Abstract-Amplification of terahertz frequency acoustic phonons by drifting electrons in three-dimensional Dirac semimetals

K. S. Bhargavi1 and S. S. Kubakaddi2,a)
http://scitation.aip.org/content/aip/journal/jap/120/12/10.1063/1.4963127
1 Department of Physics, Siddaganga Institute of Technology, Tumkur-572013, Karnataka, India
2 Department of Physics, K. L. E. Technological University, Hubballi-580031, Karnataka, India
a) Email: sskubakaddi@gmail.com
J. Appl. Phys. 120, 125705 (2016); http://dx.doi.org/10.1063/1.4963127

The amplification coefficient α of acoustic phonons is theoretically investigated in a three-dimensional Dirac semimetal (3DDS) driven by a dc electric field E causing the drift of the electrons. It is numerically studied as a function of the frequency ω q, drift velocity v d, electron concentration ne, and temperature Tin the Dirac semimetal Cd3As2. We find that the amplification of acoustic phonons (α ∼ hundreds of cm-1) takes place when the electron drift velocity v d is greater than the sound velocity v s. The amplification is found to occur at small E (∼few V/cm) due to large electron mobility. The frequency dependence of αshows amplification in the THz regime with a maximum α m occurring at the same frequency ω q m for different v d. The α m is found to increase with increasing v d. α vs ω q for different ne also shows a maximum, with α m shifting to higher ω q for larger ne. Each maximum is followed by a vanishing α at nearly “2k f cutoff,” where k f is the Fermi wave vector. It is found that α m/ne and ω q m/n e 1/3 are nearly constant. The α m ∼ ne can be used to identify the 3DDS phase as it differs from α m ∼ n e 1/3 dependence in conventional bulk Cd3As2 semiconductor.