Bloch Ferromagnetism of Composite Fermions

TL;DR

This paper reports the first experimental observation of a Bloch-like ferromagnetic transition in a two-dimensional composite fermion system, showing a sudden full spin polarization as density decreases, supported by theoretical calculations.

Contribution

It provides the first experimental evidence of Bloch ferromagnetism in a composite fermion system, demonstrating a sharp transition to full spin polarization.

Findings

Abrupt transition to full magnetization observed

Transition occurs as composite fermion density decreases

Theoretical calculations semi-quantitatively explain the phenomenon

Abstract

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. We report here the observation of an abrupt, interaction-driven transition to full magnetization, highly reminiscent of Bloch ferromagnetism that has eluded experiments for the last ninety years. Our platform is the exotic two-dimensional Fermi sea of composite fermions at half-filling of the lowest Landau level. Via quantitative measurements of the Fermi wavevector, which provides a direct measure of the spin polarization, we observe a sudden transition from a partially-spin-polarized to a fully-spin-polarized ground state as we lower the composite fermions' density. Our detailed theoretical calculations provide a semi-quantitative account of this phenomenon.