Bloch ferromagnetism of composite fermions

Composite fermions can be tuned to very low effective density in a clean two-dimensional electron gas, which allows the formation of a Bloch ferromagnet. In 1929, Felix Bloch suggested that the paramagnetic Fermi sea of electrons should make a spontaneous transition to a fully magnetized state at very low densities, because the exchange energy gained by aligning the spins exceeds the enhancement in the kinetic energy(1). However, experimental realizations of this effect have been hard to implement. Here, we report the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism. Our platform utilizes the two-dimensional Fermi sea of composite fermions near half-filling of the lowest Landau level. We measure the Fermi wavevector-which directly provides the spin polarization-and observe a sudden transition from a partially spin-polarized to a fully spin-polarized ground state as we lower the density of the composite fermions. Our theoretical calculations that take Landau level mixing into account provide a semi-quantitative account of this phenomenon.

Automated summary

Composite fermions in a clean two-dimensional electron gas can be tuned to very low densities to form a Bloch ferromagnet, with an abrupt transition to full magnetization similar to Bloch ferromagnetism observed experimentally. By measuring the Fermi wavevector, a sudden transition from partially spin-polarized to fully spin-polarized ground state was observed as the density of composite fermions decreases. Theoretical calculations considering Landau level mixing provide a semi-quantitative explanation of this phenomenon.