Unconventional optical selection rules in ZrTe5 under an in-plane magnetic field

The optical selection rules of an electron system under a magnetic field play key roles in determining its optical properties, from which the band structures and underlying symmetries can be derived. In this paper, based on a three-dimensional strong topological insulator model describing ${\mathrm{ZrTe}}₅$, we study the Landau levels (LLs) and magneto-optical conductivity under an in-plane magnetic field. We reveal that in the transverse conductivity $\mathrm{Re}({\ensuremath{\sigma}}_zz)$, the unconventional optical selection rules $n\ensuremath{\rightarrow}n\ifmmode\pm\else\textpm\fi{}2$ dominate, with $n$ being the LL index. We attribute the unconventional selection rules to the peculiar distribution of parity carried by the LLs, resulting from the chiral symmetry of the sub-Hamiltonians. Moreover, we predict that, if the strong anisotropic system is tuned to be nearly isotropic, the LLs would redistribute and the conventional selection rules $n\ensuremath{\rightarrow}n\ifmmode\pm\else\textpm\fi{}1$ can be recovered.