Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd ₃ As ₂

Abstract

Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd3As2 is carried out. An acoustic phonon (AP) mode with central frequency = 0.037 THz (i.e., 1.23 cm −1 or 0.153 meV) is unambiguously generated and detected, which we attribute to laser-induced thermal strain. An AP chirping (i.e., variation of the phonon frequency) is clearly detected, which is ascribed to heat capacity variation with time. By comparing our experimental results and the theoretical model, we obtain a chirping time constant, which is 31.2 ps at 6 K and 19.8 ps at 300 K, respectively. Significantly, we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping, instead of a Fano resonance. Moreover, we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence, as well as temperature, which endows Cd3As2 application potentials in thermoelectric devices.

Ultrafast Optical Probe of Coherent Acoustic Phonons in Dirac Semimetal Cd3As2 Film Epitaxied on GaAs(111)B Substrate

“…One can see that the coherent acoustic phonon response exhibits nearly the same frequency of 0.039 THz for the thicker Cd 3 As 2 films with different thicknesses of 40, 150, and 200 nm, only the 20 nm thin film exhibits a response at 0.049 THz. In addition, the amplitude of the coherent acoustic phonon response in the 40, 150, 200 nm Cd 3 As 2 films exhibits the same temperature-dependent trend with that reported in a recent work for bulk Cd 3 As 2 , 14 in which the generated coherent acoustic phonon is attributed to the propagating strain pulse. 16 It is noted that the characteristic frequency 0.039 THz and its temperature-dependent behavior observed in 40, 150, 200 nm Cd 3 As 2 films are nearly the same as that of the bulk Cd 3 As 2 , 14 suggesting that the coherent acoustic phonon in these thicker Cd 3 As 2 films is generated following the propagating strain pulse model.…”

“…In addition, the amplitude of the coherent acoustic phonon response in the 40, 150, 200 nm Cd 3 As 2 films exhibits the same temperature-dependent trend with that reported in a recent work for bulk Cd 3 As 2 , 14 in which the generated coherent acoustic phonon is attributed to the propagating strain pulse. 16 It is noted that the characteristic frequency 0.039 THz and its temperature-dependent behavior observed in 40, 150, 200 nm Cd 3 As 2 films are nearly the same as that of the bulk Cd 3 As 2 , 14 suggesting that the coherent acoustic phonon in these thicker Cd 3 As 2 films is generated following the propagating strain pulse model. This is further confirmed by the coherent acoustic phonon responses probed with different wavelengths as below.…”

“…5,6 There have been growing studies reported for the novel phenomena in Cd 3 As 2 , such as the linear magnetoresistance, 7 topological properties, 8 magneto-optical response, 9 Landau levels, 10 quantum Hall effect, 11 superconductivity, 12 and nonlinear optical response in the THz region. 13 Recently, coherent longitudinal acoustic phonon (CAP) 14 and coherent helix vacancy phonon 15 generation have been reported in bulk Cd 3 As 2 ; however, there is no lattice dynamics study available for the epitaxial Cd 3 As 2 films. Exploring the phonon dynamics in Cd 3 As 2 is vital to understanding the properties of phonons that govern the thermal conductivity and electron scattering, which is important for developing thermoelectric and optoelectronic devices in the future.…”

Quasiparticle dynamics and phonon softening in FeSe0.9S0.1 superconductors

“…Such oscillatory behaviour has been observed in other Fe-based superconductors investigated by the pump–probe technique. 32–35 The LA coherent phonon amplitude A 3 and damping time τ 3 as functions of temperature are shown in Fig. 4a and b.…”

Separated Electron–Phonon and Phonon–Phonon Scatterings Across Interface in Thin Film LaCoO3/SrTiO3

“…[2,3,21,22] In parallel, coherent phonon can also be generated and detected by ultrafast pump-probe spectroscopy. [23][24][25] In this work, we investigate the fluence-dependent photocarriers dynamics in a 40 nm thick LaCoO 3 film on a SrTiO 3 substrate. Two distinct relaxation processes with lifetimes 𝜏 fast = 0.2 ps and 𝜏 slow = 0.9 ps are experimentally observed.…”