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
ν=3 (Δ3) 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 density dependence of Δ3 on the
electron density was observed, while the slope of these two configurations
differs by more than a factor of two. 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
Δ3 before and after the coincidence.Comment: REVTEX 4 pages, 4 figure