Shock Waves in Nanomechanical Resonators

We study the formation of shock waves in a nanomechanical resonator with an embedded two-dimensional electron gas using surface acoustic waves. The mechanical displacement of the nanoresonator is read out via the induced acoustoelectric current. Applying acoustical standing waves, we are able to determine the so-called anomalous acoustocurrent. This current is found only in the regime of shock wave formation.

Figure 1

Top and side views of the sample geometry and experimental setup. The micrograph of the sample shows the suspended specimen placed in the delay line formed by IDTs. The suspended 2DEG interacts with the traveling SAW via the elliptic motion of clamping points, which induces acoustoelectric currents.

Figure 2

(a) Longitudinal magnetoresistance with Shubnikov–de Haas oscillations well pronounced. (b) Acoustoelectric current at 1.5 K with surface acoustic waves driving the current from the left (black) and right (red) (source-drain voltage $V_{\mathrm{DS}}=50\mu \mathrm{V}$. At $f=f_{\mathrm{saw}}$, the IDTs generate acoustical waves and the acoustoelectric current is observed as a dip in the current. Driving an acoustic excitation from the right reverses the sign of the normal acoustoelectrical current $I_n$ (traces are for power levels from 0 dBm to 10 dBm). The remarkable feature is the region in the shoulder of the acoustic bandpass: for both directions of the acoustic excitation the anomalous acoustoelectric current $I_{\mathrm{an}}$ shows no sign reversal.

Figure 3

(a) Standing wave measurements–nonlinear response: the current, sensitive to the relative phase shift $\varphi$, is induced by the acoustic standing wave proportional only to the anomalous acoustoelectric current. The maximum current appears at $180°$ phase shift. The applied rf power at the IDTs increases from $-40\mathrm{dbm}$ to $-10\mathrm{dBm}$. (b) The sinusoidal shape evolves into a $(\sin x)/x$ shape, indicating the shock front. At larger powers the resonator jumps into the next higher acoustic mode and the sinusoidal wave form reemerges.

Figure 4

(a) Full scan: At large powers the resonator jumps into the next higher acoustic mode and the sinusoidal wave form reemerges. (b) Maximum current vs SAW amplitude and comparison to model calculations.