Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

Published 12 Jul 2018 in cond-mat.mtrl-sci and cond-mat.mes-hall | (1807.04761v1)

Abstract: In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$2$MnGa, reaching $S{yx}\sim -6$ $\mu$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $\alpha_{yx}$ shows a crossover between $T$-linear and $-T \log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $\alpha_{yx}$. The $T$ dependence of $\alpha_{yx}$ in experiments and numerical calculations can be understood in terms of a quantum critical scaling function predicted by the low energy effective theory over more than a decade of temperatures. Moreover, the observation of chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.

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Summary

The paper demonstrates a giant anomalous Nernst effect in Co₂MnGa with a −6 μV/K value at room temperature.
It reveals a transition from T-linear to −T log(T) scaling in thermoelectric conductivity near a quantum Lifshitz transition of Weyl fermions.
The findings support potential thermoelectric applications and advance theoretical understanding of quantum-critical behavior in ferromagnetic semimetals.

Giant Anomalous Nernst Effect and Quantum-Critical Scaling in a Ferromagnetic Semimetal

The study under discussion presents a thorough examination of the anomalous Nernst effect (ANE) in the full-Heusler ferromagnetic semimetal Co $_2$ MnGa, highlighting significant findings in both experimental and theoretical physics. This research identifies and explores the giant ANE in Co $_2$ MnGa, which exhibits a remarkable $S_{yx}$ value of approximately $-6$ $\mu$ V/K at room temperature, indicating an enhancement by an order of magnitude over previously reported values for magnetic conductors.

Key Findings

The researchers observed that as temperature increases, the transverse thermoelectric conductivity, quantified by the Peltier coefficient $\alpha_{yx}$ , undergoes a transition between linear and logarithmic temperature dependencies ( $T$ -linear to $-T \log(T)$ ), thereby violating the Mott formula at elevated temperatures. This transition was attributed to the proximity of the system to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions. The presence of Weyl fermions was further corroborated by the experimental observation of chiral anomaly and unsaturated positive longitudinal magnetoconductance.

The study uses numerical and analytical methods to corroborate the experimental results, providing a quantum critical scaling function that accurately describes $\alpha_{yx}(T)$ over more than a decade of temperatures. The theoretical framework is rooted in the low-energy effective theory, lending credence to the scalability of these findings across different systems and conditions.

Experimental and Theoretical Implications

From a practical standpoint, the high magnitude of the ANE demonstrated in this study opens avenues for the use of magnetic Weyl semimetals in thermoelectric technologies, particularly those requiring high efficiency and minimal geometric constraints, such as devices contending with irregular heat source surfaces. The enhancement of ANE through control over the Berry curvature and Fermi energy proximity is depicted not just as a possibility but a tangible approach to material innovation in the field.

Theoretically, the work enriches the understanding of topological phases of matter, offering insights into the scaling behavior near quantum critical points. The observed logarithmic scaling underlines the bridge between low-energy theories of Weyl fermions and observable thermoelectric properties, extending the potential applicability of these theories to various systems beyond semimetals.

Prospective Directions

Future developments in this area may focus on further material innovation, expanding the catalog of Weyl semimetals exhibiting enhanced ANE, and exploring similar transitions. Additionally, the implementation of ANE in spintronic devices could offer optimized energy harvesting capabilities, informed by the fundamental insights into Weyl fermions and their associated quantum critical behaviors.

The study's findings are also likely to inspire refined computational models that better predict thermoelectric and magnetic properties in complex materials, enhancing the predictive capability of theoretical techniques concerning advanced material design.

In summary, this research elucidates the fundamental physics underlying ANE in ferromagnetic semimetals with Weyl fermions and positions Co $_2$ MnGa as a candidate for technological applications requiring efficient thermoelectric conversion.