Research

　Frustrated systems, in which spin arrangement is not uniquely determined, have attracted attention because peculiar phenomena can occur. Recently, we found a T-linear behavior of the nuclear spin-lattice relaxation rate 1/T₁ in the frustrated semiconductor YbCuS₂ with a Yb zigzag chain. This behavior indicates the presence of charge-neutral quasiparticles. To investigate the origin and characteristics of such novel quasiparticles observed in YbCuS₂, it is highly desirable to study other Yb-based zigzag-chain compounds.
　In this study, we performed nuclear magnetic resonance measurements on YbAgSe₂, which is a sister compound YbCuS₂. Our results indicate no evidence of the T-linear behavior of 1/T₁ in YbAgSe₂. This suggests that the charge-neutral quasiparticle is unique to YbCuS₂.
　The results of this study has been published in Journal of the Physical Society of Japan.

　Our paper was ranked among the Top 20 Most Downloaded Articles -- October 2024 in Journal of the Physical Society of Japan.

〇Article information
F. Hori, S. Kitagawa, K. Ishida, S. Mizutani, Y. Ohmagari, and T. Onimaru
"Gapped Spin Excitation in Magnetic Ordered State on Yb-Based Zigzag Chain Compound YbAgSe₂"
J. Phys. Soc. Jpn. 93, 114702 (2024); arXiv:2411.09325.

　Superconductivity is known as a phenomenon where electrical resistance becomes zero when a substance is cooled to a low temperature. In superconductivity, two electrons form a paired state (Cooper pairs). Since electrons have spin and orbital degrees of freedom, Cooper pairs also have spin and orbital degrees of freedom. However, in almost all superconductors discovered so far, the Cooper pairs have zero total spin and orbital angular momenta, and have no degrees of freedom. Therefore, only one superconducting (SC) state is realized. On the other hand, in SC states with spin or orbital degrees of freedom, SC multiple phases can be expected, and theoretical studies have been conducted. However, since there are very few observed examples of such superconductors and SC transition temperature is low, it has been extremely difficult to search for phenomena derived from SC multiple phases.
　We have investigated the SC spin rotation originating from the SC multiple phases in the superconductor UTe₂ by nuclear magnetic resonance (NMR) measurements in a complex extreme environment. Our results provide microscopic evidence for the remaining the spin degrees of freedom in the superconductivity of UTe2, inherent to the spin-triplet superconductivity.
　The results of this study has been published in Science Advances.

〇Article information
K. Kinjo, H. Fujibayashi, H. Matsumura, F. Hori, S. Kitagawa, K. Ishida., Y. Tokunaga, H. Sakai, S. Kambe, A. Nakamura, Y. Shimizu, Y. Homma, D. Li, F. Honda, and D. Aoki
"Superconducting spin reorientation in spin-triplet multiple superconducting phases of UTe₂"
Sci. Adv. 9, eadg2736 (2023); arXiv:2307.15784.
Kyoto University Research Information Repository, Kyoto University Press Release

　Recently, exotic ordered states and quasiparticles not known in convenrional magnetic materials have attracted attention in condensed-matter physics. In particular, it is expected that such peculiar phenomena can appear in frustrated systems where the spin arrangement of the entire systems is not uniquely determined. We focused on the magnetic semiconductor YbCuS₂, where rare-earth ytterbium atoms (Yb) form zigzag chains, and investigated the frustration effect of the rare-earth zigzag chain. Nuclear quadrupole resonance (NQR) and specific heat measurements revealed that YbCuS₂ exhibits an incommensurate antiferromagnetic (AFM) order and neutral quasiparticles exsit on AFM ordered states of YbCuS₂.
　Since the results of this study cannot be explained by the theory of conventional zigzag-chain frustrated magnets, it indicates the necessity of a new theory and suggests that YbCuS₂ is one of promising platforms for new frustrated magnets.
　The results of this study has been published in Communications Materials.

〇Article information
F. Hori, K. Kinjo, S. Kitagawa, K. Ishida, S. Mizutani, R. Yamamoto, Y. Ohmagari, and T. Onimaru
"Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain"
Commun. Mater. 4, 55 (2023) ; arXiv:2201.07563.
Kyoto University Research Information Repository, Kyoto University Press Release, Yahoo! News

　Materials are classified into two types, metals and insulators, depending on whether or not electricity flows through them. Metals have the property of easily conducting heat, while insulators have the property of being difficult to conduct heat. This is because conduction electrons, which conduct electricity in metals, carry heat. In recent years, materials have been discovered that conduct heat in the same manner as metals, although they are electrically insulating and do not conduct electricity, and have attracted attention. Since there are no conduction electrons in insulators, it is believed that these materials contain mysterious charge-neutral particles that carry only heat. YbIr₃Si₇ is one example of such materials, and the existence of charge-neutral particles has been reported from specific heat and thermal conductivity measurements at low temperatures.
　We performed nuclear magnetic resonance measurements (NMR) on YbIr₃Si₇ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. As a result, we found that the charge-neutral particles in YbIr₃Si₇ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral particles in insulators.
　The results of this study has been published in Physical Review B.

〇Article information
S. Kitagawa, T. Kobayashi, F. Hori, K. Ishida, A. H. Nevidomskyy, L. Qian, and E. Morosan
"Enhancement of charge-neutral fermionic excitations near the spin-flop transition in the magnetic Kondo material YbIr₃Si₇"
Phys. Rev. B 106, L100405 (2022) ; arXiv:2209.10844.
Kyoto University Research Information Repository