Nonclassical Longitudinal Magnetoresistance in Anisotropic Black Phosphorus

Resistivity measurements of a black phosphorus (bP) field-effect transistor 16 nm thick in parallel magnetic fields up to 45 T are reported as a function of the angle between the in-plane field and the source-drain (S-D) axis of the device. The crystallographic directions of the bP crystal are determined by Raman spectroscopy, with the zigzag axis found to be within 5° of the S-D axis and the armchair axis in the orthogonal planar direction. A transverse magnetoresistance (TMR) as well as a classically forbidden longitudinal magnetoresistance (LMR) are observed. Both are found to be strongly anisotropic and nonmonotonic with increasing in-plane field. Surprisingly, the relative magnitude (in %) of the positive LMR is larger than the TMR above ?32 T. Considering the known anisotropy of bP whose zigzag and armchair effective masses differ by a factor of ?7, the experiment strongly suggests this LMR to be a consequence of the anisotropic Fermi surface of bP.