(a) Frequency spectra of displacement signals measured at different frequencies of AC electric currents. The measurement temperature is 4.5 K. (b) Frequency spectra of displacements at different amplitudes of AC electric currents. The frequency of the AC electric current was kept at 10 kHz. The measurement temperature is 4.5 K. The current-induced signals are highlighted by green arrows. c. Current-amplitude dependence of the current-induced displacement signals (red dots) and the background (BG) signals (open squares). The background levels are estimated by the 10-kHz values of the fitted curves for the spectra: A/f + B, where f is the frequency and A and B are fitting parameters. The dotted line is just guides for eyes. This plot is obtained from the data in (b). The background levels are shaded in gray just for clarity.