Realization of nodal-ring semimetal in pressurized black phosphorus

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.