Abstract
Dirac and Weyl semimetals form an ideal platform for testing ideas developed in high energy physics to describe massless relativistic particles. One such quintessentially field-theoretic idea of chiral anomaly already resulted in the prediction and subsequent observation of the pronounced negative magnetoresistance in these novel materials for parallel electric and magnetic fields. Here we predict that the chiral anomaly occurs - and has experimentally observable consequences - when real electromagnetic fields E and B are replaced by strain-induced pseudo-electromagnetic fields e and b. For example, a uniform pseudomagnetic field b is generated when a Weyl semimetal nanowire is put under torsion. In accord with the chiral anomaly equation we predict a negative contribution to the wire resistance proportional to the square of the torsion strength. Remarkably, left and right moving chiral modes are then spatially segregated to the bulk and surface of the wire forming a "topological coaxial cable". This produces hydrodynamic flow with potentially very long relaxation time. Another effect we predict is the ultrasonic attenuation and electromagnetic emission due to a time periodic mechanical deformation causing pseudoelectric field e. These novel manifestations of the chiral anomaly are most striking in the semimetals with a single pair of Weyl nodes but also occur in Dirac semimetals such as Cd3As2 and Na3Bi and Weyl semimetals with unbroken time reversal symmetry.
Highlights
Mechanical strain that varies smoothly on the interatomic scale is known to affect the low-energy Dirac fermions in graphene in a way that is similar to the externally applied magnetic field
Refs. [6,27], we find that (i) a uniform pseudomagnetic field b 1⁄4 ∇ × a directed along the axis of the wire zis generated by applying static torsion as indicated in Consequences of the strain-induced gauge fields can be most deduced from the chiral anomaly equations
To further confirm the validity of the analytical results presented in the previous sections, we carried out extensive numerical simulations of the lattice Hamiltonian (2.4) in the presence of magnetic field B, as well as torsional and unidirectional strain implemented via Eq (2.9)
Summary
Mechanical strain that varies smoothly on the interatomic scale is known to affect the low-energy Dirac fermions in graphene in a way that is similar to the externally applied magnetic field. The first equation (1.2) is most commonly associated with the chiral anomaly, and it expresses nonconservation of the chiral charge in the presence of aligned EM or pseudo-EM fields This can be understood as pumping of charge from one Weyl point to the other—the chiral magnetic effect [23]. We see that charge transfer between the bulk and the boundary leads to interesting effects when a timedependent e field is generated, e.g., by driving a longitudinal sound wave through the crystal when a B field is present Such a sound wave will experience an anomalous attenuation that can be attributed to the chiral anomaly. It will produce charge-density oscillations in the crystal that can be observed through electric-field measurement outside the sample. Though the anomaly equations are similar in the two systems (missing the e · B term in the helium case), the suggested experimental systems and manifestations are very different—we propose torsion not rotation, and transport not force measurement
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