Quantum Melting Phase Diagram of a Two-Dimensional Electron Wigner Crystal

Strongly interacting electrons can form novel quantum states of matter, exhibiting rich phase diagrams and exotic physical phenomena. The two-dimensional (2D) electron gas provides a paradigmatic example, where competition between Coulomb repulsion and kinetic energy leads to quantum melting of an electron Wigner crystal on increasing electron density. Despite nearly a century of intensive research, however, the detailed nature of this transition is not fully understood. Here, using cryogenic reflectance and magneto-optic spectroscopy, we provide clear experimental evidence for long-standing theoretical predictions that the density-driven quantum melting of a Wigner crystal proceeds through an electronic mixture phase characterized by a microscopic coexistence of crystal and liquid states. At densities below ~3×10¹¹ cm⁻², electrons in a MoSe₂ monolayer form a two-dimensional Wigner crystal. As the electron density increases, we observe abrupt changes in the spin susceptibility, excitonic reflection spectrum, and charge-order-induced exciton scattering. These anomalies mark clear phase boundaries separating a Wigner crystal, intermediate quantum melting phase(s), and a Fermi liquid state. The experimentally determined phase diagram reveals the prominent Pomeranchuk effect arising from the enhanced stability of the crystalline phase upon heating due to its large spin entropy. Our study reveals the interplay between distinct electronic states and establishes the multi-staged nature of the quantum melting in a 2D correlated electron system. Atomically thin semiconductors thus provide a unique experimental platform to study the universal properties of quantum phase transitions in correlated electron systems in two dimensions.