[Seminar] "Topological transition by vector spin chirality on a triangular lattice" by Prof. Yusuke Nambu

Speaker

Associate Prof. Yusuke Nambu / Tohoku University

Title

Topological transition by vector spin chirality on a triangular lattice

Abstract

Geometrically frustrated systems with reduced spatial dimensionality continue to challenge our understanding of condensed matter. Triangular lattice antiferromagnet, the simplest motif among the frustrated systems, fosters unusual magnetic properties. An example is the magnetism of NiGa2S4 [1], where the correlation length associated with Ni spin is pinned at an incommensurate period. As separated from the gradual growth of the spatial correlation on cooling, the temperature dependence of temporal spin correlation indicates the formation of megahertz spin fluctuation [2]. Spins subsequently tend to be in a quasi-static quantum state that is preserved only for the integer size of spins [3], owing to possible nematic correlation [4]. We have recently performed polarised elastic neutron scattering to elucidate the anomaly toward the megahertz dynamics. The selected neutron polarisation can unambiguously detect in-plane and out-of-plane magnetic moments. Anisotropy in the spin space is found to show a reversal change, and we compared our experimental findings with theoretical Monte-Carlo simulations. Only the model that describes a dissociation of vortices formed by vector spin chirality with the Z2 homotopy group, could explain such a change. In this talk, I will start with a brief overview of spatial/temporal spin correlations and the quantum characteristics in the compound, and then discuss the possible origin of the magnetism.

[1] S. Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas, C. Broholm, H. Tsunetsugu, Y. Qiu, and Y. Maeno, Science 309, 1697 (2005).
[2] Y. Nambu, J. S. Gardner, D. E. MacLaughlin, C. Stock, H. Endo, S. Jonas, T. J. Sato, S. Nakatsuj, and C. Broholm, Phys. Rev. Lett. 115, 127202 (2015).
[3] Y. Nambu, S. Nakatsuji, Y. Maeno, E.K. Okudzeto, and J.Y. Chan, Phys. Rev. Lett. 101, 207204 (2008).
[4] M. E. Valentine, T. Higo, Y. Nambu, S. Nakatsuji, D. Chaudhuri, and N. Drichko, Phys. Rev. Lett. 125, 197201 (2020).

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