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

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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 30mK and in magnetic fields up to 45T. 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 integer quantum Hall effect regime that the ν=3 is weaker than the nearby ν=4 state. Our data indicate that the two-fold degeneracy of the CFs is lifted and an estimated valley splitting of 1K.