Research

 Here, I list some recently published results of research activity, etc.

Research

　Here, I introduce some recently published results of research activity and so on. For the list of my publications and conference presentations, see this links.

◎Experimental Identification of Superconductivity with Spin Degrees of Freedom
-New Phenomena in Spin-Triplet Superconducting Multiple Phases-

【Jul, 2023】(first author: Katsuki Kinjo)

　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

◎Discovery of Charge-Neutral Quasiparticles in a Magnetic Material with a Zigzag Chain Structure

【Jul, 2023】(first author: Fumiya Hori)

　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

◎Mysterious Neutral Particles on YbIr3Si7 Observed by NMR!

【Sep, 2022】(first author: Shunsaku Kitagawa)

　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.