Tuesday, September 27, 2016

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

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

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