High Frequency Magneto Oscillations in low Electron Density GaAs/AlGaAs Quantum Wells

We have observed exceptional magneto oscillations in modulation doped GaAs/AlGaAs quantum well structures with low ($10\%$) Al barriers. These reproducible, rapid oscillations appear in our low density samples ($n\sim6.5\times 10^{10}/cm^{2}$) after standard illumination and are robust against thermal cycling. They are periodic in \textit{inverse} magnetic field (1/B) and the amplitude increases with temperature up to $\sim600mK$. With increasing temperature, these oscillations shift from the $\nu\sim1/2$ range towards the $\nu\sim 1/3$ range of the underlying, low density 2DES. Some aspects are reminiscent of recent work by G. A. Cs\'{a}thy et. al. (PRL, \textbf{92}, 256804(2004)) Under tilt, these oscillations follow the standard $\cos\theta$ behavior of a 2D system. The period, if naively translates into density, yields $1.4\times10^{13}/cm^{2} $, which significantly outnumbers all countable carriers in the device. A backside gate affects the QHE in the 2DES as well as the rapid oscillations, although the density shifts for the latter are highly incompatible with dimensions and screening considerations.