QLC Young Colloquium

The 7th QLC young colloquium

Date & Time : Tuesday, May 31, 2022. 15:30~17:00
Speaker: Three winners of the 3rd QLC Young Researcher Award ,
     POHLE, Rico(University of Tokyo)
     KITAORI, Aki(University of Tokyo)
     OKUMA, Ryutaro(University of Oxford)
Place:online using “Zoom”

*If you wish to join this seminar, please register at this site .
*Zoom meeting ID information will be sent by the day of this colloquium to those who registered.

Speaker: Rico POHLE (University of Tokyo)
Title: Spin liquid and nematic states in the spin-1 honeycomb Kitaev model with bilinear-biquadratic interactions
 The Kitaev model on the honeycomb lattice provides an elegant realization of a quantum spin liquid, showing fractionalized excitations and topological order [1]. While mainly discussed for materials with an effective spin-orbital entangled moment S=1/2 [2], recent studies suggest that the model could also be realized for S=1 or even larger S [3,4].  S=1 spin moments are special, since they allow not only for dipolar but also quadrupolar fluctuations on a single site. Such on-site quadrupole moments very naturally support higher-order biquadratic interactions, which can lead to unconventional states, as e.g. spin nematics [5].  In this talk, we show that the Kitaev model under the influence of bilinear-biquadratic interactions hosts many unconventional ordered and disordered phases. By using semi-classical Monte Carlo simulations explicitly designed to treat spin-1 magnets [6], we obtain a comprehensive phase diagram, showing various phases of dipolar and quadrupolar magnetic order. Surprisingly, we find that the competition between Kitaev and positive biquadratic interactions also promotes dimensional reduced, 1D phases and a chiral spin liquid state. Our results suggest that spin-1 Kitaev systems host unconventional phases for realistic model Hamiltonians and motivate their exploration in real materials.

[1] A. Kitaev, Ann. Phys. 321, 2 (2006).
[2] G. Jackeli and G. Khaliullin, Phys. Rev. Letters 102, 017205 (2009).
[3] P. P. Stavropoulos, D. Pereira, and H.-Y. Kee, Phys. Rev. Letters 123, 037203 (2019).
[4] A. Scheie et al., Phys. Rev. B 100, 214421 (2019)
[5] H. Tsunetsugu and M. Arikawa, J. Phys. Soc. Jpn. 75, 083701 (2006).
[6] K. Remund, R. Pohle, Y. Akagi, J Romhányi, and N. Shannon, arXiv:2203.09819.

Speaker: Aki KITAORI (University of Tokyo)
Title: Research on emergent inductance 2022 -Significant expansion of target systems-
 Researches of “emergent inductor”, that detect and utilize the spin motive force generated by a current-driven magnetic structure as an imaginary part of voltage, are in the transitional period from the dawn to the growth period. Starting with the theoretical proposal [1] in 2019 with the spiral magnetic structure model, it was experimentally demonstrated in 2020 [2], and room temperature operation was realized in 2021 [3]. In the same year, several theoretical studies have suggested that both positive and negative inductances can appear [4-6]. And in 2022, the research area of emergent inductor will be further expanded. In this seminar, I will look back on the development of emergent inductance research over the past few years and then introduce the latest research status. The key of 2022’s emergent inductance research is “expansion of the target systems”. Past emergent inductance studies have focused on spiral magnetic structures, but last year’s Ieda-Yamane paper [5] and Yamane-Fukami-Ieda paper [6] extend the scope of application theoretically to Rashba systems and ferromagnets. Here, we have experimentally obtained results suggesting that emergent inductance can be observed in a wide range of magnets through another approach. The details will be explained on the day.

[1] N. Nagaosa, Jpn. J. Appl. Phys. 58, 12090 (2019)
[2] T. Yokouchi et al., Nature 586, 232 (2020)
[3] A. Kitaori et al., PNAS 118, e2105422118 (2021)
[4] D. Kurebayashi and N. Nagaosa, Commun. Phys. 4, 260 (2021)
[5] J. Ieda and Y. Yamane, Phys. Rev. B 103, L100402 (2021)
[6] Y. Yamane, S. Fukami and J. Ieda, arXiv:2109.03558 (2021)

Speaker: Ryutaro OKUMA (Oxford University)
Titel: Itinerant frustration in van der Waals iodide magnets
 Van der Waals (vdW) materials are a promising and rapidly growing field to search for novel quantum phenomena by the use of spectroscopic tools and exfoliation technique. Strongly correlated vdW systems are scarce and limited to chalcogenides and halides of transition metal, which limits the the access to vdW systems with magnetism and metallicity. We show that lanthanum halides LnAI (Ln = Lanthanide, A = main group element) [1] are such itinerant vdW magnets.
 In LnAI, lanthanides and main group elements form a bilayer triangular lattice and a honeycomb net separated by vdW-stacked iodine, respectively. GdGaI is a semimetal at room temperature and band folding occurs at low temperature accompanied by weak ferromagnetic ordering of Gd. The change in the band structure resembles exciton condensation and the magnetic structure is expected to be a triple-q order characterized by ordering of scalar spin chirality. By contrast, CeSiI is a good metallic conductor and Ce sublattice shows a cycloidal magnetic order with a “step” in the magnetization process [2]. We discuss the magnetism of these compounds in terms of itinerant frustration on a triangular lattice.

[1] H. Mattausch et al. Angew. Chem. Int. Ed. 37, 499 (1998). M. Lukachuk et al. Z. Naturforsch. B 62, 633 (2007).
[2] R. O, C. Ritter, G. J. Nilsen, Y. Okada Phys. Rev. Mater. 5, L121401 (2021).

Committee Chair:Hiroki WADATI(University of Hyogo)