Parallel-field Hall effect in ZrTe5

The parallel-field Hall effect is the appearance of a Hall voltage VH that is transverse to the current I when a magnetic field B is applied parallel to I (i.e., B || I perpendicular to VH). 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 ZrTe5 and was proposed to be due to Berry curvature. However, it is forbidden for the known symmetry of ZrTe5 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 ZrTe5 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.