  Topological semimetals are intriguing targets for exploring unconventional
physical properties of massless fermions. Among them, nodal line or nodal ring
semimetals have attracted attention for their unique one-dimensional band
contact in momentum space and resulting nontrivial quantum phenomena. By field
angular resolved magnetotransport measurements and theoretical calculations, we
show that pressurized black phosphorus (BP) is an ideal nodal ring semimetal
with weak spin-orbit coupling, which has a sole and carrier density-tunable
nodal ring isolated from other trivial bands. We also revealed that the large
magnetoresistance effect and its field-angular dependence in semimetallic BP
are due to highly anisotropic relaxation time. Our results establish
pressurized BP as an elemental model material for exploring nontrivial quantum
properties unique to the topological nodal ring.
