Composite-Fermion Spin Excitations as ν Approaches 1/2: Interactions in the Fermi Sea

Measurements of low-lying spin excitations by inelastic light scattering unveil a delicate balance between spin reversal and Fermi energies in the Fermi sea of composite fermions that emerges in the limit of ν→1/2. The interplays between these two fundamental quasiparticle interactions are uncovered in lowest spin-flip excitations in which the spin orientation and Landau level index of composite fermions change simultaneously. A collapse of the spin-flip excitation gap as ν→1/2 is linked to vanishing quasiparticle energy level spacings and loss of full spin polarization.