nu=5/2 Fractional Quantum Hall Effect at 10 T: Implications for the Pfaffian State

We report on the magnetotransport (including tilt fields) around Landau level filling factor nu = 5/2 in a high-purity modulation-doped GaAs/AlGaAs quantum well with twice the electron density of standard samples. A quantized 5/2 Hall plateau is observed at B approximate to 10 T, with an activation gap similar to 125 mK; the plateau can persist up to similar to 25 degrees tilt field. This finding is discussed in the context of a proposed Moore-Read Pfaffian (or anti-Pfaffian) wave function being a possible ground state at 5/2. The tilt fields induce a background resistance at 5/2 that could be either isotropic or anisotropic, depending simply on the in-plane magnetic field direction with respect to the GaAs crystalline axis. Such data indicate a substantial coupling between the 5/2 collective phases and the GaAs crystal.