  Negative compressibility generated by many-body effects in 2D electronic
systems can enhance gate capacitance. We observe capacitance enhancement in a
newly emerged 2D layered material, atomically thin black phosphorus (BP). The
encapsulation of BP by hexagonal boron nitride sheets with few-layer graphene
as a terminal ensures ultraclean heterostructure interfaces, allowing us to
observe negative compressibility at low hole carrier concentrations. We
explained the negative compressibility based on the Coulomb correlation among
in-plane charges and their image charges in a gate electrode in the framework
of Debye screening.
