Spontaneous ferromagnetism in a dilute 2D electron system

The ground states of an interacting, low-density electron system are of fundamental interest in the physics of strongly correlated electrons. Here we report experiments in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well, revealing a cascade of transitions as the density is lowered [1]. The large electron effective mass in this system allows us to reach very large values of the interaction parameter r_s, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases: a paramagnetic metallic phase at large densities, a transition to an insulating state at r_s = 27 that is stabilized by interaction and disorder, a spontaneous transition to a ferromagnetic state when r_s surpasses 35, and then a phase with strongly non-linear current-voltage characteristics, suggestive of a pinned Wigner solid, when r_s exceeds 38. Remarkably, with the application of a perpendicular magnetic field, we observe clear signatures of a fractional quantum Hall state at filling factor v = 1/3 at r_s as large as 38.

Reference: [1] Md. S. Hossain, et al., Observation of spontaneous ferromagnetism in a two-dimensional electron system, submitted to PNAS.