Spin-pairing instabilities at the coincidence of two Landau levels

The effect of interactions near the coincidence of two Landau levels with opposite spins at filling factor ½ is investigated. By mapping to composite fermions it is shown that the fluctuations of the gauge field induce an effective attractive fermion interaction. This can lead to a spin-singlet ground state that is separated from the excited states by a gap. The magnitude of the gap is evaluated. The results are consistent with the recently observed half-polarized states in the fractional quantum Hall effect at a fixed filling factor. It is suggested that similar anomalies exist for other spin configurations in degenerate spin-up and spin-down Landau levels. An experiment for testing the spin-singlet state is proposed.