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 longit udinal resistivity of two-dimensional electron systems in half-filled high Landau levels to the magnitude and orientation of an in-plane magnetic fiel d. In the third and higher Landau levels, at filling fractions v = 9/2, 11/ 2, etc., the in-plane held can lead to a striking interchange of the "hard" and "easy" transport directions. In the second Landau level the normally i sotropic resistivity and the weak v = 5/2 quantized Hall state are destroye d by a large in-plane held and the transport becomes highly anisotropic.