, we have focused on the magneto-optical effect. The magneto-optical effect can be regarded as an optical extension of the anomalous Hall effect, and is expected to be resonantly enhanced especially for optical transitions on topological electronic structures. In this seminar, I will mainly introduce the magneto-optical effect studied in the magnetic Weyl semimetal Co3Sn2S2 . In this material, topological electronic structures such as nodal rings and Weyl points exist in the vicinity of the Fermi level. Correspondingly, the magneto-optical spectra exhibit a large resonance structure in the low-energy region around 0.1 eV. The first-principles calculations indeed demonstrate that this resonance corresponds to optical transitions on the topological electronic structure. Recently, we also found that the topological magnetic structures such as skyrmions can induce a sharp resonance structure in the magneto-optical spectra . In the latter half of the seminar, we would like to show this result and discuss the electromagnetic response arising from topological objects in solids.
 Y. Okamura et al., Nat. Commun. 11, 4619 (2020).
 Y. Hayashi*, Y. Okamura* et al., Nat. Commun. 12, 5974 (2021) (*equal contribution).
Speaker：Hakuto SUZUKI（Tohoku University)
Title： Determination of pseudospin Hamiltonian in the Kitaev magnet a-RuCl3 by resonant inelastic x-ray scattering
α-RuCl3 is a major candidate for the realization of the Kitaev quantum spin liquid, as well as a prototype of two-dimensional van der Waals magnetism. Despite increasing experimental evidence for the fractionalization of spins into Majorana Fermions, its zigzag antiferromagnetic order at low temperatures indicates deviations from the Kitaev model. In this talk, I will introduce our approach to the spin Hamiltonian of α-RuCl3 by means of resonant inelastic x-ray scattering (RIXS) at the Ru L3 absorption edge . The experiment was performed with the intermediate-energy RIXS (IRIXS) spectrometer at the P01 beamline of PETRA III, DESY.
In the paramagnetic state, the quasi-elastic RIXS intensity originating from magnetic excitations has a broad maximum around the zone center without any local maxima at the zigzag magnetic Bragg wavevectors. This finding implies that the zigzag order is fragile and readily destabilized by competing ferromagnetic correlations. Based on a comparison with theoretical calculations of RIXS intensity for the extended Kitaev-Heisenberg model, we determined the spin Hamiltonian with high precision. The important consequence is that the zigzag state is achieved by quantum fluctuations, with the classical ferromagnetic state as a proximate metastable state.
As RIXS requires only small crystals with ~10 mm dimensions, the present approach has a potential to evolve into an efficient screening tool for the expanding list of Kitaev candidate materials.
Committee Chair：Hiroki WADATI（University of Hyogo）