Anomalous coincidences between valley split Landau levels in a Si/SiGe heterostructure

We have performed magneto-transport experiments on a high mobility 2DEG in a Si/SiGe heterojunction in tilted magnetic fields up to 26 T at temperatur es down to 450 mK. When tilting the sample in the magnetic held the value o f the spin splitting increases with respect to the Landau level splitting l eading to an overlap of spin-split sub-levels of different Landau levels, t he so-called coincidences. Coincidences between up to live neighbouring Lan dau levels are found. From their positions we deduce a Lande factor g* appr oximate to 3.4. Coincidences between the lowest Landau levels with fully re solved individual valley states show extremely high SdH peaks compared to t he individual SdH maxima outside the coincidence suggesting strong exchange enhancement effects in the occurrence of the coincidence. (C) 1998 Elsevie r Science B.V. All rights reserved.