セミナー（創発物性科学分野）

2022

乾 幸地氏（東大物工）

• タイトル：自動微分を用いた目的の性質を満たすハミルトニアンの逆設計
• 日時： 9/20(火) 14:00~   
• 場所： 12棟202C
• アブストラクト：物質科学の究極の目的は、望みの性質を持つ物質や材料を自在に作り出すことである。近年、目的となる物理的な性質からハミルトニアンを推定するという逆問題を解く形のアプローチが注目され[1]、ベイズ推定や生成モデル、摂動論などを用いた研究が行われている[2-5]が、計算コストや汎化性能、計算可能な物理量に制約があるといった課題が残されている。我々は機械学習の分野で活用されている自動微分を用いて、目的とする物理的な性質を示すハミルトニアンを自動で構築する汎用的なフレームワークを開発した。このフレームワークを量子異常ホール効果に適用することで、このフレームワークがHaldaneモデルを再発見するだけでなく、その6倍大きな異常ホール効果を示す新しいハミルトニアンを自動で生成できることを紹介する。また、光起電力効果[6]への応用では、太陽光の照射によって電子が非共面的なスピン配置の上を動くことによって、約900A/m2の光電流が発生することを示す[7]。コードはgithubに公開してある[8]。[1] E. Lander et al., Materials Genome Initiative Strategic Plan (National Science and Technology Council, 2021).[2] G. L. W. Hart et al., Nature Materials 4, 391 (2005).[3] R. Tamura and K. Hukushima, Phys. Rev. B 95, 064407 (2017).[4] B. Sanchez-Lengeling and A. Aspuru-Guzik, Science, 361, 6400 (2018).[5] H. Fujita et al., Phys. Rev. B 97, 075114 (2018).[6] R. C. Miller, Phys. Rev. 134, A1313 (1964).[7] K. Inui and Y. Motome, arXiv:2203.07157 (2022).[8] https://github.com/koji-inui/automatic-hamiltonian-design
• 問い合わせ先：野村悠祐（nomura_at_appi, _at_→@, appi→appi.keio.ac.jp）
  
  

Dr. Natsumi Komatsu (Rice University)

• Title: Thermoelectric and Electronic Transport Studies of Ultrahigh-Conductivity Carbon Nanotube Fibers
• Date: 2022.5.25 (Wed) 9:30-10:30
• Place: 14棟7階フォーラム
• Abstract: Ever since the discovery of carbon nanotubes (CNTs), it has long been a challenging goal to create macroscopically ordered assemblies, or crystals, of CNTs that preserve the extraordinary properties of individual CNTs on macroscopic fibers of aligned CNTs with ultrahigh electrical conductivity have been successfully fabricated, but their transport properties and mechanisms have not been systematically investigated. We first studied their electronic transport properties, and found the aligned CNT fiber to be in the weak localization regime at low temperatures, in contrast to all previously studied CNT assemblies, whose low-temperature transport behaviors were described by hopping carriers in strongly localized regimes. Indeed, we observed the phenomenon of universal conductance fluctuations, which is a hallmark of quantum coherent transport, in bundles that constitute the fibers at low temperatures. We further found a series of exceptional properties for the aligned CNT fibers: high continuous current rating, a thermal conductivity comparable to copper, and the largest thermoelectric power factor ever achieved for any CNT sample. These findings will lead to deeper understanding of dominant electronic transport mechanisms inside the fibers, and also provide a route for new opportunities for electrical, thermal, and thermoelectric applications.
• Bio: Natsumi Komatsu received a B.S. degree from Keio University, and received an M.S. degree and Ph.D. degree from Rice University. Her research interests include developing macroscopically aligned carbon nanotubes and investigating their electrical, thermal, optical, and thermoelectric properties. During her PhD at Rice, she served as a graduate student ambassador as well as a Grad STRIVE liaison. She is an alumna of two TOMODACHI programs and Funai Overseas Scholarship.
• Organizer: Tomoko Shimizu