Valley splitting of Si/Si1-xGex heterostructures in tilted magnetic fields

We have investigated the valley splitting of two-dimensional electrons in high-quality Si/Si1-xGex heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor nu=3 (Delta(3)) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear dependence of Delta(3) on the electron density was observed, while the slope of these two configurations differs by more than a factor of 2. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of Delta(3) before and after the coincidence.