The Two-flux Composite Fermion Series of Fractional Quantum Hall States in Strained Si

Magnetotransport properties are investigated in a high-mobility two-dimensional electron system in the strained Si quantum well of a (100) Si_0.75Ge_0.25/Si/Si_0.75Ge_0.25 heterostructure, at temperatures down to 30mK and in magnetic fields up to 45T. We observe around \nu=1/2 the two-flux composite fermion (CF) series of the fractional quantum Hall effect (FQHE) at \nu=2/3, 3/5, 4/7, and at \nu=4/9, 2/5, 1/3. Among these FQHE states, the \nu=1/3, 4/7 and 4/9 states are seen for the first time in the Si/SiGe system. Interestingly, of the CF series, the 3/5 state is weaker than the nearby 4/7 state and the 3/7 state is conspicuously missing, resembling the observation in the integer quantum Hall effect regime that the \nu=3 is weaker than the nearby \nu=4 state. Our data indicate that the two-fold degeneracy of the CFs is lifted and an estimated valley splitting of ~1K.