Current-bias spectroscopy of in-plane magnetoresistance on the microtesla scale in twisted monolayer-trilayer graphene*

Twisted graphene multilayers offer a powerful experimental platform to investigate the interplay between broken symmetries, orbital magnetism, and sometimes superconductivity. In this work, we report the observation of an extraordinary negative in-plane magnetoresistance response in twisted monolayer-trilayer graphene, localized to the same gate voltages where the anomalous Hall effect is also observed. Under a nanoampere AC bias, the device resistance drops by as much as 40% with the application of 1 mT of in-plane magnetic field, mirroring previous measurements in twisted double bilayer graphene. Mapping out the detailed current-voltage characteristics of the sample reveals that this nanoamp magnetoresistance peak hides an intricate pattern of sharp differential resistance peaks activated by 10's of pA bias. These differential resistance peaks move rapidly to higher bias with in-plane magnetic field, explaining the millitelsa-wide magnetoresistance peaks observed in earlier AC measurements, but are only weakly modified by out-of-plane magnetic fields an order of magnitude larger. Our findings impose strong constraints on possible mechanisms for this extreme low-field magnetoresistance, which may be universal to orbital magnets of moiré graphene.