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. [ABSTRACT FROM AUTHOR]