The parallel-field Hall effect is the appearance of a Hall voltage $V_{\mathrm{H}}$ that is transverse to the current $I$ when a 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 ${\mathrm{ZrTe}}_5$ and was proposed to be due to Berry curvature. However, it is forbidden for the known symmetry of ${\mathrm{ZrTe}}_5$ 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 ${\mathrm{ZrTe}}_5$ with careful magnetic-field alignment. We interpret this result to originate from symmetry breaking and show that a tilting of the Fermi surface allowed by broken symmetry can explain the observed signal. This work demonstrates that the parallel-field Hall effect can be a powerful tool to detect tiny symmetry breaking that is otherwise difficult to detect.

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• Anomalous Hall effect
• Hall effect
• Symmetry protected topological states
• Topological materials