DATE2026.04.28 #Press Releases

World's largest transverse magneto-thermoelectric conductivity arising from flat bands

Disclaimer: machine translated by DeepL which may contain errors.

First Demonstration of Itinerant Flat Bands in a Magnetically Ordered Material

Summary 

A research group led by Project Assistant Professor Susumu Minami (at the time of the research; now Assistant Professor at the Graduate School of Engineering, Kyoto University), doctoral student Yangming Wang (at the time of the research), doctoral student Hiroto Nakamura (at the time of the research), Lecturer Akito Sakai, and Professor Satoru Nakatsuji at the Graduate School of Science, The University of Tokyo, in collaboration with Professor Ryotaro Arita of the same university (concurrently Team Director, RIKEN Center for Emergent Matter Science), Fundamental Science Research Fellow Rikuto Oiwa at RIKEN Center for Emergent Matter Science (at the time of the research; now Lecturer at Hokkaido University), Associate Professor Seigo Souma, and Professor Takafumi Sato at the Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, observed the largest-ever transverse magneto-thermoelectric conductivity at room temperature in the ferrimagnetic material GdCo₅. The group also clarified, both experimentally and theoretically, that the origin of this giant magneto-thermoelectric effect is an itinerant flat band arising from the interference effect of wave functions.

GdCo₅ has a crystal structure in which honeycomb and kagome lattices are alternately stacked. In this study, using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations, the research group revealed from both experimental and theoretical perspectives that itinerant flat bands exist across a wide momentum space, arising from the interference effects of wave functions on this stacked honeycomb-kagome lattice (Figure 1). This is the first case in which itinerant flat bands have been observed in a magnetically ordered material. Furthermore, a giant magneto-thermoelectric effect originating from these flat bands was discovered, achieving the largest value of transverse magneto-thermoelectric conductivity at room temperature. By exploiting the giant magneto-thermoelectric effect found in this work, it is expected that this will lead to the development of thermoelectric and spintronics devices.

Fig. 1 (Left) Schematic of the interference effect of the wave function due to Co atoms arranged on a stacked honeycomb-kagome lattice.(Right) Itinerant flat band observed by angle-resolved photoemission spectroscopy (ARPES) and theoretical calculation. The red plane represents the theoretically calculated itinerant flat band, and the red circle and the color map represent the itinerant flat band obtained by ARPES.

Publication Information

Journal name	Advanced Materials
Title of paper	Evidence for itinerant ferromagnetic flat bands producing large transverse responses
Author(s)	Susumu Minami, Yangming Wang, Seigo Souma, Hiroto Nakamura, Akito Sakai, Takumi Osumi, Hang Su, Hikaru Watanabe, Shun'ichiro Kurosawa, Rikuto Oiwa, Daisuke Nishio-Hamane, Kosuke Nakayama, Takuya Nomoto, Ryotaro Arita, Takafumi Sato, Satoru Nakatsuji
DOI	10.1002/adma.202517521