Parallel-Field Hall effect in ZrTe${}_5$

Parallel-field Hall effect is the appearance of a Hall voltage $V_{\mathrm{H}}$ that is transverse to the current $I$ when the magnetic field $B$ is applied parallel to $I$ (i.e. $B\parallel I\perp V_{\mathrm{H}}$). Such an effect is symmetry forbidden in most cases and hence is very unusual. Interestingly, the existence of a finite parallel-field Hall effect was reported for the layered topological semimetal ZrTe${}_5$ and was proposed to be due to Berry curvature. However, it is forbidden for the known symmetry of ZrTe${}_5$ and the possible existence of a misaligned out-of-plane magnetic field was not completely ruled out. Here, we elucidate the existence of the parallel-field Hall effect in ZrTe${}_5$ with careful magnetic-field alignment. We interpret this result to originate from symmetry breaking and quantitatively explain the observed parallel-field Hall signal by considering a tilting of the Fermi surface allowed by broken symmetry.