Valley-degenerate two-dimensional electrons in the lowest Landau level

Abstract

We report low temperature magnetotransport measurements on a high mobility ($\ensuremath{\mu}=325\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$) two-dimensional electron system on a H-terminated Si(111) surface. We observe the integral quantum Hall effect at all filling factors $\ensuremath{\nu}\ensuremath{\le}6$ and find that $\ensuremath{\nu}=2$ develops in an unusually narrow temperature range. An extended, exclusively even numerator, fractional quantum Hall hierarchy occurs surrounding $\ensuremath{\nu}=3/2$, consistent with twofold valley-degenerate composite fermions (CFs). We determine activation energies and estimate the CF mass.