Experimental Evidence for a Spin-Polarized Ground State in the v=5/2 Fractional Quantum Hall Effect

We study the v=5/2 even-denominator fractional quantum Hall effect (FQHE) over a wide range of magnetic field in a heterojunction insulated-gate field-effect transistor (HIGFET). The electron density can be tuned from n=0 to 7.6 x 10 sup 11 cm sup -2 with a peak mobility mu = 5.5 x 10 tup 6 cm sup 2 / Vs. The v=5/2 state shows a strong minimum in diagonal resistance and a developing Hall plateau at magnetic fields (B) as high as 12.6T. The strength of the energy gap varies smoothly with B-field. We interpret these observations as strong evidence for a spin-polarized ground state at v=5/2.