Fractional Quantization in ac Conductance ofAlxGa1−xAsCapacitors

Minima are found in the ac conductance (100 kHz) of $n^{-}-\mathrm{GaAs}-{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}-n^{+}-\mathrm{GaAs}$ capacitors at voltages corresponding to fractional fillings, frac13; and frac23;, of the lowest spin-split Landau level of an accumulation layer on $n^{-}-\mathrm{GaAs}$. These are the first observations of a fractional quantum effect in which electron motion is perpendicular rather than parallel to the $n^{-}-\frac{\mathrm{GaAs}}{{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}}$ interface.