Nanotechnologies / Materials
Smart device, Clean energy

Exploration of innovative functional quantum materials
by high-pressure synthesis
Department of Materials Engineering Science, Graduate School of Engineering Science
Professor Shintaro Ishiwata
https://researchmap.jp/ishiwata

Nanotechnologies / Materials

Abstract
We are exploring functional quantum materials such as high-temperature superconductors, spintronics materials, and thermoelectric materials under extreme environments such as high pressureof several GPa and strong oxidation atmospheres. One group ofmaterials that we are currently focusing on is a system called topological semimetals, which are expected to become high-speed
and energy-saving electronic materials that outperform existing
semiconductor materials because they contain conduction electrons with ultra-high mobility. The discovery of magnetic materials
with such properties as topological semimetals could lead to thedevelopment of novel spintronics materials, but there have beenvery limited examples of such studies. Recently, we have succeeded in synthesizing single crystals of a new type of topologicalsemimetal α-EuP3 with magnetic properties under high pressures.
Furthermore, by measuring various physical properties under magnetic fields and performing first-principles calculations, we have revealed that this material is a system that gives rise to multiple topological semimetallic phases with different properties depending on
the magnetic field orientation with respect to the mirror symmetry
plane of the crystal structure.

formation technology applications such as ultra energy-saving andhigh-speed electronic devices. In the future, by actively utilizing
first-principles calculations and informatics, we aim to strategicallydevelop functional quantum materials using high-pressure synthesis methods.

▍ Background & Results
Recently, a group of materials called topological semimetals,
typified by graphene, have attracted much attention. These materials contain unique electrons that behave much like masslessneutrinos. If these electrons can be generated and controlled bya magnetic field, it could lead to new spintronics and super energy-saving electronic devices. However, the relationship between
topological quantum phases and magnetism is not simple, andits realization has been difficult. Recently, we have succeeded in
growing single crystals of a new layered magnetic semimetal α-EuP3 under high pressure of several GPa, and by controlling thedirection of the large magnetic moment of europium in the magnetic field, we have selectively generated a nodal-line semimetal
phase showing a giant anomalous Hall effect and a Weyl semimetal
phase showing a negative magnetoresistance effect. Furthermore,a close examination of the band structure under a magnetic fieldobtained by first-principles calculations reveals that the selectivegeneration of the topological quantum phase by a magnetic fieldfound in α-EuP3 is due to the subtle coupling between the magnetic field controllable magnetism of europium and the conducting
electrons in the phosphorus layer, and that the nontrivial topologyof the band structure is protected by the mirror symmetry of thesystem.

Crystal structure of magnetic phosphorus compound α-EuP3 (left upper) and its
single crystal (right upper).
Selective generation of topological quantum phases by magnetic fields.

▍ Significance of the research and Future perspective
This study proposes a realistic solution for the generation andmanipulation of nontrivial band topology by magnetic fields, whichis expected to accelerate the development of materials for new in-Multi-anvil-type high-pressure apparatus.

P a t e n t
Tr e a t i s eAlex Hiro Mayo; Takahashi, Hidefumi; Mohammad Saeed Bahramy et al. Magnetic Generation and Switching of Topological Quantum Phases in a Trivial
Semimetal α−EuP3. Phys. Rev. X, 2022, 12, 011033, doi: 10.1103/PhysRevX.12.011033

URL
K e y w o r d topological semimetal, high pressure synthesis, spintronics