Inelastic Tunneling into Multipolaronic Bound States in Single-Layer MoS2

Polarons are quasiparticles that arise from the interaction of electrons or holes with lattice vibrations. Though polarons are well studied across multiple disciplines, experimental observations of polarons in two-dimensional crystals are sparse. We use scanning tunneling microscopy and spectroscopy to measure inelastic excitations of polaronic bound states emerging from coupling of nonpolar zone-boundary phonons to Bloch electrons in n-doped metallic single-layer MoS2. The latter is kept chemically pristine via contactless chemical doping. Tunneling into the vibrationally coupled polaronic states leads to a series of evenly spaced peaks in the differential conductance on either side of the Fermi level. Combining density functional (perturbation) theory with a recently developed electron-lattice downfolding technique, we show that the energy spacing stems from the longitudinal-acoustic phonon mode that flattens at the Brillouin zone edge and is responsible for the formation of stable multipolarons in metallic MoS2.