High-Frequency Magneto-Oscillations in $\mathrm{GaAs}/\mathrm{AlGaAs}$ Quantum Wells

We have observed very high-frequency, highly reproducible magneto-oscillations in modulation doped $\mathrm{GaAs}/\mathrm{AlGaAs}$ quantum well structures. The oscillations are periodic in an inverse magnetic field ($1/B$) and their amplitude increases with temperature up to $T\sim 700\mathrm{mK}$. Being initially most pronounced around the filling factor $\nu =1/2$, they move towards lower $\nu$ with increasing $T$. Front and back-gating data imply that these oscillations require a coupling to a parallel conducting layer. A comparison with existing oscillation models renders no explanation.

Figure 1

(a) Magnetoresistance, $R_{xx}$, traces between $T=50\mathrm{mK}$ and 1.4 K. Except for 50 mK, the $R_{xx}$ values increase with $T$. The rapid oscillations have the strongest amplitude at 700 mK. For increasing $T$, the most pronounced oscillation features shift from $\nu \sim 1/2$ to $\nu \sim 1/3$. Inset: parallel layer density, $n_{\parallel }$ (▪, left-hand scale) estimated from precise $R_{xy}$ measurements [12], and the average oscillation amplitude (○, right-hand scale) vs $T$. (b) Differences of neighboring $B$ or $1/B$ values [$\Delta B$ or $\Delta (1/B)$] vs $B$ shows these oscillations to be periodic in $1/B$ rather than in $B$.

Figure 2

A close-up of the oscillations from 260 to 700 mK.

Figure 3

(a) Gray scale plot of $B$ vs back-gate voltage after slowly varying background has been removed. Thick black lines represent the FQHE from the 2DES. Thin ripples represent the high-frequency oscillations, which form a set of nearly parallel lines. (b) Analysis of $\Delta (1/B)$ shows a clear period change with back-gating. (c) Gray scale plot of $B$ vs front-gate voltage. The 2DES is screened by a parallel conduction layer. Ripples can be turned off by applying negative bias to the front-gate.