arXiv subjects

Yuki Kojima

Publications and source records attributed to Yuki Kojima.

5 recordsLinked to original sources

arXiv2026

NMR study on equilateral triangular lattice antiferromagnet Ba2La2CoTe2O12

We report a 139La-NMR study of Ba2La2CoTe2O12, S = 1/2 equilateral triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at TN = 3.26 K into an ordered state with the 120 degree spin structure. Under magnetic fields above 3T, TN splits into TN1 and TN2, which correspond to the transitions from the paramagnetic phase to the up-up-down (uud) phase and from the uud phase to the triangular coplanar phase, respectively. The NMR spin-lattice relaxation rate 1/T1 exhibits a critical divergence at TN1, indicating the onset of long-range magnetic order. At TN2, the NMR-linewidth measured at 5.4 T exhibits an anomalous decrease, which we attribute to a change in the spin structure from the uud to the triangular coplanar phase.

cond-mat.stat-mech↗

arXiv2025

On homomorphisms from finite subgroups of $SU(2)$ to Langlands dual pairs of groups

Let $N(\Gamma,G)$ be the number of homomorphisms from $\Gamma$ to $G$ up to conjugation by $G$. Physics of four-dimensional $\mathcal{N}=4$ supersymmetric gauge theories predicts that $N(\Gamma,G)=N(\Gamma , \tilde G)$ when $\Gamma$ is a finite subgroup of $SU(2)$, $G$ is a connected compact simple Lie group and $\tilde G$ is its Langlands dual. This statement is known to be true when $\Gamma=\mathbb{Z}_n$, but the statement for non-Abelian $\Gamma$ is new, to the knowledge of the authors. To lend credence to this conjecture, we prove this equality in a couple of examples, namely $(G,\tilde G)=(SU(n),PU(n))$ and $(Sp(n),SO(2n+1))$ for arbitrary $\Gamma$, and $(PSp(n),Spin(2n+1))$ for exceptional $\Gamma$. A more refined version of the conjecture, together with proofs of some concrete cases, will also be presented. The authors would like to ask mathematicians to provide a more uniform proof applicable to all cases.

math.RT↗

arXiv2023

A closed manifold is a fat CW complex

The main purpose of this paper is to introduce a new smooth version of a CW complex named a fat CW complex, and to show that it includes all closed manifolds, because existing smooth versions of CW complexes (e.g. [Iwa22]) do not have such property. We also verify that de Rham theorem holds for a fat CW complex and that a regular CW complex is reflexive in the sense of Y. Karshon, J. Watts and P. I-Zemmour. Further, any topological CW complex is topologically homotopy equivalent to a fat CW complex. So, a fat CW complex enjoys many nice properties.

math.GT↗

arXiv2021

Magnons and Spinons in $\mathrm{Ba}_2\mathrm{CoTeO}_6 $: A Composite System of Isolated Spin-$1/2$ Triangular Heisenberg-like and Frustrated Honeycomb Ising-like Antiferromagnets

We report the neutron scattering results on magnetic orderings and excitations in $\mathrm{Ba}_2\mathrm{CoTeO}_6$ composed of two almost isolated subsystems A and B, which are described as an $S\,{=}\,1/2$ triangular Heisenberg-like antiferromagnet and a frustrated honeycomb Ising-like antiferromagnet, respectively. Stripy ordering of subsystem B was confirmed below $T_{\rm N1}\,{=}\,12.0$ K, whereas sharp streaks were observed along $(1/3, 1/3, L)$ and $(2/3, 2/3, L)$ at 0.3 K (${\ll}\,T_{\rm N2}\,{=}\,3.0$ K). This indicates the two-dimensional nature of ordering in subsystem A. It was found that the excitation spectra of both subsystems are well separated and independent of each other. The excitation spectrum of subsystem A is composed of two single-magnon branches with roton-like minima at the M point and a clearly structured intense continuum, as similarly observed in $\mathrm{Ba}_3\mathrm{CoSb}_2\mathrm{O}_9$, which is strongly indicative of spinon excitations. Dispersion curves for subsystem B can be described by linear spin wave theory within the third-neighbor exchange interaction.

cond-mat.str-el↗

arXiv2018

Quantum magnetic properties of the spin-1/2 triangular-lattice antiferromagnet Ba$_2$La$_2$CoTe$_2$O$_{12}$

We report the crystal structure of Ba$_2$La$_2$CoTe$_2$O$_{12}$ determined by Rietveld analysis using X-ray powder diffraction data. It was found from magnetic measurements that Ba$_2$La$_2$CoTe$_2$O$_{12}$ can be described as a spin-1\2 triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at $T_{\rm N}\,{=}\,3.26$ K to an ordered state with the $120^{\circ}$ structure. The magnetization curve exhibits the one-third plateau characteristic of triangular-lattice quantum antiferromagnets. The antiferromagnetic exchange interaction and the $g$ factors parallel and perpendicular to the $c$ axis were evaluated to be $J/k_{\rm B}\,{=}\,22$ K, $g_{\parallel}\,{=}\,3.5$ and $g_{\perp}\,{=}\,4.5$, respectively.

cond-mat.str-el↗