We analyze the linear response of a half filled Landau level to long wavelength and low frequency driving forces, using Fermi liquid theory for composite fermions. This response is determined by the composite fermions quasi--particle effective mass, $m^*$, and quasi--particle Landau interaction function $f(\theta-\theta')$. Analyzing infra--red divergences of perturbation theory, we get an exact expression for $m^*$, and conjecture the form of the $f(\theta-\theta')$. We then conclude that in the limit of infinite cyclotron frequency, and small ${ \bf q},\omega$, the composite fermion excitation spectrum is continuous for $0<\omega<\gamma \frac{ e^2}{ \epsilon h}q$, with $\gamma$ an unknown number. For fractional quantum Hall states near a half filled Landau level, we derive an exact expression for the energy gap.
Categories: cond-mat