  Two-dimensional layered semiconductor black phosphorus (BP), a promising
pressure induced Dirac system as predicted by band structure calculations, has
been studied by $^{31}$P-nuclear magnetic resonance. Band calculations have
been also carried out to estimate the density of states $D(E)$. The temperature
and pressure dependences of nuclear spin lattice relaxation rate $1/T_1$ in the
semiconducting phase are well reproduced using the derived $D(E)$, and the
resultant pressure dependence of semiconducting gap is in good accordance with
previous reports, giving a good confirmation that the band calculation on BP is
fairly reliable. The present analysis of $1/T_1$ data with the complemental
theoretical calculations allows us to extract essential information, such as
the pressure dependences of $D(E)$ and chemical potential, as well as to
decompose observed $1/T_1$ into intrinsic and extrinsic contributions. An
abrupt increase in $1/T_1$ at 1.63GPa indicates that the semiconducting gap
closes, resulting in an enhancement of conductivity.
