Density Induced Interchange of Anisotropy Axes at Half-Filled High Landau Levels

We observe density induced 90${}^{\circ }$ rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ($\nu$=9/2, 11/2, etc). Using a field effect transistor, we find the transition density to be $2.9\times {10}^{11}$cm${}^{-2}$ at $\nu$=9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor-magnetic field plane in the regime $4.4<\nu <4.6$. An in-plane magnetic field applied along either anisotropy axis always stabilizes the low density orientation of the stripes.