  A strong anisotropy in the longitudinal resistivity of a 2D electron system
has been observed at half-filled high Landau levels. We report on detailed
Hartree-Fock calculations of the unidirectional charge density wave (UCDW)
orientation energy induced by a tilted magnetic field. We find that stripes can
orient both parallel or perpendicular to the in-plane field depending on the
sample parameters and field strength. The close agreement between complex
experimental data on different sample geometries and our theoretical results
strongly support the UCDW picture as the origin of the observed anisotropies in
high Landau levels.
