▎ 摘　　要

We present a theoretical investigation on the hippersound generation and amplification in n-type graphene in the presence of a dc driving electric field. By investigating the consequences of electron-acoustic phonon interaction in a gated graphene, we have found the conditions for phonon amplification to be obtained, namely v(D) > (v) over tilde (s); v(D) being the drift velocity of electrons in the dc field and and vs being the effective sound velocity in the graphene sheet and R-q > Gamma(q), where R-q is the phonon generation rate and C-q is the phonon linear losses. By considering two extreme values of the electron mobility in graphene, namely mu = 20 000 cm(2)/Vs and 1.2V/cm for mu = 190 000 cm(2)/Vs, the critical external electric fields for hippersound amplification were obtained as much as 10 V/cm 1.2 V/cm, respectively. These threshold dc fields are smaller when compared with the corresponding applied fields for phonon amplification in semiconducting bulk and nanostructures. The study is relevant to the application of graphene as acoustoelectric devices as well as in the construction of a high-frequency phonon spectrometer. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4748174]