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Anomalous Scaling Behavior of Transverse Thermoelectric Conductivity in Ferromagnet CoMnSb

Nakatsuji Group

Transverse transport effects such as anomalous Hall and Nernst effects are effective probes to topological texture in electronic band structures. In particular, large Berry curvature originating from Weyl nodes or nodal line structures is known to enhance these effects significantly. In contrast to the Hall conductivity σyx, which measures the summation of the Berry curvature below Fermi energy, the transverse thermoelectric conductivity αyx sensitively captures the Berry curvature in the vicinity of the Fermi energy [1]. Therefore, the Nernst effect provides complementary information about the characteristics of the topological band structure. Moreover, scaling behavior αyx ~ -T logT is recently reported in ferromagnet Co2MnGa [2]. This anomaly is attributed to the Weyl semimetal state tuned close to the quantum Lifshitz critical point. However, the universality of this critical behavior is still yet to be confirmed experimentally.

Here, we report magnetic, transport, and thermoelectric properties of single crystals of ferromagnetic CoMnSb with a high Curie temperature of 470 K [3]. The actual experimental crystal structure of CoMnSb is a 2×2×2 superstructure of the previously predicted half-Heusler structure. Since theoretical understanding of the transverse transport phenomena in the superstructure CoMnSb is lacking, we performed first-principles calculation of superstructure CoMnSb to compare with experimental results.

We find a sizable anomalous Hall conductivity σyx and transverse thermoelectric conductivity αyx experimentally. We observe substantial anomalous Hall conductivity σyx and transverse thermoelectric conductivity αyx. The experimental σyx (T ) is well described by DFT calculation for the chemical potential E - EF ≈ -130 meV (Fig. 1(a)), which is consistent with the valence electron count estimated from the sample composition measured by Inductively Coupled Plasma (ICP) method. Moreover, αyx exhibits -T logT critical behavior instead of the conventional Fermi liquid behavior αyx ~ T over a decade of temperature between 10 and 400 K (Fig. 1(b)). This behavior is similar to ferromagnetic Weyl semimetal Co2MnGa and nodal-line semimetals Fe3X (X = Al, Ga) [2, 5]. Our theoretical calculation shows that in the vicinity of the chemical potential E - EF ≈ -130 meV, the scaling behavior αyx ~ -T logT arises from Weyl nodes in the band structure (Fig. 2).

In summary, we have successfully synthesized single-crystal samples of superstructure CoMnSb and explored their transverse transport properties. The observed critical scaling behavior in the transverse thermoelectric conductivity σyx ~ -T logT clearly deviates from the conventional Fermi liquid behavior, providing a hallmark of topological texture in the band structure. The comparison of experimental results with DFT calculations shows that this critical behavior results from Weyl nodes. This result indicates that the logarithmic criticality in the transverse thermoelectric conductivity provides a highly sensitive signature to reveal the topological band texture.

  • [1] G. Sharma, P. Goswami, and S. Tewari, Phys. Rev. B 93, 035116 (2016).
  • [2] A. Sakai et al., Nat. Phys. 14, 1119 (2018).
  • [3] H. Nakamura et al., Phys. Rev. B 104, L161114 (2021).
  • [4] S. Minami et al., Appl. Phys. Lett. 113, 032403 (2018).
  • [5] A. Sakai et al., Nature 581, 53 (2020).
  • H. Nakamura, S. Minamia, T. Tomita, A. A. Nugrohob, and S. Nakatsuji
  • aThe University of Tokyo
  • bInstitut Teknologi Bandung