Observation of Inter-Valley Gap Anomaly of Two Dimensional electrons in   Silicon
  We report here a systematic study of the energy gaps at the odd-integer quantum Hall states $\nu=3$ and 5 under tilted magnetic (B) fields in a high quality Si two-dimensional electron system. Out of the coincidence region, the valley splitting is independent of the in-plane B fields. However, the $\nu=3$ valley gap differs by about a factor of 3 ($\Delta_v\sim$ 0.4K vs. 1.2K) at either side of the coincidence. More surprisingly, instead of reducing to zero, the energy gaps at $\nu=3$ and 5 rise rapidly when approaching the coincidence angles. We believe that such anomaly is related to strong coupling of the nearly degenerate Landau levels. 
K. Lai, W. Pan, D.C. Tsui, S. Lyon, M. M\"uhlberger, F. Sch\"affler