Two-Flux Composite Fermion Series of the 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) Si0.75Ge0.25/Si/Si0.75Ge0.25 heterostructure, at temperatures down to 30 mK and in magnetic fields up to 45 T. We observe around ν=1/2 the two-flux composite fermion (CF) series of the fractional quantum Hall effect (FQHE) at ν=2/3, 3/5, 4/7, and at ν=4/9,2/5,1/3. Among these FQHE states, the ν=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 IQHE regime that the ν=3 is weaker than the nearby ν=4 state. Our results can be quantitatively understood in the picture of CF's with the valley degree of freedom.