[Paper Review] Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet

Asaba, T., Ivanov, V.+5 more|arXiv (Cornell University)|Apr 19, 2021

Topological Materials and Phenomena131 citations

TL;DR

This study reports a colossal anomalous Nernst effect (ANE) of 23 µV/K in the ferromagnetic kagome metal UCo0.8Ru0.2Al, driven by strong electronic correlations and spin-orbit coupling in uranium's 5f electrons. The effect arises from a dense network of at least 148 Weyl nodes and nodal lines near the Fermi level, generating extreme Berry curvature that amplifies the transverse thermoelectric response beyond conventional materials.

ABSTRACT

Analogous to the Hall effect, the Nernst effect is the generation of a transverse voltage due to a temperature gradient in the presence of a perpendicular magnetic field. The Nernst effect has promise for thermoelectric applications and as a probe of electronic structure. In magnetic materials, a so-called anomalous Nernst effect (ANE) is possible in zero magnetic field. Here we report a colossal ANE reaching 23 $\mu$V/K in the ferromagnetic metal UCo$_{0.8}$Ru$_{0.2}$Al. Uranium's $5f$ electrons provide strong electronic correlations that lead to narrow bands, which are a known route to producing a large thermoelectric response. Additionally, the large nuclear charge of uranium generates strong spin-orbit coupling, which produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that at least 148 Weyl nodes and two nodal lines exist within $\pm$ 60 meV of the Fermi level in UCo$_{0.8}$Ru$_{0.2}$Al. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Motivation & Objective

• To explore materials with enhanced anomalous Nernst effect (ANE) for thermoelectric applications.
• To identify systems combining strong electronic correlations, large spin-orbit coupling, and topological band structures to maximize Berry curvature contributions.
• To investigate the role of kagome lattice symmetry and noncentrosymmetry in hosting Weyl nodes and nodal lines.
• To demonstrate that actinide-based correlated ferromagnets can exhibit giant ANE responses.
• To establish a link between quantum-critical Weyl nodes and extreme thermoelectric transverse responses.

Proposed method

• First-principles electronic structure calculations with spin-orbit coupling to map the Berry curvature and Weyl node distribution in UCo0.8Ru0.2Al.
• Measurement of longitudinal resistivity ρxx and Seebeck coefficient Sxx to identify the ferromagnetic transition at T_C = 56 K.
• Use of the Mott formula to relate the anomalous Nernst coefficient to the Berry curvature via the conductivity and thermopower tensors.
• Theoretical modeling of the anomalous Hall conductivity (AHC) using the Berry curvature integral over the Brillouin zone.
• Analysis of the electronic structure using spin-polarized density functional theory (DFT) with Hubbard U corrections to account for 5f electron correlations.
• Identification of topological features such as Weyl nodes and nodal lines via symmetry and band structure analysis.

Experimental results

Research questions

• RQ1 Can a correlated noncentrosymmetric kagome ferromagnet host a colossal anomalous Nernst effect?
• RQ2 What is the role of Weyl nodes and nodal lines in enhancing the Berry curvature and transverse thermoelectric response?
• RQ3 How do strong electronic correlations and spin-orbit coupling in uranium 5f electrons amplify the anomalous Nernst effect?
• RQ4 Is the observed ANE in UCo0.8Ru0.2Al primarily intrinsic and driven by Berry curvature?
• RQ5 Can the presence of a large number of Weyl nodes (≥148) near the Fermi level explain the record-breaking ANE value?

Key findings

• The anomalous Nernst effect reaches 23 µV/K in UCo0.8Ru0.2Al, the largest reported value in any material to date.
• Theoretical calculations reveal at least 148 Weyl nodes and two nodal lines within ±60 meV of the Fermi level, indicating a highly nontrivial topological electronic structure.
• The anomalous Hall conductivity due to Berry curvature is calculated at ∼2,000 Ω⁻¹cm⁻¹, confirming a strong intrinsic contribution.
• The large ANE is attributed to the combination of strong 5f electronic correlations, large spin-orbit coupling, and the kagome lattice geometry, which collectively enhance Berry curvature.
• The ferromagnetic transition at T_C = 56 K is marked by a kink in both ρxx and Sxx, indicating a change in electronic scattering and thermopower.
• The system exhibits a moderately dirty metallic state with a residual resistivity of 105 µΩcm and a longitudinal conductivity of ∼9,500 Ω⁻¹cm⁻¹.