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 ν0ex0ex=0ex0ex9/2, 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 ν0ex0ex=0ex0ex5/2 quantized Hall state are destroyed by a large in-plane field and the transport becomes highly anisotropic.