Signatures of the Chiral Anomaly in Phonon Dynamics

Abstract

Discovered in high-energy physics, the chiral anomaly has recently made way to materials science by virtue of Weyl semimetals (WSM). Thus far, the main efforts to probe the chiral anomaly in WSM have concentrated on electronic phenomena. Here, we show that the chiral anomaly can have a large impact in the A1 phonons of enantiomorphic WSM. In these materials, the chiral anomaly produces an unusual magnetic-field-induced resonance in the effective phonon charge, which in turn leads to anomalies in the phonon dispersion, optical reflectivity, and the Raman scattering.Introduction.-A major recent development in quantum materials has been the discovery of threedimensional Weyl semimetals (WSM) 1 . These materials contain Weyl nodes, i.e. topologically robust points of contact in momentum space between two nondegenerate and linearly-dispersing electronic bands. Weyl nodes are characterized by a quantum number called chirality, referring to the parallel or anti-parallel locking between momentum and spin. WSM have their low-energy electrodynamics governed by pairs of Weyl nodes with the Hamiltonian

Temperature-tunable Fano resonance induced by strong coupling between Weyl fermions and phonons in TaAs

“…Our findings not only suggest a new approach for investigating exotic quantum phenomena in WSMs, such as the chiral anomaly, through the behaviour of phonons, but also imply a broad range of potential applications, such as in thermo-, magneto- and electro-optical devices. Please note that, after our manuscript was submitted, we noticed two preprints (refs 33, 34) that theoretically propose detecting the chiral anomaly in WSMs through phonon dynamics.…”

Anomalous phonon-mode dependence in polarized Raman spectroscopy of the topological Weyl semimetal TaP

“…Topological Weyl semimetal (WSM) is a novel phase of matter that offers the realization of the Weyl fermion, i.e. a massless solution to the Dirac equation with definite chirality [1][2][3][4][5] be demonstrated, where the quantum-mechanical breaking of the classical chiral symmetry [6,7] leads to numerous novel observable phenomena, such as negative magnetoresistance [8][9][10], nonlocal transport [11], unconventional plasmon mode [12], phonon anomaly [13], secondharmonic generation [14], and circular photogalvanic effect [15,16]. Such exotic optical and electronic properties suggest that WSMs could be a promising platform for next-generation electronics and optoelectronics [17][18][19][20][21].…”

“…Theoretically, TaP should give the same Raman intensity when the laser is polarized along the a-or b-axis. In the Raman study by Liu et al on TaAs [34], another WSM with tetragonal space group, the measured Raman intensities of the B 1 1 and B 1 2 modes do not show observable disparity along the a-and b-axes for the same experimental configuration.Though they employed a single 514.5 nm laser as the excitation, different from our choice of13 wavelengths. More importantly, even though TaP and TaAs belong to the same space group and have the same crystal structure, they do not necessarily have the same Raman tensor because of different chemical compositions.…”

Hall viscosity for optical phonons

“…It is important to note that, in the present case, the photons do not develop an effective electric dipole through the anomaly, or the induced mixing between phonons (as in Refs. [31,32]) but this phonon Hall viscosity appears due to the chiral vector nature of the electron-phonon coupling.…”

“…Optical phonons, in contrast, display a behavior that strongly depends on the particular underlying lattice structure of the material, not following a universal lowenergy dynamics. Nevertheless, the impact of the chiral anomaly has been already studied in some systems where the lattice structure allows for a pseudoscalar representation of phonons that couples as a chiral charge imbalance, activating the non-equilibrium CME [30][31][32] leading to the coupling between acoustic phonons and plasmons. In the present paper, we develop a theory of coupling between Weyl electrons and optical phonon modes through chiral vector fields taking a recently proposed model of Weyl fermions in a crystal of Porphyrin [33,34].…”