Anisotropic states of two-dimensional electron systems in high landau levels: Effect of an in-plane magnetic field

We report the observation of an acute sensitivity of the anisotropic longitudinal resistivity of two-dimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic field. In the third and higher Landau levels, at filling fractions ν=9/2, 11/2, etc., the in-plane field can lead to a striking interchange of the "hard" and "easy" transport directions. In the second Landau level the normally isotropic resistivity and the weak ν=5/2 quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic.