In Fig.4, higher AC frequency will contribute to the photon-assisted tunneling and gives a wide region of DC conductivity at about the threshold voltage. In Fig.5, resonance is obvious but what’s more interesting is the finite conductivity at \(\omega=0\). This is intuitive since the CDW is depinned when \(V_{dc}>V_t\) and the more complete of the depinning implies that the more responsive to AC signals.

The combined AC and DC conductivity is shown in Fig.6 with the z-axis being either AC conductivity (with fixed DC voltage) or DC conductivity (with fixed AC frequency). The resonance “ridge” can be seen at about the resonance frequency \(\omega/2\pi = 113 MHz\) for \(V_{dc}>V_t\). In the low field region where \(V_{dc} \approx 10mV\), the resonant conductivity is not clear since the dominant contribution to conductivity is the photon-assisted tunneling (the red plateau in low field and high frequency domain).