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Phonon splitting of the longitudinal optical and transverse optical modes(LO-TO splitting), a ubiquitous phenomenon in three-dimensional (3D) polarmaterials, is essential for the formation of the 3D phonon polaritons. Theoriespredict that the LO-TO splitting will break down in two-dimensional (2D) polarsystems, but direct experimental verification is still missing. Here, usingmonolayer hexagonal boron nitride (h-BN) as a prototypical example, we reportthe direct observation of the breakdown of LO-TO splitting and the finite slopeof the LO phonons at the center of the Brillouin zone in 2D polar materials byinelastic electron scattering spectroscopy. Interestingly, the slope of the LOphonon in our measurements is lower than the theoretically predicted value fora freestanding monolayer due to the screening of the Cu foil substrate. Thisenables the phonon polaritons (PhPs) in monolayer h-BN/Cu foil to exhibitultra-slow group velocity (~ 5 x 10^-6 c, c is the speed of light) andultra-high confinement (~ 4000 times smaller wavelength than that of light).Our work reveals the universal law of the LO phonons in 2D polar materials andlays a physical foundation for future research on 2D PhPs.

Observation of the Breakdown of Optical Phonon Splitting in a Two-dimensional Polar Monolayer