Suppression of the Landau-level coincidence: A phase transition in tilted magnetic fields

Magnetotransport studies of ${\mathrm{Ga}}_{\mathit{x}}$ ${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/InP heterostructures in strong parallel magnetic fields at mK temperatures reveal a suppression of the coincidence of two Landau levels with opposite spin at filling factor 2. This phenomenon is explained by the occurrence of a phase transition from a spin-unpolarized state (at small tilt angles) to a spin-polarized state (at large tilt angles).