Quantum Melting in a Germanium Quantum Dot

Strongly interacting electrons can crystalize into a Wigner Crystal phase, which undergoes a phase transition into a Fermi liquid when kinetic energy dominates. While this transition is a cornerstone of condensed matter physics, this melting process remains difficult to probe due to long-range potential fluctuations in bulk systems. When confined to the nanometer scale, a small number of particles can maintain an ordered state known as a Wigner molecule—a microscopic analogue of the Wigner crystal where disorder-induced effects are largely suppressed. However, the quantum melting of Wigner molecules has yet to be observed. In this work, we demonstrate the formation and quantum melting of Wigner molecules in a gate-defined germanium quantum dot. By precisely modulating the hole occupancy and the confinement potential, we provide unambiguous evidence that below a critical hole density, Coulomb repulsion localizes individual holes into discrete lattice sites, establishing a spin-polarized Wigner molecular state, well consistent with previous theory. As the density increases, we observe the melting of this ordered structure into a Fermi liquid. Crucially, our measurements resolve an intermediate regime characterized by the coexistence of ordered and disordered phases within a narrow effective density window. Whereas this intermediate state is missing in the present theoretical framework of Wigner molecules, it appears in a range similar to that of liquid-solid coexistence in Wigner crystal. These results provide a new platform for further exploration of the microscopic features of strongly correlated physics, shedding new light on the intermediate regime in the quantum melting of Wigner crystals and open an avenue to exploit the application of Wigner molecules for quantum information in a very promising spin qubit platform.