  Magnetoresistance in single crystals of black phosphorus is studied at
ambient and hydrostatic pressures. In the semiconducting states at pressures
below 0.71 GPa, the magnetoresistance shows periodic oscillations, which can be
ascribed to the magneto-phonon resonance that is characteristic of high
mobility semiconductors. In the metallic state above 1.64 GPa, the both
transverse and longitudinal magnetoresistance show titanic increase with
exhibiting superposed Shubnikov-de Haas oscillations. The observed small Fermi
surfaces, high mobilities and light effective masses of carriers in
semimetallic black phosphorus are comparable to those in the representative
elemental semimetals of bismuth and graphite.
