arXiv subjects

Tatsuya Iwasaki

Publications and source records attributed to Tatsuya Iwasaki.

4 recordsLinked to original sources

arXiv2026

Non-Collinear and Non-Coplanar Magnetic Orders in 1/1 Periodic Approximant to the Icosahedral Quasicrystal

Ground-state properties of rare-earth based 1/1 periodic approximants of icosahedral quasicrystal are clarified theoretically on the basis of an effective model for magnetism taking into account uniaxial anisotropy arising from crystalline electric field. By performing numerically-exact calculation on the 1/1 approximant crystal with a lattice constant $a$=14.725 Å, we have determined the ground-state phase diagram for ferromagnetic interactions. The result shows that eight kinds of noncollinear and noncoplanar magnetic structures are stabilized, whose magnetic space groups are identified as $I_{\rm P}m'\bar{3}'$, $C2'/m'$ and $R\bar{3}$. We have clarified the degeneracy of each ground state, which is expected to be reflected in the numbers of the domains. By analyzing each state, the magnetic as well as topological properties are revealed. Our results are shown to explain the measured magnetic structures in the 1/1 approximants and the effective model is discussed to be useful for understanding the magnetic structures and their topological properties in broad range of rare-earth based 1/1 approximants.

cond-mat.str-el↗

arXiv2026

Effect of magnetic field on whirling-anti-whirling order in icosahedral-quasicrystal approximant

Recent neutron measurement in the icosahedral quasicrystal approximant Au-SM-Tb (SM=Al, Ga) has revealed unique noncollinear magnetic order ``whirling-anti-whirling states''. Here, we report theoretical analysis on the magnetic-field-direction dependence on the whirling-anti-whirling order in the 1/1 approximant crystal. By performing exact-diagonalization calculation for the effective model taking into account the uniaxial magnetic anisotropy arising from the crystalline electric field, we show the metamagnetic transition takes place simultaneously with the topological transition under the magnetic field along the (111) direction. After the metamagnetic transition, the emergent fictious magnetic field induced by the chirality of noncoplanar magnetic moments appears, the analysis of which concludes that the topological Hall effect is expected to be observed in the electrical conductivity $σ_{xy}$ and $σ_{yz}$ for the applied field direction from (111) to (001).

cond-mat.str-el↗

arXiv2023

Crystalline Electronic Field and Magnetic Anisotropy in Dy-based Icosahedral Quasicrystal and Approximant

The lack of the theory of the crystalline electric field (CEF) in rare-earth based quasicrystal (QC) and approximant crystal (AC) has prevented us from understanding the electronic states. Recent success of the formulation of the CEF theory on the basis of the point charge model has made it possible to analyze the CEF microscopically. Here, by applying this formulation to the QC Au-SM-Dy (SM=Si, Ge, Al, and Ga) and AC, we theoretically analyze the CEF. In the Dy$^{3+}$ ion with $4f^9$ configuration, the CEF Hamiltonian is diagonalized by the basis set for the total angular momentum $J=15/2$. The ratio of the valences of the screened ligand ions $α=Z_{\rm SM}/Z_{\rm Au}$ plays an important role in characterizing the CEF ground state. For $0\leα<0.30$, the magnetic easy axis for the CEF ground state is shown to be perpendicular to the mirror plane. On the other hand, for $α>0.30$, the magnetic easy axis is shown to be lying in the mirror plane and as $α$ increases, the easy axis rotates to the clockwise direction in the mirror plane at the Dy site and tends to approach the pseudo 5 fold axis. Possible relevance of these results to experiments is discussed.

cond-mat.str-el↗

arXiv2011

Verbal Characterization of Probabilistic Clusters using Minimal Discriminative Propositions

In a knowledge discovery process, interpretation and evaluation of the mined results are indispensable in practice. In the case of data clustering, however, it is often difficult to see in what aspect each cluster has been formed. This paper proposes a method for automatic and objective characterization or "verbalization" of the clusters obtained by mixture models, in which we collect conjunctions of propositions (attribute-value pairs) that help us interpret or evaluate the clusters. The proposed method provides us with a new, in-depth and consistent tool for cluster interpretation/evaluation, and works for various types of datasets including continuous attributes and missing values. Experimental results with a couple of standard datasets exhibit the utility of the proposed method, and the importance of the feedbacks from the interpretation/evaluation step.

cs.AI↗