Is the charge-density wave in (TaSe4)2I an axion insulator? · revision history

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Revision 1: Is the charge-density wave in (TaSe4)2I an axion insulator
Revision 2: Is the charge-density wave in (TaSe4)2I an axion insulator?
Plain
When a charge-density wave (a periodic ripple in electron density) gaps out the Weyl points of (TaSe4)2I, its sliding motion should act like the hypothetical axion particle; one experiment reported this and later work questioned it.
Precise
Gooth et al. (Nature 575, 315, 2019, doi:10.1038/s41586-019-1630-4) reported a positive magnetoconductance of the sliding charge-density wave in (TaSe4)2I for E parallel to B, attributed to the phason (sliding mode) acting as a dynamical axion field $\theta(r,t)$ coupled to E.B; Sinchenko et al. (Applied Physics Letters, 2022, doi:10.1063/5.0080380) questioned this interpretation. Determine whether the charge-density-wave phase is an axion insulator with dynamical $\theta$: the test is a phason-carried response proportional to E.B, absent for E perpendicular to B, with magnitude set by the number of Weyl-node pairs. An answer is yes or no with that quantitative comparison.
Settled by
Angle-resolved nonlinear transport of the sliding mode with independent control of E and B on several samples, plus spectroscopic measurement of the phason mass and its coupling to light.
Kind
well-posed
Answer type
yes-no
References
(empty)
Literature status
contested
Status note
Questioned in 2022; a 2023 Nature Materials study (Kim et al., doi:10.1038/s41563-023-01504-5) observed a massive phason in (TaSe4)2I, consistent with but not proof of axion dynamics.
Posed since
2019

Decisions