  Phonons, the fundamental vibrational modes of a crystal lattice, play a
crucial role in determining electronic properties of materials through
electron-phonon interaction. However, it has proved difficult to directly probe
the phonon modes of materials in electrical measurements. Here, we report the
observation of giant quantized phonon peaks of the K and K out-of-plane phonon
in graphene monolayer in magnetic fields via tunneling spectra, which are
usually used to measure local electronic properties of materials. A
perpendicular magnetic field quantizes massless Dirac fermions in graphene into
discrete Landau levels (LLs). We demonstrate that emission or absorption of
phonons of quasiparticles in the LLs of graphene generates a new sequence of
discrete states: the quantized phonon modes. In our tunneling spectra, the
intensity of the observed phonon peaks is about 50 times larger than that of
the LLs because that the K and K out-of-plane phonon opens an inelastic
tunneling channel. We also show that it is possible to switch on off the
quantized phonon modes at nanoscale by controlling interactions between
graphene and the supporting substrate.
