Giant acoustoelectric current in suspended quantum point contacts

We present results on the acoustoelectric current driven through a quantum point contact (QPC) placed on a suspended nanobridge, which is subject to surface acoustic waves (SAWs). The magnitude of this current is much larger than that of a two-dimensional gas, and the system has enhanced sensitivity to perturbations. In particular, the current oscillations as a function of magnetic field exhibit additional features associated with the spin splitting. Furthermore, the negative voltage applied to the QPC gates induces the oscillations revealing the subband structure not seen in transport measurements at the elevated temperatures of our experiment. Near the pinch off conditions, the acoustoelectric current becomes negative which we attribute to the enhanced backscattering caused by the SAW-phonons’ absorption and emission.

Figure 1

Scanning electron micrograph (SEM) images of the device. (a) Full view of the etched acoustic waveguide, dark regions indicate the suspended membrane. Scale bar is 2 μm. (b) Close up image of center region, yellow indicates the Ti/Au electrodes which make up the QPC pairs. Scale bar is 300 nm. (c) Schematic diagram of the device and measurement setup, not drawn to scale.

Figure 2

Hall measurement data (taken with floating gates): (a) Standard Hall measurement as taken with a four-point probe and standard lock-in techniques using a source-drain bias. (b) The acoustoelectric current $I_{\mathrm{AEC}}$ was produced using a SAW with no dc bias applied. (c) Theoretical curve for the acoustoelectric current.

Figure 3

Applied dc source-drain bias pinch off with $V_{\mathrm{SD}}$ ranging from 0.1 to 1 mV in 0.1 mV increments. Inset shows a magnified view of the current with the gate voltage ranging from −0.69 to −0.66 V. Measurements are performed at zero magnetic field.

Figure 4

(a) Acoustoelectric current at magnetic fields of 0, 0.5, 1.0, 1.5, and 2.0 T. Applied RF power of −10 dBm and a frequency of 840 MHz. Inset shows a subset of the data detailing the spin splitting that starts to become visible at 1 T. (b) Inelastic scattering of an electron emitting $n$ phonons of $\hslash \omega$, causing backscattering from the QPC gate potential $V_g$. (c) Inelastic scattering of an electron absorbing $n$ phonons of $\hslash \omega$, allowing it to scatter forward across the QPC gate potential $V_g$. (d) Electron absorbing $n$ phonons and scattering to the left.