Observation of Wigner crystal of electrons in a monolayer semiconductor

If the Coulomb repulsion between the electrons becomes significantly stronger than their kinetic energy, the itinerant electrons in the two-dimensional systems are expected to form a Wigner crystal (WC). Here we provide a direct evidence that the electrons at densities < 3×10¹¹ cm^-2 in a MoSe₂ monolayer spontaneously break continuous translation symmetry and form a WC even at B = 0 [*]. This is revealed by our low-temperature (T = 80 mK) optical experiments that utilize a newly developed technique of detecting a charge order. This method is based on the observation of the exciton umklapp or, equivalently, Bragg scattering off the WC that gives rise to emergence of a new, umklapp peak in the optical excitation spectrum. By tracing the evolution of this peak with the magnetic field and electron density we determined the WC phase properties at fields up to B = 16 T. We provide a qualitative explanation of these findings with a tentative phase-diagram obtained from our Hartree-Fock calculations. Altogether, our results evidence that gate-controlled TMD monolayers allow for exploration of strongly-correlated electronic states in previously inaccessible regime where inter-electron interactions are much stronger than the kinetic energy.

[*] T. Smolenski et al., arXiv:2010.03078 (2020)