Nanotechnologies / Materials
Energy harvesting, Smart device, Material development

Exploration of thermoelectric and thermomagnetic materials
hosting relativistic quasi-particles
Department of Physics, Graduate School of Science
Associate Professor Hideaki Sakai
https://researchmap.jp/7000018618

Nanotechnologies / Materials

Abstract
The so-called Dirac materials, whose electronic state is described by the relativistic equations of motion, have attracted muchattention owing to their ultrahigh mobility. Such prominent conduction properties are promising not only for electronics applications
but also for thermoelectric power generation. However, so far, there
have been few bulk Dirac materials that can be applied to thermoelectric power generation. Recently, our group has synthesized alayered bulk material consisting of alternating layers of Dirac fermion layer and insulating block layer. By partially substituting theelements in the block layer, we have succeeded in controlling awide range of carrier concentration while keeping the high mobilityof ~20,000 cm2/Vs. As a result, we have shown that the thermoelectric performance can be optimized by the carrier concentration,
demonstrating great potential as a thermoelectric material.

properties associated with the Dirac fermions as a function of carrier density (from p-type to n-type, see Figure 2). The experimental
variation of thermopower is roughly reproduced by the first-principles calculation, achieving the optimized power factor of more than
100 μW/K2cm. On the other hand, the Fermi energy dependenceof Nernst signal shows an anomalous increase toward the Diracpoint, which suggests the field-induced gap reduction of the Diracband due the exchange interaction with the Eu moments.

▍ Significance of the research and Future perspective
Our findings demonstrate the validity of the block layer concept
for developing the thermoelectric Dirac/Weyl materials, providing
a clear-cut guide for the future material design. This will inspire further exploration of the relevant topological materials as a new classof thermoelectrics.

▍ Background & Results
Recently, the so-called Dirac/Weyl semimetals, 3D bulk analog ofgraphene, are among the most active topics in materials science.
In this class of materials, ultrahigh mobility due to the protectionfrom backscattering is promising for potential applications. Whilethe electronic application was an early target, the thermoelectricand transverse thermoelectric (Nernst) power generation has received current attention. To optimize the power factor, it is essential
to finely tune the Fermi energy by chemical substitution.
Here we demonstrate chemically tunable Dirac fermions in layered material EuMnBi2, which consists of the alternative stack of
the 2D Dirac fermion layer and the insulating block layer (Figure 1).
A major advantage of this material is that the spatially-separated
block layer serves as a charge reservoir to the Dirac fermion layer.
We have succeeded in controlling the Fermi energy across thecharge-neutral Dirac point by partially substituting Eu2+ with Gd3+
in the block layer. Importantly, even in the doped samples, the highmobility is retained and hence clear quantum oscillations wereobserved, from which we have quantitatively estimated the carrier
density. As a result, we have revealed the overall thermoelectric

Figure 1

Figure 2
P a t e n t
Tsuruda, Keigo; Sakai, Hideaki et al. Enhancing thermopower and Nernst signal of high-mobility Dirac carriers by Fermi level tuning in the layered magnet
EuMnBi2. Adv. Funct. Mater. 2021, Volume 31, p. 2102275, doi: 10.1002/adfm.202102275
Sakai, Hideaki. High-field Studies on Layered Magnetic and Polar Dirac Metals. J. Phys. Soc. Jpn. 2022, Volume 91, p. 101001,
doi: 10.7566/JPSJ.91.101001
UR
L http://hide-sakai.net
K e y w o r d thermoelectric effect, nernst effect, high mobility, dirac fermion, topological materials

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