Cerenkov emission of acoustic phonons electrically generated from three-dimensional Dirac semimetals

Cerenkov acoustic phonon emission is theoretically investigated in a three-dimensional Dirac semi metal (3DDS) when it is driven by a dc electric field E. Numerical calculations are made for Cd3As2 in which mobility and electron concentration are large. We find that Cerenkov emission of acoustic phonons takes place when the electron drift velocity v(d) is greater than the sound velocity v(s). This occurs at small E (similar to few V/cm) due to large mobility. Frequency (omega(g)) and angular (theta) distribution of phonon emission spectrum P(omega(q), theta) are studied for different electron drift velocities v(d) (i.e., different E) and electron concentrations tie. The frequency dependence of P(omega(q), theta) shows a maximum P-m(omega(q), theta) at about omega(m) approximate to 1 THz and is found to increase with the increasing v(d) and n(e). The value of omega(m), shifts to higher region for larger n(e). It is found that omega(m)/n(e)(1/3) and P-m(omega(q), theta)/n(e)(2/3) are nearly constants. The latter is in contrast with the P-m(omega(q), theta)/n(e)(1/2) = constant in conventional bulk semiconductor. Each maximum is followed by a vanishing spectrum at nearly "2k(f) cutoff," where k(f) is the Fermi wave vector. Angular dependence of P(omega(q), theta) and the intensity P(theta) of the phonon emission shows a maximum at an emission angle 45 and is found to increase with increasing v(d). P(theta) is found to increase linearly with n(e), giving the ratio P(theta)/(n(e)v(d)) nearly a constant. We suggest that it is possible to have the controlled Cerenkov emission and generation of acoustic phonons with the proper choice of E, theta, and n(e). 3DDS with large n(e) and mobility can be a good source of acoustic phonon generation in THz regime. Published by ALP Publishing.