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Valley Stoner instability of the composite Fermi sea


Sodemann,  Inti
Max Planck Institute for the Physics of Complex Systems, Max Planck Society;

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(Preprint), 999KB

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Zhu, Z., Sheng, D. N., Fu, L., & Sodemann, I. (2018). Valley Stoner instability of the composite Fermi sea. Physical Review B, 98(15): 155104. doi:10.1103/PhysRevB.98.155104.

We study two-component electrons in the lowest Landau level at total filling factor nu(T) = 1/2 with anisotropic mass tensors and principal axes rotated by pi/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 beyond a critical mass anisotropy of m(x)/m(y) similar to 7. 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 wave functions 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 of AlAs is the analog of an itinerant Stoner magnet with a finite spontaneous valley polarization. We pinpoint experimental evidence indicating the presence of Stoner magnetism in the Jain states surrounding nu = 1/2.