Spectroscopy of Wigner crystal polarons in an atomically thin semiconductor

Key Points:

• Researchers have directly observed Wigner crystal polarons (WPs) in a WSe2 monolayer, revealing new light–matter excitations formed by strong coupling between excitons and collective excitations of a Wigner crystal (WC) at low temperatures and electron densities.
• WPs manifest as umklapp replicas of attractive exciton–polarons (APs), exhibiting a sizable blueshift even at low electron densities, indicating their origin from both Bragg scattering and polaron dressing by WC collective modes.
• The stability of the WC in WSe2 is enhanced compared to MoSe2, persisting to higher electron densities (~7 × 10^11 cm^−2) and melting around 30 K, likely due to disorder effects that reduce the required Coulomb-to-kinetic energy ratio.
• Experiments demonstrate that WP intensities depend on the spin state of the WC, controllable via external magnetic fields or optical spin pumping, and that WPs share spin-valley polarization characteristics with corresponding APs.
• Theoretical modeling using a mean-field charge-density wave approach and polaron hybridization explains the observed spectral features, including the energy offsets and doping dependence of WPs, establishing WPs as universal quasiparticles in strongly correlated electronic crystals accessible via optical spectroscopy.