Faculty of International Social Sciences

Yo Machida

  (町田 洋)

Profile Information

Affiliation
Professor, Faculty of Science, Gakushuin University
Degree
Doctor of Science

Researcher number
40514740
J-GLOBAL ID
200901036313769260
researchmap Member ID
6000012446

Research Interests

 1

Papers

 35
  • Takuya Kawabata, Kosuke Shimura, Yuto Ishii, Minatsu Koike, Kentaro Yoshida, Shu Yonehara, Kohei Yokoi, Alaska Subedi, Kamran Behnia, Yo Machida
    Physical Review Research, 7(3) 033017, Jul 7, 2025  Peer-reviewed
    When an ideal insulator is cooled, four regimes of thermal conductivity are expected to emerge one after another. Two of these, the Ziman and the Poiseuille, are hydrodynamic regimes in which collision among phonons are mostly normal. It has been difficult to observe them, save for a few insulators with high levels of isotopic and chemical purity. Our thermal transport measurements, covering four decades of temperatures between 0.1 K and 900 K, reveal that sapphire displays all four regimes, despite its isotopic impurity. In the Ziman regime, the thermal conductivity exponentially increases, attaining an amplitude as large as 35 000 W/Km. We show that the peak thermal conductivity of ultrapure, simple insulators, including diamond, silicon, and solid helium, is set by a universal scaling depending on isotopic purity. The thermal conductivity of sapphire is an order of magnitude higher than what is expected by this scaling. We argue that this may be caused by the proximity of optical and acoustic phonon modes, as a consequence of the large number of atoms in the primitive cell. Published by the American Physical Society 2025
  • Yo Machida, Valentina Martelli, Alexandre Jaoui, Benoît Fauqué, Kamran Behnia
    Fizyka Nyzkykh Temperatur, 50(7) 638-648, May 24, 2024  Peer-reviewed
  • Yuna Nakajima, Yuichi Akahama, Yo Machida
    Physical Review Research, 6(1) 013125, Jan 31, 2024  Peer-reviewed
  • Xiaokang Li, Yo Machida, Alaska Subedi, Zengwei Zhu, Liang Li, Kamran Behnia
    Nature Communications, 14(1) 1027, Feb 23, 2023  Peer-reviewed
    Abstract The origin of phonon thermal Hall Effect (THE) observed in a variety of insulators is yet to be identified. Here, we report on the observation of a thermal Hall conductivity in a non-magnetic elemental insulator, with an amplitude exceeding what has been previously observed. In black phosphorus (BP), the longitudinal (κii), and the transverse, κij, thermal conductivities peak at the same temperature and at this peak temperature, the κijjj/B is ≈ 10−4−10−3 T−1. Both these features are shared by other insulators displaying THE, despite an absolute amplitude spreading over three orders of magnitude. The absence of correlation between the thermal Hall angle and the phonon mean-free-path imposes a severe constraint for theoretical scenarios of THE. We show that in BP a longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes. The anisotropic charge distribution surrounding atomic bonds can pave the way for coupling between phonons and the magnetic field.
  • Taiki Uehara, Takumi Ohtsuki, Masafumi Udagawa, Satoru Nakatsuji, Yo Machida
    Nature Communications, 13(1) 4604, Aug 6, 2022  Peer-reviewed
    Abstract It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal κxx and transverse κxy thermal conductivity in Pr2Ir2O7, which is a metallic analog of spin ice. Despite the presence of mobile charge carriers, we find that both κxx and κxy are dominated by phonons. A T/H scaling of κxx unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and κxx deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the κxx and κxy data clearly shows that κxy correlates with κxx in its response to magnetic field including a success of the T/H scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.

Misc.

 3

Books and Other Publications

 1

Professional Memberships

 1

Research Projects

 9