Evidence of Composite Fermion interactions in the Fermi sea at $\nu\rightarrow 1/2$

Interactions between composite fermions with two attached flux quanta ($^2$CFs) are explored at filling factors of the fractional quantum Hall effect $\nu\rightarrow 1/2$. Low-lying ($\omega < $1meV) spin flip excitations modes, in which spin orientation and Landau level index of composite fermions change simultaneously, are measured in resonant inelastic light scattering experiments. The measurements uncover a delicate balance between spin reversal and Fermi energies in the Fermi sea of composite fermions that emerges in the limit of $\nu\rightarrow$1/2. A collapse of the spin-flip excitation gap as $\nu\rightarrow$1/2 is linked with vanishing quasiparticle energy level spacings and loss of full spin polarization. This work is supported by the NSF under Award Number DMR-03-52738 and by the DoE award DE-AIO2-04ER46133. It is also supported by a research grant of the W. M. Keck Foundation.