Thermal transport in Yb-based 1-2-20 materials

Lattice thermal transport properties of heavy fermion YbT$_2$Zn$_{20}$-based (T = Co, Rh, and Ir) 1-2-20 compounds are calculated with single-mode relaxation-time approximation and full solution of linearized phonon Boltzmann transport equation from first-principles anharmonic phonon calculations. We predict low lattice thermal conductivity $\kappa_L$ with a maximum value of $\sim24.16\,$mW/m-K at 85.14 K, 137.29 mW/m-K at 9.67 K, and 23.55 mW/m-K at 67.50 K and $\sim12.02\,$, 40.04, and 10.30 mW/m-K at room temperature for YbCo$_2$Zn$_{20}$, YbRh$_2$Zn$_{20}$, and YbIr$_2$Zn$_{20}$, respectively. Based on the analysis of the cumulative $\kappa_L$ as a function of the phonon mean-free path and the frequency-dependent joint density of states at various temperatures, we attribute the low $\kappa_L$ to a rattling mode with an average characteristic rattling frequency $\omega\sim2.67\,$THz that enhanced phonon scattering processes, which reduced the phonon mean-free paths, suppressed the phonon lifetime, and enhanced the probability of three-phonon scattering events. The predicted low $\kappa_L$, especially in YbCo$_2$Zn$_{20}$ and YbIr$_2$Zn$_{20}$, are promising for thermoelectric and thermal management applications.