Spin-dependent properties of the fractional quantum Hall effect at odd denominators

We present results of tilted-field experiments on the fractional quantized Hall effect at v= 8 5 and v= 5 3 filling factors. At v= 8 5 we observe a sudden transition in the energy gap as a function of tilt angle which is consistent with a phase transition from a spin-unpolarized to a spin-polarized ground state. By contrast, the v= 5 3 gap under tilt lacks the remarkable features of the v= 8 5 state. The angular dependence of its energy gap casts doubt on a simple generalization of the standard spin-polarized hierarchical scheme to electron systems with small Zeeman energy.