Reorientation of Anisotropy in a Square Well Quantum Hall Sample
  We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $\nu$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model. 
W. Pan, T. Jungwirth, H.L. Stormer, D.C. Tsui, A.H. MacDonald, S.M.   Girvin, L. Smrcka, L.N. Pfeiffer, K.W. Baldwin, and K....