The discovery of dynamic chiral anomaly in a Weyl semimetal NbAs

Chiral anomaly, the breaking of chiral symmetry upon quantization of relativistic fermions, leads to non-conservation of chiral charges. Weyl semimetals, characterized by band crossings in 3D reciprocal space, offer a platform to study this phenomenon due to their Weyl fermions with well-defined chirality. The unique electromagnetic response of Weyl semimetals is captured by the θE⋅B term in the electromagnetic Lagrangian, where a non-vanishing θE⋅B field induces anomalous Hall effects and chiral anomaly. Previous studies primarily focused on the DC regime (static E⋅B). Accessing the dynamic E⋅B term is crucial for understanding Weyl fermion behavior in the AC regime and exploring light-matter interactions. However, achieving oscillating E⋅B with external fields is challenging in semimetals. This paper addresses this challenge by using the electric field driven by internal phonon potential combined with a static magnetic field to realize dynamic E⋅B field, enabling the study of dynamic chiral anomaly in the AC regime. The authors predicted and experimentally observed an anomalous phonon transition as a magnetic-field-dependent phonon activity, specifically its angle dependence in magneto-infrared spectroscopy, supporting the existence of the dynamic chiral anomaly.

The discovery of Weyl semimetals has opened avenues for studying Weyl physics in condensed matter. The chiral magnetic effect, a consequence of chiral anomaly, has been observed in the DC regime, manifested as negative longitudinal magnetoresistance. However, experimental exploration in the dynamic AC regime has been limited due to the challenges in generating oscillating E⋅B fields in semimetals. Theoretical work has predicted the possibility of observing signatures of chiral anomaly through phonon dynamics, suggesting that the coupling between Weyl fermions and phonons could reveal further information about chiral anomaly. This paper builds on this prior work, both experimental and theoretical, to investigate the dynamic chiral anomaly.

The research employed magneto-optical spectroscopy, a technique suitable for studying phonon modes and topological condensed matter systems in the presence of a magnetic field. NbAs was selected due to its well-studied strong electron-phonon interaction, fulfilling the conditions for observing the dynamic chiral anomaly. Raman and infrared spectroscopy were initially used to characterize the phonon modes at zero magnetic field. Magneto-infrared spectroscopy measurements were then performed in Voigt geometry, with the magnetic field parallel to the (001) surface and infrared light propagating perpendicularly. The polarization of the light was manipulated to investigate the angular dependence of the phonon activity. The experiments involved varying the magnetic field strength and the angle between the magnetic field and the polarization of the incident light to observe the magnetic field induced phonon activity. Kramers-Kronig analysis was performed to extract the real part of optical conductivity from the reflectivity data. The results were compared to theoretical predictions based on symmetry analysis and the dynamic chiral anomaly. Additionally, the experiments were repeated with a representative Dirac semimetal, Cd3As2, to assess the generality of the observed phenomenon.

The key findings of this study include: (1) The experimental realization of dynamic chiral anomaly in Weyl semimetal NbAs by using the electric field driven by internal phonon potential combined with an external magnetic field. (2) Observation of a magnetic-field-induced anomalous phonon activity, specifically an A1 phonon mode initially inactive in the infrared spectrum at zero field becomes active in the presence of a magnetic field. (3) This phonon activity exhibits a strong angle dependence, being observable only when the oscillating electric field of the light has a component parallel to the applied magnetic field. (4) The frequency of the field-induced phonon mode does not shift with the magnetic field, differentiating it from typical inter-Landau-level transitions. (5) The intensity of the field-induced phonon mode exhibits a B4.3 dependence, indicating a possible magnetic field tunable electron-phonon coupling. (6) Similar behavior was observed in Cd3As2 thin flakes, suggesting a potential universality of this phenomenon in both Weyl and Dirac semimetals.

The observed magnetic-field-induced phonon activity directly supports the theoretical predictions of dynamic chiral anomaly. The angle dependence of the phenomenon, specifically its dependence on the relative orientation of the electric field and the magnetic field, provides strong evidence that the observed phonon activation originates from the chiral magnetic effect. The consistent observation of the phenomenon in both NbAs and Cd3As2 suggests that the dynamic chiral anomaly may be a universal feature in Weyl and Dirac semimetals. The magnetic field dependence of the phonon intensity could provide a novel way to tune the strength of electron-phonon coupling in these materials. Further studies are needed to fully explore this possibility. The discovery opens new pathways for manipulating and studying light-matter interactions in topological materials.

This research successfully demonstrated the experimental realization of dynamic chiral anomaly in a Weyl semimetal, using magneto-infrared spectroscopy. The magnetic-field-induced phonon activity exhibits a strong angle dependence consistent with theoretical predictions. The findings highlight a novel approach to study Weyl fermions' unique electromagnetic responses in the AC regime and suggest potential avenues for manipulating electron-phonon coupling in topological materials. Future research could focus on exploring the interplay between the field-induced phonon mode and inter-Landau-level transitions in systems with stronger electron-phonon coupling and investigating the potential for controlling electron-phonon interactions using magnetic fields.

The study focused on specific Weyl and Dirac semimetals. Further investigation is needed to determine the generality of the findings across a broader range of topological materials. The intensity of the observed phonon activity is relatively weak, suggesting the need for further optimization of experimental techniques to enhance signal strength. The theoretical model used simplified assumptions, and more sophisticated theoretical treatments could provide a deeper understanding of the underlying physics.