Melting of a 2D quantum electron solid in high magnetic field

The melting temperature T-m of a solid is generally determined by its solid - liquid transition on being heated at a fixed pressure, usually ambient pressure. It is also determined indirectly by the density n by means of the equation of state. This remains true even for solid helium(1), in which quantum effects often lead to unusual properties(2). Here, we present experimental evidence to show that for a two-dimensional (2D) solid formed by electrons in a semiconductor sample under a strong perpendicular magnetic field(3) (B), T-m is not controlled by n, but effectively by the quantum correlation between the electrons through the Landau level filling factor nu= nh/eB ( where h is the Planck constant and e is the electronic charge). Such melting behaviour, different from that of all other known solids ( including a classical 2D electron solid at zero magnetic field(4)), suggests the quantum nature of the magnetic-field-induced electron solid. Moreover, T-m increases with the strength of the sample-dependent disorder that tends to pin the electron solid in place.