arXiv subjects

Shingo Takahashi

Publications and source records attributed to Shingo Takahashi.

4 recordsLinked to original sources

arXiv2026

Observation of Altermagnetic Order Switching in Bulk MnTe by Polarized Neutron Diffraction

Altermagnetic order, characterized by the N\'{e}el vector, breaks time-reversal symmetry (TRS) even in the nonrelativistic limit. Although spin-polarized and anomalous transport phenomena emerge with this order, they are mutually compensated by TRS-connected antiphase domains with opposite N\'{e}el vectors. Here we employ polarized neutron diffraction to directly probe the altermagnetic order in MnTe. Pronounced nuclear-magnetic interference terms were observed, providing direct evidence of a net N\'{e}el vector in the bulk crystal. Moreover, a weak ferromagnetic moment (WFM), originating from relativistic spin-orbit coupling, was found to be coupled with the altermagnetic order. Both the altermagnetic order and the WFM can be switched by milli-Tesla-scale magnetic field cooling.

cond-mat.str-el↗

arXiv2026

Magnetic order and excitations in the magnetically intercalated van der Waals material Cr$_{\frac{1}{4}}$NbSe$_2$

Cr$_{\frac{1}{4}}$NbSe$_2$ is a triangular lattice magnet in which magnetic Cr$^{3+}$ ions are intercalated to form triangular lattices between NbSe$_2$ van der Waals layers stacked along the c axis. By unpolarized and polarized neutron scattering experiments, we have revealed that the magnetic ground state of this system is a 120$^{\circ}$-type antiferromagnetic order characterized by the magnetic propagation wave vector of $q=(\frac{1}{3}, \frac{1}{3}, 0)$. We also performed inelastic neutron scattering measurements using co-aligned single crystals, and determined dispersion relations of magnetic excitations at low temperatures. Comparing the observed spectra with calculations based on the linear spin-wave theory, we revealed that the out-of-plane ferromagnetic interaction is fairly strong as compared to the in-plane nearest neighbor antiferromagnetic interaction. Although the crystal structure of this system is composed of two-dimensional van der Waals layers, the magnetic order has a three dimensional character, which would be attributed to long-range magnetic interactions mediated by conduction electrons.

cond-mat.str-el↗

arXiv2025

First Constraint on P-odd/T-odd Cross Section in Polarized Neutron Transmission through Transversely Polarized $^{139}$La

We report the first constraint on time-reversal invariance violating (TRIV) effects in polarized neutron transmission through a transversely polarized $^{139}$La target. We formulate the transmission asymmetry within the density matrix formalism, explicitly incorporating the forward scattering amplitude of $^{139}$La including tensor polarization terms up to third-rank. The formalism is applied to existing transmission data originally obtained to measure the spin-dependent cross section near the $0.75$~eV $p$-wave resonance. Since these data were not optimized for P-odd/T-odd observables, the attainable sensitivity is intrinsically limited; nevertheless, they provide a useful test of the formalism on real experimental data. No statistically significant TRIV signal is observed. By analyzing the global $\chi^2$ structure in the parameter space, we obtain an upper limit of $|W_T|<15~\mathrm{eV}$ at the 90\% confidence level. This corresponds to an upper limit on the resonance-averaged TRIV cross section of $|\Delta\sigma_{\not{T}\not{P}}|<8.3\times10^2~\mathrm{b}$. These results validate the present theoretical framework and provide guidance for future dedicated TRIV searches in polarized neutron transmission experiments.

nucl-ex↗

arXiv2018

Solving Non-identifiable Latent Feature Models

Latent feature models (LFM)s are widely employed for extracting latent structures of data. While offering high, parameter estimation is difficult with LFMs because of the combinational nature of latent features, and non-identifiability is a particularly difficult problem when parameter estimation is not unique and there exists equivalent solutions. In this paper, a necessary and sufficient condition for non-identifiability is shown. The condition is significantly related to dependency of features, and this implies that non-identifiability may often occur in real-world applications. A novel method for parameter estimation that solves the non-identifiability problem is also proposed. This method can be combined as a post-process with existing methods and can find an appropriate solution by hopping efficiently through equivalent solutions. We have evaluated the effectiveness of the method on both synthetic and real-world datasets.

cs.LG↗