Experimental signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP

TL;DR

This paper reports experimental evidence of the mixed axial-gravitational anomaly in the Weyl semimetal NbP, observed through thermoelectric transport measurements, confirming a long-standing theoretical prediction in quantum physics.

Contribution

The study provides the first experimental detection of the mixed axial-gravitational anomaly in a Weyl semimetal, linking theoretical predictions to observable thermoelectric effects.

Findings

Observation of positive longitudinal magnetothermoelectric conductance in NbP

The conductance vanishes in the ultra quantum limit

Evidence supports the existence of a mixed axial-gravitational anomaly

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.