  By measuring the angles at which the Landau levels overlap in tilted magnetic
fields (the coincidence method), we determine the splitting of the
conduction-band valleys in high-mobility two-dimensional (2D) electrons
confined to AlAs quantum wells. The data reveal that, while the valleys are
nearly degenerate in the absence of magnetic field, they split as a function of
perpendicular magnetic field. The splitting appears to depend primarily on the
magnitude of the perpendicular component of the magnetic field, suggesting
electron-electron interaction as its origin.
