On the collective mode spectrum for composite fermions at 1/3 filling factor

The collective mode spectrum of the composite fermion state at 1/3 filling factor is evaluated. At zero momentum, the result coincides with the cyclotron energy at the external magnetic field value, and not at the effective field, in spite of the fact that only the former enter in the equations, thus, the Kohn theorem is satisfied. Unexpectedly, in place of the magneto-roton minimum, the collective mode gets a threshold indicating the instability of the mean field composite fermion state under the formation of crystalline structures. However, the question about if this outcome only appears within the mean field approximation should be further considered. (author). 9 refs, 1 fig