Observation of an Unconventional Anomalous Hall Effect in a New Ferromagnetic Weyl Semimetal CeAlSi*

In the new ferromagnetic Weyl semimetal (FM WSM) CeAlSi, we find both a conventional and an unconventional anomalous Hall effect (AHE), that we name the loop Hall effect (LHE). The conventional AHE features a concurrent step increase in both Hall resistivity and magnetization when the applied field is along the magnetic easy-axis, similar to the AHE in the existing FM WSMs hosting finite Berry curvature. However, when the field is along the hard-axis, we observe the LHE, which does not exhibit a parallel field dependence between Hall resistivity and magnetization, in sharp contrast to the conventional AHE. More interestingly, by combining DFT calculation and quantum oscillation experiment, we find that the LHE only appears in the samples where the Fermi level lies within a short distance of 1 meV of the Weyl nodes. CeAlSi is also special in a way that both inversion and time-reversal symmetries are intrinsically broken which leads to a skewed distribution of Weyl nodes; this property may account for the unprecedentedly close connection between the LHE and Weyl nodes in CeAlSi.