  Two-dimensional (2D) crystals have emerged as a class of materials with
tuneable carrier density. Carrier doping to 2D semiconductors can be used to
modulate manybody interactions and to explore novel composite particles.
Holstein polaron is a small composite particle of an electron carrying a cloud
of self-induced lattice deformation (or phonons), which has been proposed to
play a key role in high-temperature superconductivity and carrier mobility in
devices. Here, we report the discovery of Holstein polarons in a surface-doped
layered semiconductor, MoS2, where a puzzling 2D superconducting dome with the
critical temperature of 12 K was found recently. Using a high-resolution band
mapping of charge carriers, we found strong band renormalizations collectively
identified as a hitherto unobserved spectral function of Holstein polarons. The
unexpected short-range nature of electron-phonon (e-ph) coupling in MoS2 can be
explained by its valley degeneracy that enables strong intervalley coupling
mediated by acoustic phonons. The coupling strength is found to gradually
increase along the superconducting dome up to the intermediate regime,
suggesting bipolaronic pairing in 2D superconductivity.
