Hot electron cooling in three-dimensional Dirac fermion systems at low temperature: effect of screening

Hot electron cooling rate P, due to acoustic phonons, is investigated in three-dimensional Dirac fermion systems at low temperature taking account of screening of electron-acoustic phonon interaction. P is studied as a function of electron temperature T_e and electron concentration n_e. Screening is found to suppress P very significantly for about T_e 1K in Cd_3As_2 . In Bloch-Gruniesen (BG) regime, for screened (unscreened) case T_e dependence is P ~ T_e^ 9 (T_e^ 5) and ne dependence gives P ~ n_e ^-5/3(n_e^-1/3). The T_e dependence is characteristics of 3D phonons and n_e dependence is characteristics of 3D Dirac fermions. In BG regime, screening effect is found to enhance for larger n_e. Screening is found to reduce the range of validity of BG regime temperature. Plot of P / T_e^ 4 vs T_e shows a maximum at temperature T_e m which shifts to higher values for larger n_e. Interesting observation is that maximum of P / T_e^4 is nearly same for different n_e and T_em / n_e^ 1/3 appears to be nearly constant. More importantly, we propose, the n_e dependent measurements of P would provide a clearer signature to identify 3D Dirac semimetal phase.