Coherent acoustic phonon dynamics facilitating acoustic deformation potential characterization of Mg3Sb2

Acoustic deformation potential (ADP) quantifies carrier-acoustic phonon coupling and is essential for dissecting transport physics in thermoelectrics. Herein, we report the use of ultrafast spectroscopy of coherent acoustic phonons (CAPs) to characterize the ADP of thermoelectric materials, using ${\mathrm{Mg}}_{3}{\mathrm{Sb}}_{2}$ as an example. The photon energy-dependent amplitudes of the CAP-induced oscillatory reflectance were used to determine the ADP coupling constant, agreeing well with that from first-principles calculations. This method relies on the transient Coulombic interaction between carriers and acoustic phonons, free of influence from other scattering channels. It is shown that the method is particularly feasible for the study of thermoelectric materials, because their common features of strong phonon anharmonicity and small band gaps make the measurement insensitive to the uncertainty of carrier diffusion coefficients, ensuring its accuracy.