The Composite Fermi Sea of Aluminum Arsenide

We study two-component electrons in the lowest Landau level at total filling factor 1/2 with anisotropic mass tensors whose principal axes are rotated by π / 2 as realized in Aluminum Arsenide (AlAs) quantum wells. Combining exact diagonalization and the density matrix renormalization group we demonstrate that the system undergoes a quantum phase transition from a gapless state in which both flavors are equally populated to another gapless state in which all the electrons spontaneously polarize into a single flavor as a function of mass anisotropy. We propose that this phase transition is a form of itinerant Stoner transition between a two-component and a single-component composite fermi sea states and describe a set of trial wavefunctions which successfully capture the quantum numbers and shell filling effects in finite size systems as well as providing a physical picture for the energetics of these states. Our estimates indicate that the composite fermi sea state in AlAs is indeed a fully polarized itinerant Stoner-type magnet.