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Valley Degenerate 2D Electrons in the Lowest Landau Level

We report low temperature magnetotransport measurements on a high mobility (μ=325,000 cm^2/V sec) 2D electron system on a H-terminated Si(111) surface. While low magnetic field data indicate a six-fold valley degenerate system, we observe the integral quantum Hall effect at all filling factors ν<=6 and find that ν=2 develops in an unusually narrow temperature range. An extended, exclusively even numerator, fractional quantum Hall hierarchy occurs surrounding ν=3/2, consistent with two-fold valley-degenerate composite fermions (CFs). We determine activation energies and estimate the CF mass.