Direct measurements of the spin gap in the two-dimensional electron gas of AlGaAs-GaAs heterojunctions

Using magnetocapacitance data, we directly determine the chemical potential jump at low temperatures of about 25 mK when the filling factor traverses the spin gap at nu = 1 and the cyclotron gap at nu = 2. The chemical potential jump for the cyclotron gap is found to increase proportionally to the magnetic field with a slope that is determined by an effective mass 0.071m(0). The data yield a spin gap that also increases proportionally with magnetic field and is described by an enhanced Lande factor g approximate to 5.2. This result has not been explained by existing theoretical models.