Evolution of the second lowest extended state as a function of the effective magnetic field in the fractional quantum Hall regime

It has been shown that, at a Landau level filling factor v=1/2, a two-dimensional electron system can be mathematically transformed into a composite fermion system interacting with a Chern-Simons gauge field. At v=1/2. the average of this Chern-Simons gauge field cancels the external magnetic field B_ext so that the effective magnetic field B_eff acting on the composite fermions is zero. Away from v=1/2, the composite fermions experience a net effective magnetic field B_eff. We present the first study of the evolution of the second lowest extended state in a vanishing effective magnetic field in the fractional quantum Hall regime. Our result shows that the evolution of the second lowest extended state has a good linear dependence on the effective magnetic field B_eff within the composite fermion picture.