Manifestation of the exchange enhancement of valley splitting in the quantum Hall effect regime

We report a ``dip'' effect in the Hall resistance, Rxy, of a Si metal-oxide-semiconductor field-effect transistor in the quantum Hall effect regime. With increasing magnetic field, the Hall resistance moves from the plateau at Landau filling factor ν=6 directly to the plateau at ν=4, skipping the plateau at ν=5. However, when the filling factor approaches ν=5, the Hall resistance sharply ``dives'' to the value 1∕5(h∕e²) characteristic of the ν=5 plateau, and then returns to 1∕4(h∕e²). This is interpreted as a manifestation of the oscillating exchange enhancement of the valley splitting when the Fermi level is in the middle between two adjacent valley-split Landau bands with the asymmetric position of the extended states.