Excitation of coherent phonons in thin films of the Dirac semimetal SrMnSb₂

When a laser beam is incident on an object, some of the energy from the light is transferred to the atoms and causes their positions to oscillate. A sub-picosecond pulse incident on a crystalline structure, like the Dirac semimetal SrMnSb₂, may cause the atoms to oscillate uniformly in what is called a coherent phonon. This is significant because coherent phonons temporarily change the semimetal’s electron band structure, which changes the material’s optical and electric properties. Theory predicts that one particular coherent phonon would open and close the band gap. We studied these coherent phonons in various MBE-grown thin-film samples of SrMnSb₂, some with and some without arsenic capping-layers to protect them against oxidation, by pump-probe spectroscopy and Raman spectroscopy. We were able to excite the coherent phonon in every film we studied, though often we had to remove the arsenic cap first. The presence of the coherent phonon, already known from studies of bulk crystals, helps establish the bulk-like properties of these films, enabling future studies of the phonon’s dynamics and its effect on the material’s Dirac electrons. Successful bulk-like growth on the films’ InAs substrate helps to advance the integration of Dirac semimetals into III-V based electronics.