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

Abstract

The conservation laws, such as those of charge, energy and momentum, have a central role in physics. In some special cases, classical conservation laws are broken at the quantum level by quantum fluctuations, in which case the theory is said to have quantum anomalies. One of the most prominent examples is the chiral anomaly, which involves massless chiral fermions. These particles have their spin, or internal angular momentum, aligned either parallel or antiparallel with their linear momentum, labelled as left and right chirality, respectively. In three spatial dimensions, the chiral anomaly is the breakdown (as a result of externally applied parallel electric and magnetic fields) of the classical conservation law that dictates that the number of massless fermions of each chirality are separately conserved. The current that measures the difference between left- and right-handed particles is called the axial current and is not conserved at the quantum level. In addition, an underlying curved space-time provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial-gravitational anomaly, but this anomaly has yet to be confirmed experimentally. However, the presence of a mixed gauge-gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field, even in flat space-time, suggesting that such types of mixed anomaly could be experimentally probed in condensed matter systems known as Weyl semimetals. Here, using a temperature gradient, we observe experimentally a positive magneto-thermoelectric conductance in the Weyl semimetal niobium phosphide (NbP) for collinear temperature gradients and magnetic fields that vanishes in the ultra-quantum limit, when only a single Landau level is occupied. This observation is consistent with the presence of a mixed axial-gravitational anomaly, providing clear evidence for a theoretical concept that has so far eluded experimental detection.

Nonmonotonic magnetoresistance of a two-dimensional viscous electron-hole fluid in a confined geometry

“…Our results are relevant to a wide range of novel materials including compensated semimetals 11 , topological insulators, and multilayer graphenes. Our main qualitative conclusions are independent of the details of the quasiparticle spectrum and are applicable also to systems with the linear (Dirac) spectrum such as the monolayer graphene.…”

“…Large positive (and often linear) MR was reported in graphene and topological insulators close to charge neutrality [14][15][16][17][18][19][20][21][22] [36][37][38][39] that this effect could be a direct condensed matter manifestation of the Adler-Bell-Jackiw chiral anomaly [40][41][42] . However, the unexpected variety of materials exhibiting negative MR [8][9][10][11][31][32][33][34][35] does not support the idea that such measurements may provide a "smoking gun" for detecting a Weyl semimetal 37,39 . Rather, there may be several different mechanisms of negative MR similarly to the case of linear positive MR that can appear, e.g., due to disorder 19,43 , in the extreme quantum limit 24,44,45 , or in compensated two-component systems 20,22,[46][47][48][49] .…”

“…At the same time, viscous one-component systems are characterized by negative MR 11,12 . Here we show that neutral two-component systems may exhibit both types of behavior at the same time so that the MR is non-monotonous: in weak magnetic fields, we find the negative, parabolic MR, while in strong fields the resistance grows with the field eventually approaching the linear dependence.…”

Semiclassical theory of anomalous transport in type-II topological Weyl semimetals

“…In bulk thermoelectric transport, Weyl semimetals have been predicted to have a number of distinct signatures [41][42][43] , most notably a Nernst thermopower at zero applied magnetic field in time-reversal breaking Weyl semimetals 42 . Recent experiments have shown extraordinary thermoelectric properties of NbP, including a large ordinary Nernst effect 44 and evidence for a mixed axial-gravitational anomaly 45 [56][57][58] , Mn 3 (Ge,Sn) is instead a weakly canted antiferromagnet. It has been suggested that the real-space magnetic texture can account for a large anomalous Hall effect in Mn 3 Ge if the spins are non-coplanar 59 , however experiments have shown that the Mn 3 (Ge,Sn) system does possess a large anomalous Hall effect in the planar magnetic phase with a Hall coefficient that is much larger than its weakly canted moment would suggest 52 .…”

The strange topology that is reshaping physics

“…These 'quasiparticle' states may have properties that are not present in any known elementary particle (see page 324) 6 . They could even mimic particles that physicists have yet to discover.…”