Amplification of surface acoustic waves through interaction with drifting conduction electrons in gate voltage-controlled bilayer graphene

A theoretical study is presented on the amplification of surface acoustic waves (SAWs) due to their interaction with conduction electrons in gate voltage-controlled bilayer graphene (BLG) in the presence of a dc electric field applied to the BLG sample. Using the Green’s function method, the SAW gain is calculated depending on the density of conduction electrons and the band gap in the electronic spectrum of BLG. It is found that the bias voltage-induced band gap opening in BLG significantly increases the SAW gain, which can be almost four times that of unbiased BLG at room temperature, provided the electron density is not too high ($\lesssim 10^{12}$ cm⁻²). The theory developed also shows that the electron density dependence of the SAW gain is non-monotonic: as the electron density increases from $10^{11}$ to $10^{13}$ cm⁻², the gain first increases, reaching a maximum, and then decreases.