Spontaneous ferromagnetism in two-dimensional electron systems


What is the ground state of an electron system when its density is very low? At sufficiently low densities, Coulomb (interaction) energy dominates over the kinetic (Fermi) energy, and the electrons should spontaneously align their spins and become fully magnetized - a classic, textbook problem discussed by Bloch (in 1929), and later on by Stoner (in 1947), that eluded experiments for nearly a century. The key challenge was to make the electron system very dilute, and yet keep the disorder to a minimum level. This talk presents the experimental realization of the long-sought, interaction-driven ferromagnetism. First, I will discuss our observation of spontaneous polarization of electron’s degrees of freedom, namely spin and conduction band valley, as we lower the electron density. I will show that at extremely low densities, we find hints of Wigner solid, the ultimate fate of a low-density electron system. The second part of the talk features the manifestation of Bloch ferromagnetism in a system of unusual suspects, namely a two-dimensional Fermi sea of composite fermions (quasiparticles composed of an electron and two flux quanta). Most phenomena in theory and experiments involving the composite fermions are well understood without any interaction between the composite fermions. Therefore, one would not expect to find signatures of strong interactions here. Surprisingly, however, our measurements of spin polarization reveal a sudden transition from a partially-spin-polarized to a fully-spin-polarized ground state as we lower the density, signaling the Bloch ferromagnetism of composite fermions.