The Integer and Fractional Quantum Hall Effect in the Lowest Landau Level of Valley Degenerate 2D Electrons on Hydrogen Terminated Si(111)

We report low temperature magnetotransport measurements on a high mobility ($\mu=325\,000\,$cm$^{2}$/V$\,$sec) 2D electron system on a H-terminated Si(111) surface. In Si(111), there are six degenerate, anisotropic valleys which can affect the magnetotransport in unexpected ways. While low magnetic field data indeed show a six-fold valley degenerate system, we observe the integral quantum Hall effect at all filling factors $\nu\leq 6$, indicating a magnetic-field-induced breaking of the valley degeneracy. Additionally, we find that $\nu=2$ develops in an unusually narrow temperature range, which might indicate the existence of a novel broken-symmetry valley phase. Finally, we observe an extended, exclusively even numerator, fractional quantum Hall hierarchy surrounding $\nu=3/2$ with denominators up to 15. This hierarchy is consistent with two-fold valley-degenerate composite fermions. We determine activation energies and provide the first estimate the composite fermion mass in a multi-valley system.