Fano Line Shape and Chiral Phonons in Thin Films of Weyl Semimetal TaAs.

Tantalum arsenide (TaAs) has emerged as an important Weyl semimetal because of its broken inversion symmetry, the realization of Weyl fermions, and the presence of topologically protected Fermi arc surface states. In this work, we have performed angle-resolved Raman spectroscopy on thin films of TaAs grown on GaAs and characterized the A₁, B₁(1), B₁(2) phonon modes in the ab- plane. We compare these results to that of bulk TaAs and GaAs crystals.  In the thin film, the B₁(1) mode exhibits an asymmetric lineshape that is well described by the Breit–Wigner–Fano (BWF) function. The Fano resonance arises from strong coupling between the infrared-active phonon and electronic transitions near the Weyl points [1]. From the fitted BWF function, the asymmetry parameter q is determined.  A nonzero ∣1/q∣ confirms the asymmetric nature of the Raman phonon lineshape. Temperature-dependent Raman spectroscopy shows that the asymmetry becomes more pronounced with decreasing temperatures. Circularly polarized Raman spectroscopy further reveals non-zero Circular Intensity Differential (CID) [2], confirming the handedness of the mode, and either the phonon is an axial phonon having a well-defined angular momentum or it’s a chiral phonon with broken improper rotational symmetry [3]. Temperature-dependent CID measurements are underway and may reveal interesting information about TaAs.

[1] Xu, B. et al., Temperature-tunable Fano resonance induced by strong coupling between Weyl fermions and phonons in TaAs, Nat Commun 8, 14933 (2017).

[2]   Norman, M.R., Vector optical activity in the Weyl semimetal TaAs, Phys. Rev. B 92, 241116(R) (2015).

[3] Juraschek, D.M. et al., Chiral phonons, Nature Physics, 21, 1532 (2025).