Exploring the apparent violation of the Mott relation in a noncentrosymmetric kagome ferromagnet

TL;DR

This work investigates an apparent violation of the Mott relation in the correlated topological metal UCo$_{1-x}$Ru$_x$Al. By combining doping-dependent transport measurements with a three-band Hubbard-augmented tight-binding model, the authors show that a strongly correlated, flat U-5f band pins the Fermi energy to the Weyl-point manifold, causing the anomalous Hall and Nernst signals to peak at the same doping while the Mott relation remains valid when viewed versus energy. The key mechanism, dubbed "Fermi surfing," explains the tandem motion of Weyl points and $E_F$ with doping and resolves the apparent discrepancy between experiment and a rigid-band Mott expectation. The results imply that careful treatment of magnetism and DOS is essential for interpreting topological transport under doping and offer a design principle for creating doping-resilient topological devices.

Abstract

In magnetic topological materials, time-reversal symmetry breaking gives rise to topological point and line nodes with distinctive signatures in the anomalous Hall and anomalous Nernst conductivity that satisfy the well-known Mott relation. However, this relationship can fail for doping-dependent transport measurements of materials with complex magnetism, topology, and electronic correlations. In this work, we present transport measurements of the correlated topological metal UCoAl doped with Ru, which appear to violate the Mott relation. We develop a model that captures the evolution of Stoner magnetism and topological Weyl points as a function of doping. Using this model, we show how the correlated flat band in this material pins the Weyl points to the Fermi energy, and demonstrate how this explains the unusual doping-dependent behavior of the anomalous Hall and anomalous Nernst conductivities in this material, while the Mott relation is in fact satisfied at each doping level.

Figures (4)

• Figure 1: Left: overview of experimental setup used for anomalous Hall and Nernst measurements and structure of UCo$_{1-x}$Ru$_{x}$Al. Right: cartoon showing the effect of doping on transport properties; as doping increases, $E_F$ moves towards the Weyl points (red/blue circles), $E_F$ and $E_\text{Weyl}$ become pinned in the "Fermi Surfing" region, and then $E_F$ moves away from the Weyl points.
• Figure 2: (a) Measured anomalous Nernst coefficient $\alpha_{xy}(T)$ and (b) anomalous Hall conductivity $\sigma_{xy}(T)$ of UCo$_{1-x}$Ru$_{x}$Al below 65 K. (c) $\sigma_{xy}$ and $\alpha_{xy}$ as a function of Ru concentration at 3K, peaking at $x_\text{Ru}=20$%. Dashed green line shows $\alpha_{xy}$ expected from the Mott relation using computed density of states. (d) Magnetization as a function of doping, reproduced from Ref. Andreev1996_onset_of_ferromagnetism.
• Figure 3:
• Figure 5: Self-consistent bands (right)& density of states (left) for the model in the text at (a) $n=3.2$, (b) $n=3.0$, and (c) $n=2.95$. The trivial band (black) crosses two bands (blue) forming the Weyl points (circled) above $E_F$ (dashed line).