June 2018 Issue
Topics

Insights into quantum limits of materials: First observation of 100% valley-polarization in solids

Yuki Fuseya, Department of Engineering Science, University of Electro-Communications, Tokyo.

"A fundamental understanding of spin orbit coupling in crystals is a major area of research in modern solid state physics," explains Yuki Fuseya. "Such knowledge plays a critical role in the development of devices made using materials including topological insulators, and multilayers composites for spintronics and multiferroics. I am looking at the transport properties of bismuth to shed light on spin orbit coupling in crystals."

Specifically, the quantum limit is the state of electrons under high magnetic fields, where the properties of solids are believed to be drastically changed due to quantum effects. However, it has been challenging to carry out precise measurements under high magnetic fields, so that, the properties of solids at the quantum limit still require clarification.

With this background, Yuki Fuseya and colleagues at Wuhan National High Magnetic Field Center, China, Los Alamos National Laboratory, USA, and Ecole Supérieure de Physique et de Chimie Industrielles, France, discovered an unexpected sharp increase in the conductivity of bismuth. The researchers proved this to be the 100% valley-polarized state, which has not been achieved in any other solid placed under magnetic fields.

In their experiments, the researchers measured the transverse magnetoresistance for magnetic fields up to 65 Tesla (T) under rotating external magnetic fields maintained to be perpendicular to the direction of electric current that was passed through the bismuth. The angular dependence of magnetoresistance obtained by these measurements was compared with theoretical analysis based on the extended Dirac electron model.

According to the previous theorical reports, the conductivity of bismuth is believed to "decrease" rapidly at the quantum limit. In contrast, the current measurements showed the conductivity to "increase" sharply at approximately 50 T, and more detailed theoretical analysis showed this observation to be due to the fact that one or two of the valleys become totally empty, that is, 100% valley-polarized state.

"This findings offer new possibilities of controlling the degrees of freedom of valleys of bismuth, and new innovations in the field of 'valleytronics'."

Angle resolved magnetoresistance of bismuth.
Angle resolved magnetoresistance of bismuth.

Reference

  • Authors: Zengwei Zhu, Jinhua Wang, Huakun Zuo, Benoît Fauqué, Ross D. McDonald, Yuki Fuseya, and Kamran Behnia
  • Title of original paper: Emptying Dirac valleys in bismuth using high magnetic field
  • Journal, volume, pages and year: Nature Communications 8, 15297 (2017).
  • Digital Object Identifier (DOI): DOI: 10.1038/ncomms15297
  • Affiliations: Department of Engineering Science, University of Electro-Communications
Yuki Fuseya
  • Associate Professor: Yuki, FUSEYA (Dr. Sc. From Osaka University 2004/03)
  • Current research areas: Theoretical condensed matter physics, solid state physics
  • Current research subjects: Spin-orbit coupling in solids, quantum transport phenomena
  • Personal website: http://www.kookai.pc.uec.ac.jp/