Experimental signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP (1703.10682v1)

Abstract: Weyl semimetals are materials where electrons behave effectively as a kind of massless relativistic particles known asWeyl fermions. These particles occur in two flavours, or chiralities, and are subject to quantum anomalies, the breaking of a conservation law by quantum fluctuations. For instance, the number of Weyl fermions of each chirality is not independently conserved in parallel electric and magnetic field, a phenomenon known as the chiral anomaly. In addition, an underlying curved spacetime provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial-gravitational anomaly, which remains experimentally elusive. However, the presence of a mixed gauge-gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field, even in flat spacetime, opening the door to experimentally probe such type of anomalies in Weyl semimetals. Using a temperature gradient, we experimentally observe a positive longitudinal magnetothermoelectric conductance (PMTC) in the Weyl semimetal NbP for collinear temperature gradients and magnetic fields (DT || B) that vanishes in the ultra quantum limit. This observation is consistent with the presence of a mixed axial-gravitational anomaly. Our work provides clear experimental evidence for the existence of a mixed axial-gravitational anomaly of Weyl fermions, an outstanding theoretical concept that has so far eluded experimental detection.

• The paper provides the first experimental observation of a mixed axial-gravitational anomaly via positive longitudinal magneto-thermoelectric conductance in NbP.
• It employs collinear temperature gradients and magnetic fields to precisely measure transport coefficients, affirming theoretical predictions.
• The findings challenge traditional conservation laws and the Wiedemann-Franz law, opening pathways for further quantum anomaly research.

Insights into the Mixed Axial-Gravitational Anomaly in Weyl Semimetals

The paper under examination presents a pivotal contribution to understanding the mixed axial-gravitational anomaly as observed in the Weyl semimetal niobium phosphide (NbP). The paper is anchored on the experimental observation of a positive longitudinal magneto-thermoelectric conductance (PMTC) in NbP, which inherently corroborates the theoretical presence of the said anomaly. This revelation marks a significant step away from the previously encountered difficulties in exploring such anomalies within quantum field theoretical frameworks.

The research builds on the fundamental quantum anomalies that arise in Weyl semimetals due to the peculiar behaviors of Weyl fermions. These fermionic quasiparticles are characterized as possessing chiral characteristics, implying a differential stability with respect to their corresponding chirality in external electromagnetic fields—a phenomenon labeled as the chiral anomaly. Consequently, the paper investigates the less explored terrain of mixed gauge-gravitational anomalies that manifest impressive implications on thermoelectrical transport phenomena despite occurring in flat spacetime.

Through controlled experiments utilizing temperature gradients aligned with magnetic fields, the authors reveal a discernible PMTC in NbP. The choice of a collinear setup ensures precise measurements of the transport coefficients, aligning with predictions that a positive thermoelectric response is indicative of mixed axial-gravitational anomalies in Weyl materials. Notably, these anomalies disrupt the conventional covariant conservation laws related to axial current and energy-momentum tensors, which are essential in quantum field theory.

A salient feature of the paper is the detailed examination of how these anomalies affect transport properties through reconciling theoretical computations with experimental data. Different theoretical approaches, such as the Kubo formalism and hydrodynamic frameworks, consistently demonstrate the thermoelectrical implications of these anomalies. The consistency across these theoretical underpinnings reinforces the argument for their presence in flat spacetime conditions. Additionally, the observed experimental results provide clarity on the effect of the magnetic field wherein the PMTC activated by the anomaly diminishes as field strength escalates into the ultra-quantum limit.

The paper's insights into the gravitational anomaly's association with thermoelectric transport point out fundamental links between thermal and electrical conductiveness influenced by the presence of an axial-gravitational anomaly. Moreover, the results suggest a deviation from the expected Wiedemann-Franz law, traditionally tying electrical and thermal conductivities, hinting at the profound impact of topological and geometric factors intrinsic to Weyl semimetals.

This work has implications extending beyond the confines of condensed matter physics. The experimental detection of gravitational anomalies in condensed matter systems forms a bridge to astrophysical and cosmological models where such anomalies have been linked to phenomena in extreme environments like neutron stars or quark-gluon plasmas. Future investigations may focus on improving the understanding of these relationships under varied experimental conditions and uncovering new materials exhibiting these properties.

In conclusion, this paper significantly enhances the discourse on anomalies in quantum systems, pioneering an empirical approach to an area previously dominated by theoretical speculation. It demonstrates the coexistence of quantum field theory concepts within tangible condensed matter systems, offering a framework for future experimental explorations of anomalies with potentially broader applications in both applied and theoretical domains.