The chiral anomaly and thermopower of Weyl fermions in the half-Heusler GdPtBi (1602.07219v2)

Abstract: The Dirac and Weyl semimetals are unusual materials in which the nodes of the bulk states are protected against gap formation by crystalline symmetry. The chiral anomaly~\cite{Adler,Bell}, predicted to occur in both systems, was recently observed as a negative longitudinal magnetoresistance (LMR) in Na$3$Bi and in TaAs. An important issue is whether Weyl physics appears in a broader class of materials. We report evidence for the chiral anomaly in the half-Heusler GdPtBi. In zero field, GdPtBi is a zero-gap semiconductor with quadratic bands. In a magnetic field, the Zeeman energy leads to Weyl nodes. We have observed a large negative LMR with the field-steering properties specific to the chiral anomaly. The chiral anomaly also induces strong suppression of the thermopower. We report a detailed study of the thermoelectric response function $\alpha{xx}$ of Weyl fermions. The scheme of creating Weyl nodes from quadratic bands suggests that the chiral anomaly may be observable in a broad class of semimetals.

• The paper reports experimental observation of the chiral anomaly in Weyl fermions of GdPtBi via distinct negative longitudinal magnetoresistance.
• It presents detailed thermoelectric measurements showing suppressed thermopower and anisotropic responses in the presence of magnetic fields.
• Angular studies confirm field-direction dependence, advancing theoretical predictions and suggesting applications in novel semiconductor devices.

Overview of "The chiral anomaly and thermopower of Weyl fermions in the half-Heusler GdPtBi"

The paper presented by Hirschberger et al. investigates the manifestation of the chiral anomaly in Weyl fermions within the half-Heusler compound GdPtBi, a material with zero-gap semiconductor behavior. The paper provides an experimental observation of the chiral anomaly, predominantly demonstrated through the negative longitudinal magnetoresistance (LMR). This research advances the understanding of Weyl physics in materials, suggesting a broader class of semimetals wherein such phenomena could be observable.

Key Findings

1. Chiral Anomaly in GdPtBi: The authors report a significant negative LMR in GdPtBi, attributing it to the chiral anomaly associated with Weyl fermions. In a magnetic field, the Zeeman energy enables the formation of Weyl nodes, a prerequisite for observing the chiral anomaly. The experimental observations were made in samples where the Fermi energy was close to the Weyl nodes.
2. Negative LMR Characteristics: The paper provides detailed measurements showing a distinct negative LMR at low temperatures, which persists at fields below 150 K. This characteristic bell-shaped negative LMR profile is linked to the alignment of magnetic fields with the current, decreasing sharply for transverse fields.
3. Thermopower and Thermoelectric Response: The paper explores the suppression of thermopower induced by the chiral anomaly, offering a detailed report of the thermoelectric response function αxx of Weyl fermions. The field suppression of thermopower demonstrates directional dependence, suggesting complex underlying mechanisms tied to Weyl physics.
4. Anisotropy and Field Steering: Through angular dependence studies, the paper details the anisotropic nature of the LMR, which aligns with theoretical expectations of the anisotropy in Weyl node formation. The LMR is most prominent when the magnetic field aligns parallel to the current, showcasing a "plume" effect consistent with the chiral anomaly.

Numerical Results and Implications

• The paper presents compelling quantitative data depicting a strong negative LMR, with the relative resistivity reduction as large as 11 times when the magnetic field aligns correctly with the current.
• The suppression of thermopower at high fields and the detailed analysis of the thermoelectric response function αxx reveal significant anisotropy, with results offering strong tests of theoretical predictions for αxx in intermediate field regimes.

Theoretical and Practical Implications

• Theoretical: The research advances theoretical understanding by suggesting that field-induced band crossings can generate Weyl nodes and thus the observable chiral anomaly in semimetals. This implies that semiconductors with significant spin-orbit coupling are promising candidates for further exploration.
• Practical: Exploring the chiral anomaly in materials such as GdPtBi has significant implications for developing novel devices based on anomalous magnetotransport phenomena. These findings could stimulate interest in electronic applications that exploit the high conductivity and unique electronic states at Weyl nodes.

Future Prospectives

Future research might focus on extending the observations to other related compounds, such as gray tin and HgCdTe, to explore practical implementations and more intricate theoretical models that consider disorder and interaction effects on the observed phenomena. Additionally, further efforts could aim to exploit the robust electronic responses due to the Weyl nodes, potentially paving the route for high-efficiency thermoelectric devices and other spintronic applications.