Evidence of the melting of Wigner crystals*

Electron systems governed by Coulomb interaction manifest remarkable correlated phenomena among which a Wigner crystal (WC) has been a long-sought-after example. Because a WC is sustained by dominating Coulomb interaction, experimental observations require systems with ultra-low carrier densities (down to 10⁹cm^-2 or r_s ∼40), a limit where interaction effect at such tiny energy scales is easily overwhelmed by disorder. As a result, a WC has not been previously demonstrated. Most detections made by transport and resonance techniques found only softly pinned modes that undergo second-order-like melting transition. Because the corresponding translational correlation length ξ is small, they are reasonably explained as intermediate/mixed phases.(i.e. hexatic and glass phases). Utilizing ultra-pure 2D systems, this study demonstrates for the first time nonlinear dc-IV results in r_s>40 limit at T down to 10 mK. Enormous pinning characterized by GΩ resistance found below a critical temperature of T_c~30mK supports a WC on almost a macroscopic scale of ξ. The thermal melting follows a two-stage process resembling the Kosterlitz-Thouless model, except that a striking discontinuity appearing in the differential resistance at T_c suggests a first-order transition.