Figure 3. Phonon MFP and thermoelectric figure of merit for the phonon-drag effect.

(a) Temperature dependence of the phonon MFP, l_κ, evaluated from the thermal conductivity with Fourier's law. Broken lines are phonon MFP values (l_b described by the equation (1)) evaluated by considering scattering at crystal boundaries²⁹. Inset shows the sample shape with length scales used for equation (1). (b) Comparison of thermal (l_κ) and phonon-drag (l_s) phonon MFPs at 15 K (triangles) and 20 K (circles)⁹. (c) Phonon-drag dimensionless figure of merit z_pT with respect to l_s at 15 K (blue line) and 20 K (red line). z_pT also depends on the electron effective mass m* as shown by the broken lines.