Wigner Crystal Melting Phase Diagram of GaAs Two-dimensional Holes*

In a strongly interacting two-dimensional (2D) electron system, when the Coulomb energy dominates over the kinetic energy, the electrons tend to arrange periodically and form a so-called Wigner crystal (WC) ground state. In a GaAs 2D electron system, a magnetic-field-induced quantum WC forms at very low temperature and high magnetic field near filling factor v = 1/5 when the kinetic (Fermi) energy is quenched and the Coulomb energy dominates. In an GaAs 2D hole system, on the other hand, the WC phase forms near v = 1/3 because of the significant Landau level mixing caused by the large hole effective mass. Despite the fact that the 2D hole system has been a subject of intense interest for many years, a WC melting temperature vs filling factor phase diagram has been missing. In this work, we map out such phase diagram using a newly developed technique which probes the melting of the WC via its screening efficiency. The phase diagram shows rich features of the WC phases at low fillings and a clear reentrant behavior around v = 1/3.