Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=0.037 THz（i.e.,1.23 cm^（-1）or 0.153 meV）is unambiguously ge...Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=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 Cd_3As_2application potentials in thermoelectric devices.展开更多
基金Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA0303603,2016YFA0300303,2017YFA0302901 and 2016YFA0300604the National Natural Science Foundation of China under Grant Nos11774408,11574383 and 11774399+1 种基金the External Cooperation Program of Chinese Academy of Sciences under Grant No GJHZ1826the Interdisciplinary Innovation Team of Chinese Academy of Sciences
文摘Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_（2 ）is carried out.An acoustic phonon（AP）mode with central frequency f=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 Cd_3As_2application potentials in thermoelectric devices.
