  The recent renaissance of black phosphorus (BP) as a two-dimensional 2D
layered material has generated tremendous interest in its tunable electronic
band gap and highly anisotropic transport properties that offer new
opportunities for device applications. Many of these outstanding properties are
attributed to its unique structural characters that still need elucidation.
Here we show Raman measurements that reveal an ultralow-frequency collective
compression mode (CCM), which is unprecedented among similar 2D layered
materials. This novel CCM indicates an unusually strong interlayer coupling in
BP, which is quantitatively supported by a phonon frequency analysis and
first-principles calculations. Moreover, the CCM and another branch of
low-frequency Raman modes shift sensitively with changing number of layers,
allowing an accurate determination of the thickness up to tens of atomic
layers, which is considerably higher than those previously achieved by using
high-frequency Raman modes. These results offer fundamental insights and
practical tools for exploring multilayer BP in new device applications.
